Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
We’ll finish off the “work” week with another Attenborough video, featuring a remarkable oasis in the Sahara that isn’t what it seems. Fortunately, the swallows have evolved an adaptation to the oasis’s trick. I wonder if the birds’ ingestion of flies is an evolved rather than a learned trait (I suspect it’s the former given the mortality produced by drinking the saline water).
I’m itching to do biology posts, and have at least one or two papers on my desk, but of course those are the hardest posts of all and get few comments. Nevertheless, I’m proceeding, but every day I get about eight or none political posts that are less work. Well, I do my best.
In lieu of a real science post, here’s some natural history by David Attenborough, showing a spider that can put a web 2 meters across hung 25 meters across a river. (Rivers, of course, are good places to catch insects. What always amazes me about spiders is that their brains are so small yet are complex enough to encode very sophisticated behaviors, including weaving webs of intricate and reproducible shape.
This spider has a different skill set, but still as amazing.
It’s Friday afternoon, the ducks are fed and watered for the weekend (it’s hot today but will cool down) and I’m soon off to hear about the fate of Botany Pond. This all means that it’s time for ani animal video.
How does it do this? See the next video, which shows that the shrimp actually packs a double punch, with the second involving boiling water.
The explanation from Wikipedia:
Mantis shrimp are commonly separated into many (most fall into spears and smashers but there are some outliers)[9] distinct groups determined by the type of claws they possess:
- Smashers possess a much more developed club and a more rudimentary spear (which is nevertheless quite sharp and still used in fights between their own kind); the club is used to bludgeon and smash their meals apart. The inner aspect of the terminal portion of the appendage can also possess a sharp edge, used to cut prey while the mantis shrimp swims.
- Spearers are armed with spiny appendages – the spines having barbed tips – used to stab and snag prey.
Both types strike by rapidly unfolding and swinging their raptorial claws at the prey, and can inflict serious damage on victims significantly greater in size than themselves. In smashers, these two weapons are employed with blinding quickness, with an acceleration of 10,400 g (102,000 m/s2 or 335,000 ft/s2) and speeds of 23 m/s (83 km/h; 51 mph) from a standing start.[10] Because they strike so rapidly, they generate vapor-filled bubbles in the water between the appendage and the striking surface—known as cavitation bubbles.[10] The collapse of these cavitation bubbles produces measurable forces on their prey in addition to the instantaneous forces of 1,500 newtons that are caused by the impact of the appendage against the striking surface, which means that the prey is hit twice by a single strike; first by the claw and then by the collapsing cavitation bubbles that immediately follow.[11] Even if the initial strike misses the prey, the resulting shock wave can be enough to stun or kill.
Smashers use this ability to attack crabs, snails, rock oysters, and other molluscs, their blunt clubs enabling them to crack the shells of their prey into pieces. Spearers, however, prefer the meat of softer animals, such as fish, which their barbed claws can more easily slice and snag.
Send in your photos, folks: I have about two days’ worth left.
Today we have a combination story-and-photo piece about pollination from reader Athayde Tonhasca Júnion. His notes are indented, and you can enlarge the photos by clicking on them.
Reluctant givers and industrious takers
For bees, pollen is an indispensable source of protein for egg production and larval development. So if they had it their way, bees would scoop up every pollen grain from a flower. And they are good at it, taking 95 to 99% of the powdery stuff back to their nests. The ‘wasted’ 1 to 5% of pollen that bees accidentally drop off or is left clinging to the bees’ hairs, is all a plant has for pollination.
Bees such as honey bees (Apis spp.) and bumble bees (Bombus spp.) carry almost all the pollen they gather in their corbiculae, or pollen baskets. From the Latin diminutive of corbis (basket), the corbicula is a shallow leg cavity surrounded by a fringe of elongated setae (‘hairs’). These bees, unsurprisingly called corbiculate bees, moisten the pollen with regurgitated nectar and saliva, so that it can be bundled up nicely for transport and easily unloaded once bees are back at their nests.
A European honey bee’s pollen basket © Gilles San Martin, Wikimedia Commons:
A corbiculate bee grooms herself regularly to remove stray pollen grains stuck to her body: most of them will be scooped up and stored securely © Ragesoss, Wikimedia Commons:
Other bees carry pollen attached to their scopa (Latin for ‘broom’), an area of dense, stiff hairs on the hind legs (typically in the families Halictidae and Andrenidae) or on the underside of the abdomen (mostly in the family Megachilidae). These non-corbiculate bees are not as tidy as their corbiculate counterparts: they do not wet and compress the pollen, but instead take it away just like dust particles clinging to the bristles of a brush or a broom.
The scopa of a megachilid or leaf-cutter bee © Pollinator, Wikimedia Commons:
Transporting pollen on the corbiculae or scopa makes a world of difference for pollination. Pollen tightly packed in the corbiculae is not easily stripped off by floral structures when the bee visits another plant, and it quickly loses its reproductive viability because it has been wet. Pollen on a scopa is kept dry and loosely attached to the bee, so it has a greater probability of being dislodged and resulting in plant fertilisation.
A load of pollen in a bumble bee’s pollen basket © Tony Wills, Wikimedia Commons:
A chocolate mining bee (Andrena scotica) with pollen loosely attached to its legs:
Regardless of how pollen is hauled away, bees’ efficiency puts plants in a jam. They need flower visitors for sexual reproduction, but the greedy blighters want it all for themselves. Pollen is metabolically expensive, so a plant can’t afford to produce lots of it and then lose most to palynivores (pollen eaters). But if it produces too little, bees may not be interested in dropping by.
To deal with this dilemma, plants have evolved strategies to keep visitors coming and at the same time not making it easy for them, thus minimising pollen profligacy. One cunning way to do this is to interfere with bees’ ability to groom themselves, so that more pollen grains are likely to be missed and end up on a receptive flower. To do this, there’s nothing better than nototribic flowers, which are built with an elaborate lever mechanism that makes stamens and style touch the dorsal surface of a visiting insect. This device is common in sage, mint and rosemary plants (family Lamiaceae), and in figworts (family Scrophulariaceae).
When a male Anthophora dufourii probes a Salvia hierosolymitana flower for nectar, its stamens are lowered to deposit pollen on the bee’s back © Gideon Pisanty, Wikimedia Commons:
Bees use their front legs to wipe their heads and antenna, and their middle and hind legs to clean their thoraxes and abdomens – you may have watched a bee or other insect doing these cleaning manoeuvres. But the space between their wings is a blind spot: think about an itch right between your shoulder blades, and you will understand the bee’s pickle. The pollen grains deposited on this hard-to-reach area are likely to escape grooming efforts and be taken to another flower.
Pollen of meadow clary (Salvia pratensis) seen under UV light on the back of B. terrestris © Koch et al., 2017:
Some flowers hide pollen at the bottom of their corollas, and visitors such as the fork-tailed flower bee (Anthophora furcata) must creep into these narrow, tubular structures that don’t allow much moving about. The bee vibrates her flight muscles to release the pollen, which gets attached to her head. She pulls out of the flower and scoops up the pollen with her front legs, but not all of it. Some grains become stuck to the thick, curved hairs sticking out between her antennae; these grains could end up on another flower.
A fork-tailed flower bee has to use her head – literally – to pollinate © Dick Belgers, Wikimedia Commons:
The common hollyhock (Alcea rosea) and other mallows (family Malvaceae) use a different tactic: they induce some bees to be less efficient gatherers thanks to their echinate pollen. Besides being prickly (echinate: covered with spines or bristles), these pollen grains are relatively large, thus difficult to handle and to mould into neat pellets. These features constitute a headache for corbiculate bees, the proficient packers, but are less of a problem for sloppy pollen harvesters such as solitary bees. As a result, more pollen grains are likely to be dislodged from bees who bother visiting these plants, increasing their chances of pollination.
Echinate pollen grains from three Malvaceae species © Konzmann et al. 2019:
Plants have developed other adaptations to minimise pollen harvesting, such as complex flower structures or progressive pollen release to force pollinators to make repeated visits. Some species hide pollen inside poricidal anthers, others produce indigestible or even toxic pollen so that only a few specialised pollinators can get to it; the palynivore hoi polloi is kept at bay. Many plants such as orchids are downright cheats: they lure pollinators with scent or visual mimicry but do not give away any nectar or pollen in return.
All these adaptations demonstrate that pollination is a negotiation between parties with conflicting interests. There is nothing altruistic here, bees and flowers are taking advantage of each other in an evolutionary give and take. Granted, this mutual exploitation has been fine-tuned in order to avoid disastrous imbalances. Plants can’t afford giving away too much pollen but can’t risk being too stingy; bees would take all the pollen they could handle, but settle for what’s available as long it’s worth their time and energy. Overly parsimonious plants and overly rapacious bees would collapse the relationship. Every plant-pollinator combination is an example of a mutually beneficial compromise; it’s natural selection as its best.
The Oxford English Dictionary gives three relevant definitions of the adjective “sentient”:
a.) That feels or is capable of feeling; having the power or function of sensation or of perception by the senses.
b.) Conscious or percipient of something.
c.) Physiology. Of organs or tissues: Responsive to sensory stimuli.
(“Sentience” itself is defined only as “The condition or quality of being sentient, consciousness, susceptibility to sensation.”)
The question that the Scientific American article below asks (and for once it’s written by a scientist in this field) is whether insects fit the definition of the first two definitions: do they have feelings and sensations experiencing qualia like pain, joy, pleasure, or the sensation of “redness”? Or are insects merely chitinous robots that are programmed by evolution to act (to us) as if they have feelings—programmed reactions that we anthropormophize as similar to our own sensations? After all, you can be “responsive to sensory stimuli” (the third sense above) without actually feeling the sensory stimuli the way humans do.
Answering the question of whether a bee or a fly is sentient in the first two senses, or has consciousness (the ability to be sentient and perceive stimuli), is difficult. Some would say it’s impossible. After all, we all know that we ourselves have consciousness and feel pain and joy, because we experience those things personally. But can I prove that, say, another person is conscious? Not directly, because we can’t get inside their brains. We infer that they’re conscious because they tell us they are; they are physically constructed with the same neurons that give us consciousness; and they act as if they experience qualia. It’s inference, but of a Bayesian sort, and the question has high priors.
But can we extend this to other species? Chittka uses the example of dogs:
Although there is still no universally accepted, single experimental proof for pain experiences in any animal, common sense dictates that as we accumulate ever more pieces of evidence that insects can feel, the probability that they are indeed sentient increases. For example, if a dog with an injured paw whimpers, licks the wound, limps, lowers pressure on the paw while walking, learns to avoid the place where the injury happened and seeks out analgesics when offered, we have reasonable grounds to assume that the dog is indeed experiencing something unpleasant.
This is a Bayesian approach to the question, and it’s really the only way to go. Yes, I think it’s highly probable that dogs, and most mammals, feel pain. But what about insects, reptiles and amphibians? They certainly avoid unpleasant stimuli and gravitate towards pleasant ones, which you could interpret as feeling joy, pleasure, or pain, but do they feel these sensations? If you say that the behavior denotes sentience, well remember that protozoans do these things, too (see below).
I’m fully aware that philosophers of mind have probably discussed this issue at length, and I haven’t followed that literature, so my musings here may seem childish to these philosophers. But this Sci. Am. article (click below to read, or find it archived here) is not written for philosophers of mind but for people like me: folks interested in science and wanting to see what’s happening in other fields. I found the article quite interesting, and for me it slightly raised the probability that insects can feel pain. But the answer remains far from settled—or even of having a high probability. And the author admits that. But he cites a number of cool studies.
Here are the lines of evidence that, to Chittka, raise the Bayesian probability that insects have sentience: experiencing pain, pleasure, and even joy.
a.) They learn and can do really smart things. (All quotes from Chittka are indented):
The conventional wisdom about insects has been that they are automatons—unthinking, unfeeling creatures whose behavior is entirely hardwired. But in the 1990s researchers began making startling discoveries about insect minds. It’s not just the bees. Some species of wasps recognize their nest mates’ faces and acquire impressive social skills. For example, they can infer the fighting strengths of other wasps relative to their own just by watching other wasps fight among themselves. Ants rescue nest mates buried under rubble, digging away only over trapped (and thus invisible) body parts, inferring the body dimension from those parts that are visible above the surface. Flies immersed in virtual reality display attention and awareness of the passing of time. Locusts can visually estimate rung distances when walking on a ladder and then plan their step width accordingly (even when the target is hidden from sight after the movement is initiated).
All of these responses, of course, could come from computers programmed to learn from experience, which is exactly what we and other animals are. Natural selection has endowed us with a neuronal network that will make us behave in ways to further our reproduction (or, sometimes, that of our group—like an ant colony). We can program computers to do this, too: robots that avoid aversive stimuli and gravitate towards good ones. And clearly we behave in such a way that furthers our reproduction, of which survival is one component. But do insects experience the world, with its pleasures and pains, by having qualia similar to ours?
A related question is this: is consciousness like we have (feeling pain and joy) something that’s merely an epiphenomenon of having evolved a sufficiently complex nervous system, or is consciousness itself a product of natural selection to further our reproduction? We don’t know the answer, but it’s pretty clear that some of our conscious experiences, like pain, have evolved by selection. People who can’t feel pain as a result of neurological conditions or disease (like Hansen’s disease) quickly start getting infections, hurting their bodies without being aware, losing fingers, and the like. If you didn’t experience pain when putting your hand in boiling water, you’d damage your body. But if consciousness is just an epiphenomenon of a complex evolved nervous system, then we can’t automatically say that bees that act as if they’re conscious really are conscious.
I’m prepared to believe, based on what I said above, that mammals feel pain. Maybe even reptiles or amphibians, though there are suggestions that fish don’t feel pain, at least in the way we do. All these creatures gravitate towards adaptive things and avoid nonadaptive ones, but again, they could be programmed to do so without the ancillary conscious experience that we have.
More evidence from Chittka:
b.) Insects act as if they can alter their consciousness:
Many plants contain bitter substances such as nicotine and caffeine to deter herbivores, but these substances are also found in low concentrations in some floral nectars. Researchers wondered whether pollinators might be deterred by such nectars, but they discovered the opposite. Bees actively seek out drugs such as nicotine and caffeine when given the choice and even self-medicate with nicotine when sick. Male fruit flies stressed by being deprived of mating opportunities prefer food containing alcohol (naturally present in fermenting fruit), and bees even show withdrawal symptoms when weaned off an alcohol-rich diet.
Again, seeking out things that are good for you, like curing you of illness or infection, could be programmed, either directly or as part of programs involved in “learning what gets rid of harmful conditions”. Now if bees are partial to coffee and cigarettes because it gets them high, then yes, it seems to show that they want to alter their consciousness, which implies that they have consciousness. But these facts aren’t that convincing to me, because nicotine and caffeine may have other beneficial physiological effects.
c.) Bees appear to be “optimistic”. Here’s the experiment Chittka adduces to support that:
We trained one group of bees to associate the color blue with a sugary reward and green with no reward, and another group of bees to make the opposite association. We then presented the bees with a turquoise color, a shade intermediate between blue and green. A lucky subset of bees received a surprise sugar treat right before seeing the turquoise color; the other bees did not. The bees’ response to the ambiguous stimulus depended on whether they received a treat before the test: those that got the pretest sugar approached the intermediate color faster than those that didn’t.
The results indicate that when the bees were surprised with a reward, they experienced an optimistic state of mind. This state, which was found to be related to the neurotransmitter dopamine, made the bees more upbeat, if you will, about ambiguous stimuli—they approached it as they would the blue or green colors they were trained to associate with a reward.
This is not a meaningless experiment, but to me shows only that bees conditioned to approach a color after a sugar reward are more likely to approach something like that color than those who weren’t conditioned. To call this “optimism” seems to me hyperbolically anthropomorphic.
d). Bees appear to experience “joy”. This experiment is more suggestive to me:
Other work suggests that bees can experience not only optimism but also joy. Some years ago we trained bumblebees to roll tiny balls to a goal area to obtain a nectar reward—a form of object manipulation equivalent to human usage of a coin in a vending machine. In the course of these experiments, we noticed that some bees rolled the balls around even when no sugar reward was being offered. We suspected that this might be a form of play behavior.
Recently we confirmed this hunch experimentally. We connected a bumblebee colony to an arena equipped with mobile balls on one side, immobile balls on the other, and an unobstructed path through the middle that led to a feeding station containing freely available sugar solution and pollen. Bees went out of their way to return again and again to a “play area” where they rolled the mobile balls in all directions and often for extended periods without a sugar reward, even though plenty of food was provided nearby. There seemed to be something inherently enjoyable in the activity itself. In line with what other researchers have observed in vertebrate creatures at play, young bees engaged more often with the balls than older ones. And males played more than females (male bumblebees don’t work for the colony and therefore have a lot more time on their hands). These experiments are not merely cute—they provide further evidence of positive emotionlike states in bees.
It’s hard to understand these results without thinking that bees, like panda cubs, are playful, messing around with balls that give them pleasure. And since bees don’t experience balls in their natural state, they could be enjoying the novelty. On the other hand, they could simply be encountering something they haven’t experienced, and are following neuronal instructions to manipulate it to see how it operates, which could be useful knowledge in the future. This second interpretation means that no “pleasure” need be involved. Remember, play behavior in animals is often there to prepare them for what happens when they become adults, and isn’t just there for pleasure.
Again, it’s hard to judge from such studies whether bees are feeling pleasure in the way we do. But to me this makes it marginally more likely.
Finally,
e). Bees appear to weigh pain against pleasure, and change their behaviors when the balance is altered. Here’s another experiment:
We decided to do an experiment with only moderately unpleasant stimuli, not injurious ones—and one in which bees could freely choose whether to experience these stimuli.
We gave bees a choice between two types of artificial flowers. Some were heated to 55 degrees Celsius (lower than your cup of coffee but still hot), and others were not. We varied the rewards given for visiting the flowers. Bees clearly avoided the heat when rewards for both flower types were equal. On its own, such a reaction could be interpreted as resulting from a simple reflex, without an “ouch-like” experience. But a hallmark of pain in humans is that it is not just an automatic, reflexlike response. Instead one may opt to grit one’s teeth and bear the discomfort—for example, if a reward is at stake. It turns out that bees have just this kind of flexibility. When the rewards at the heated flowers were high, the bees chose to land on them. Apparently it was worth their while to endure the discomfort. They did not have to rely on concurrent stimuli to make this trade-off. Even when heat and reward were removed from the flowers, bees judged the advantages and disadvantages of each flower type from memory and were thus able to make comparisons of the options in their minds.
To me, this really shows nothing more than that animals are attracted to adaptive stimuli and repelled by harmful ones, with the addition of being able to balance harms versus advantages. (This is like the “flight distance” of animals, with some individuals able to give more weight to attractive stimuli. That’s probably how cats got domesticated!) But it doesn’t tell us whether animals are feeling the pain or attraction the way we do.
And we should remember that even protozoans show avoidance of some external stimuli and can be induced by electrical shocks to avoid light. So these animals can be trained. Do they feel pain or pleasure? I doubt it—not protozoa! They may not show “play” behavior, but perhaps they can be trained to weigh aversive versus adaptive stimuli, as in section “d” above. I doubt anybody would conclude with confidence that protozoa feel pain the way we do (they don’t have a nervous system) or are even conscious.
Against the doubts that I’ve raised, Chittka offers a counterargument:
Critics could argue that each of the behaviors described earlier could also be programmed into a nonconscious robot. But nature cannot afford to generate beings that just pretend to be sentient. Although there is still no universally accepted, single experimental proof for pain experiences in any animal, common sense dictates that as we accumulate ever more pieces of evidence that insects can feel, the probability that they are indeed sentient increases.
The first sentence is what I have said already. And I’m willing to go along with the third sentence, too: as we learn more, the Bayesian probability that other species experience pain or pleasure can increase or decrease.
But I’m not willing to go along with the idea that “nature cannot afford to generate beings that just pretend to be sentient.” What does he mean by “afford”? My interpretation is this: he’s saying that natural selection cannot produce organisms that act as if they’re sentient unless they really are sentient. And I cannot see any support for that, for we already know that protozoans act as if they experience qualia, but almost certainly don’t. And saying “pretend to be sentient” is pretty anthropormorphic! It implies, for example, that programmed robots that do what bees do are “pretending to be sentient” when in fact we know they are NOT sentient.
Finally, that leads to the Big AI Question: if we generate robots sufficiently complex that they respond exactly as humans do in complex situations requiring consciousness, does that mean that they have become conscious? I say “no”, but others disagree. After all, there are those panpsychists who say that even electrons and rocks have a rudimentary form of consciousness.
I’m writing this on the fly, so forgive me if my thoughts are half-baked. I do think that Chtittka’s experiments are clever, and, over time, may give us a sense of sentience in other species. But I’m not yet ready to throw in with him on the claim that insects are conscious. It’s enough for me now to realize that they do experience some aspects of the environment as things to be avoided. And that is why I have always anesthetized my fruit flies before killing them. (When I was an undergrad I used to take them to the biology department roof and let them go, but my advisor Bruce Grant nixed that on the grounds that I was polluting the natural gene pool of Drosophila.)
The last bit of Chittka’s paper is a thoughtful analysis of how these kinds of studies should condition our behavior towards insects. But even if they don’t feel pain, aversion or attraction itself should help us confect a philosophy of “insect ethics.”
h/t: Howard, who brought this paper to my attention and wanted my take on it. I’m sending him this link as my take.
Everyone loves puffins, and there’s a live PuffinCam on YouTube that operates 24 hours a day off the coast of Maine. Here are the facts:
This live puffin cam overlooks the “loafing ledge” on Seal Island, 21 miles off the coast of Maine. The loafing ledge is a prime spot for puffins to congregate, with plenty of “exit routes” in every direction in case a hawk or gull attacks.
Click the video and loaf with the Atlantic puffins (Fratercula arctica). Right now (12:30 pm) I think most of them are feeding, but later on they’ll be schmoozing on the ledge. Be sure to notice how they fly: it’s amazing these things can even take to the air! You’ll see other species loafing with the puffins, too.
Machias Seal Island is in fact territory (20 acres) disputed between the U.S. and Canada!
Machias Seal Island is a barren island and devoid of trees. Because of its location at the boundary between the Gulf of Maine and the Bay of Fundy, Machias Seal Island is fog-bound for many days of the year. It is also a sanctuary for seabirds such as Atlantic puffins, razorbills, common murres, common and Arctic terns, Leach’s storm-petrels, and common eiders.
Here’s the island on the horizon:
h/t: Jean
Nature red in claw and tentacle! Here an octopus takes on a crab in a battle to the death. Before you start the video, can you guess who wins?
A frog pollinating a flower? That would be remarkable, and a paper supposedly describing the phenomenon was recently published. It got a lot of attention, including a large piece in (guess where?) Scientific American. I was prepared to write about it based on the publicity, but I needed to see the paper first. When I read it, I was sorely disappointed. The evidence for frog pollination, which would be the very first described case of an amphibian effecting pollination—was as thin as a piece of paper.
The relevant paper, from the journal Food Webs, is not widely available, even through the University of Chicago library. Fortunately, a kind reader somehow got hold of the pdf (you can too, through judicious inquiry), and just a bit of it is online, which you can see by clicking the link below.
I’ll be brief (well, I’ll try). This group of investigators from Brazil report that the tropical treefrog Xenohyla truncata was observed eating fruit, petals of flowers, and sipping nectar during one four-hour observation period. This itself is unusual because frogs are mostly carnivorous and insectivorous. This species (and one congeneric relative) were reported earlier to be omnivorous, eating both fruits and invertebrates.
The popularized result? One (count it, one) frog was found with pollen on its back after sticking itself into a flower to eat. Was it observed pollinating another flower? No. We don’t even know if the plant is self-compatible, so that a frog could even effect cross-pollination on the same plant. Did the frog visit more than one plant, so that cross pollination was possible? No.
The upshot s that all the publicity given to this frog is comes from the observation of a single individual exiting a flower with pollen on its back. That says virtually nothing about whether it is a pollinator, and even less about whether it’s an important pollinator.
Here are the data described in the paper:
We conducted in situ observations of a breeding population of X. truncata on 15 December 2020, in a Restinga vegetation area in the municipality of Búzios, state of Rio de Janeiro, southeastern Brazil (22◦46′13.94”S, 41◦57′4.47” W; WGS84; 2 m a.s.l.), for approximately four hours (from 6:00 to 10:00 pm). Air temperature was 25.8 ◦C. We observed five individuals of X. truncata in feeding activity on two plant species between 7:00 and 9:00 pm.
. . .Around 8:00 pm we observed other X. truncata individuals leaving bromeliads and climbing a Brazilian milk fruit tree [JAC: Cordia taguahyensis] full of fruits and flowers. Three individuals (sex undetermined) clustered around a ripe fruit and began a dispute to get close to the fruit, pushing each other away as they tried to bite the fruit (Fig. 1D; Video S2). After approximately five min, two individuals gave in and remained perched on branches close to the fruit, while the third began to nibble the fruit, increasing a pre-existing hole to gain access to the pulp (Fig. 1D). While this individual fed on the fruit, the others no longer disturbed it. The same individual remained nibbling and sucking the fruit pulp for about 10 min, the others eventually approaching to feed. x
. . . On the other side of the same tree, we observed a X. truncata individual that climbed a branch and entered an open flower (Fig. 1E), where it remained for approximately 5 min performing suction-like movements (Video S3). Upon leaving, pollen grains were adhered to its back (Fig. 1F).
This is the first report of a frog species actively feeding on nectar and flowers in nature and the first evidence that it may act as pollinator.
Here are the two pictures mentioned above, along with their captions from the paper:
The frog was also observed feeding on an “alien” (THEIR WORD) species, the beareded Iris, Iris x germanica (see video below; the “x” indicates the species is of hybrid origin).
So what’s the evidence that this frog actually effected any pollination? None that I can see. What is the evidence that it’s a regular and important pollinator of the native milk fruit tree? None, nada, zippo. Now it is possible that this frog could still pollinate other flowers on the same tree (if it’s self-compatible) or on other trees, but we don’t have that evidence. What we have is the authors’ speculation, eagerly and breathlessly snapped up and regurgitated by the press. More from the paper (my emphasis)
As mentioned, the relationship between X. truncata and the native C. taguahyensis is remarkable. The flower structure of C. taguahyensis allows X. truncata to enter and exit the flower, and to carry pollen grains after the visit. In this case, X. truncata could act as a pollinator of this species, or even of other plant species with similar floral structure. However, to play the pollinator role of C. taguahyensis, this frog should visit another flower or another plant individual on the same night. We lack information about the breeding system of C. taguahyensis, but some Cordia species are self-compatible, whereas others are self-incompatible (Opler et al., 1975; Machado and Loiola, 2000; Mcmullen, 2011; Wang et al., 2020). As X. truncata wanders from one plant to another before it settles in a bromeliad for daytime shelter (our pers. obs.), it is likely that the above mentioned scenario about its pollinator role actually occurs.
How likely is it? We don’t know.
Species in the genus Cordia are visited by a wide variety of invertebrates, such as bees, butterflies, beetles, wasps and flies (Opler et al., 1975; Machado and Loiola, 2000; Lopes et al., 2015), as well as vertebrates such as bats (Alvarez and Quintero, 1970) and birds (Opler et al., 1975; Dalsgaard, 2011; Wang et al., 2020). Thus, C. taguahyensis is likely pollinated by multiple animal species and the treefrog X. truncata is now a potential pollinator candidate.
“May”, “might”, “seems likely”, and so on it goes. To show pollination, you need to show pollination, not speculate that it’s likely. One way to do this is dust a flower with fluorescent pigment, like those used in making black-light posters, and then see if any fluorescent pollen makes its way to another flower (and, of course, that the flowers are reproductively compatible). This wasn’t done (we used this method to mark Drosophila in the wild.) Ergo, while we have new evidence that X. truncata is omnivorous and eats petals and nectar, and that pollen adheres to its back, that’s all we have. It’s possible that a cross pollination occurs occasionally, but even that is speculation, and doesn’t show that the frog, as opposed to the many insects that visit the plant, is of any importance as a pollinator. As the authors say, “the treefrog X. truncata is now a potential pollinator candidate.”
Well, at least there’s a video of the cute little frog eating from a flower (but the “alien” plant, not the milkfruit flower); this comes from IFL Science. Cute little bugger, isn’t it? The nectar and plant material may be a valuable supplement to the diet of this frog.
The journals that mentioned this paper as a possible case of amphibian pollination include Science, the New York Times, IFL Science, and other places. But the first place that comes up when you google “pollinating frog” is Scientific American.
Sofia Quaglia at the NYT gives the most critical take on this paper, saying this:
But more observations are needed to say the frogs really are pollinating plants.
“We cannot say that these frogs are actually pollinators,” said Felipe Amorim, a pollination ecologist at São Paulo State University who was not involved in the research. “They are flower visitors, they are flower-visitor frogs. We have a lot to learn about this novel interaction.”
For instance, the mucus secreted by the frog’s skin needs to be tested to confirm it doesn’t spoil the pollen before it gets to its destination. Scientists also need to work out whether the pollen is ever delivered to other flowers and if it does successfully fertilize and germinate them. It’s also still unclear why this frog species has developed a liking for flora over fauna in the first place.
And at least Sci. Am. mentions some of these problems. But really, the publicity given this observation, which has an interesting part (some frogs eat flowers and nectar) and a not-so-interesting part (one frog got pollen on its back) far exceeds its scientific novelty. This is what happen when either a university publicity machine goes into action or journalists copy each other’s content. Two colleagues to whom I sent the paper both found the claims of possible pollination by the press (and by the authors, too) wildly exaggerated.
As Kurt Vonnegut said, “So it goes.”
Someone called this Big Think piece to my attention because some quotes from me are in it. And they are, but that’s not the important part, which is the evolutionary biology of giving up, and I guess I’m the Expert Evolutionist in this take. The piece is by Julia Keller, a prolific author and journalist who won a Pulitzer Prize for feature writing in 2004, and this is an excerpt from her new book Quitting: A Life Strategy: The Myth of Perseverance and How the New Science of Giving Up Can Set You Free. which came out April 18.
Although I had some association with Julia when she wrote for the Chicago Tribune (I think she helped me get a free-speech op-ed published), I don’t remember even speaking to her on this topic, but it must have been quite a while back. At any rate, I certainly want to be set free from my maladaptive compulsions, which include persisting when I should give up, so I’ll be reading her book.
Click on the screenshot to read:
The science involved is largely evolutionary: it pays you to give up when you leave more offspring by quitting than by persisting. Or to couch it more accurately, genes that enable you to assess a situation (consciously or not) and give up at the right point—right before the relative reproductive gain from persisting turns into a relative loss compared to other gene forms affecting quitting—will come to dominate over the “nevertheless she persisted” genes. Keller engages the reader by drawing at the outset a comparison between Simone Biles stopping her gymnastic performance in the 2021 Tokyo games, and, on the other hand, a honeybee deciding whether or not to sting a potential predator of the nest.
If the bee does sting, she invariably dies (her innards are ripped out with the sting), and can no longer protect the nest. But if that suicidal act drives away a potential predator, copies of the “sting now” gene are saved in all the other nest’s workers, who are her half sisters. (And of course they’re saved in her mother—the queen, the only female who can pass on her genes.) If a worker doesn’t sting, every copy of that gene might be lost if the nest is destroyed, for if the nest goes, so goes the queen, and every gene is lost. On the other hand, a potential predator might not actually prey on a nest, so why give up your life if it has no result? You have to know when stinging is liable to pay off and when it isn’t.
Inexorably, natural selection will preserve genes that succeed in this reproductive calculus by promoting stinging at the right time and place—or, on the other hand refraining from stinging if it’s liable to have no effect on colony (ergo queen) survival. And in fact, as you see below, honeybees, while they surely don’t consciously do this calculus, they behave as if they do, and they do it correctly. Often natural selection favors animals making “decisions” that cannot be conscious, but have been molded by selection to look as if they were conscious.
As for Simone Biles, well, you can read about her. Her decision was clearly a conscious one, but also bred in us by selection—selection to avoid damaging our bodies, which of course can severely limit our chance to pass on our genes. This is why we usually flee danger when there is nothing to gain by meeting it. (She did have something to gain—gold medals—which is why she’s like the bees.)
Why do young men street race their cars on the street, a dangerous practice? What do they have to gain? Well, risk-taking is particularly prevalent in postpubescent males compared to females, and I bet you can guess why.
I’ll first be a bit self aggrandizing and show how I’m quoted on evolution, and then get to the very cool bee story. It’s a short piece, and you might think of other “quitting vs. non quitting” behaviors of animals that could have evolved. (Hint: one involves cat domestication.)
“Perseverance, in a biological sense, doesn’t make sense unless it’s working.”
That’s Jerry Coyne, emeritus professor at the University of Chicago, one of the top evolutionary biologists of his generation. [JAC: a BIT overstated, but I appreciate it.] I’ve called Coyne to ask him about animals and quitting. I want to know why human beings tend to adhere to the Gospel of Grit—while other creatures on this magnificently diverse earth of ours follow a different strategy. Their lives are marked by purposeful halts, fortuitous side steps, canny retreats, nick‑of‑time recalculations, wily workarounds, and deliberate do‑overs, not to mention loops, pivots, and complete reversals.
Other animals, that is, quit on a regular basis. And they don’t obsess about it, either.
In the wild, Coyne points out, perseverance has no special status. Animals do what they do because it furthers their agenda: to last long enough to reproduce, ensuring the continuation of their genetic material.
We’re animals, too, of course. And despite all the complex wonders that human beings have created—from Audis to algebra, from hot-fudge sundaes to haiku, from suspension bridges to Bridgerton—at bottom our instincts are always goading us toward the same basic, no‑nonsense goal: to stick around so that we can pass along little copies of ourselves. [JAC: note how this is an individual-centric view rather than the correct gene-centric one, but it’s good enough.] It’s axiomatic: the best way to survive is to give up on whatever’s not contributing to survival. To waste as few resources as possible on the ineffective. “Human behavior has been molded to help us obtain a favorable outcome,” Coyne tells me. We go for what works. We’re biased toward results. Yet somewhere between the impulse to follow what strikes us as the most promising path—which means quitting an unpromising path—and the simple act of giving up, something often gets in the way. And that’s the mystery that intrigues me: When quitting is the right thing to do, why don’t we always do it?
Well, who ever said that every aspect of human behavior was molded by natural selection? Please don’t think that I was implying that it was, as we have a cultural veneer on top of the behaviors conditioned by our genes. In this piece Keller doesn’t get to the subject of why we don’t quit when we should. I’m sure that’s in the book.
Now the very cool bee story:
Justin O. Schmidt is a renowned entomologist and author of The Sting of the Wild, a nifty book about a nasty thing: stinging insects. Living creatures, he tells me, echoing Coyne, have two goals, and those goals are rock-bottom rudimentary: “To eat and not be eaten.” If something’s not working, an animal stops doing it—and with a notable absence of fuss or excuse-making. . . .
. . . For a honeybee, the drive to survive carries within it the commitment to make sure there will be more honeybees. And so she defends her colony with reckless abandon. When a honeybee stings a potential predator, she dies, because the sting eviscerates her. (Only the females sting.) Given those odds—a 100 percent mortality rate after stinging—what honeybee in her right mind would make the decision to sting if it didn’t bring some benefit?
That’s why, Schmidt explains to me from his lab in Tucson, sometimes she stands down. When a creature that may pose a threat approaches the colony, the honeybee might very well not sting. She chooses, in effect, to quit—to not take the next step and rush forward to defend the nest, at the cost of her life.
His experiments, the results of which he published in 2020 in Insectes Sociaux, an international scientific journal focusing on social insects such as bees, ants, and wasps, reveal that honeybees make a calculation on the fly, as it were. They decide if a predator is close enough to the colony to be a legitimate threat and, further, if the colony has enough reproductive potential at that point to warrant her ultimate sacrifice. If the moment meets those criteria—genuine peril (check), fertile colony (check)—the honeybees are fierce fighters, happy to perish for the greater good.
But if not… well, no. They don’t engage. “Bees must make life‑or‑death decisions based on risk-benefit evaluations,” Schmidt tells me. Like a gymnast facing a dizzyingly difficult maneuver that could prove to be lethal, they weigh the danger of their next move against what’s at stake, measuring the imminent peril against the chances of success and the potential reward. They calculate odds.
And if the ratio doesn’t make sense, they quit.
That’s a bit oversimplified, for the calculus is not only unconscious (I doubt bees can weigh threats this way), but the decision capability has been molded by competition over evolutionary time between different forms of genes with different propensities to sting or give up. Further, individual worker bees are sterile, and so what’s at stake is the number of gene copies in the nest as a whole—and especially in the queen. The asymmetrical relatedness between the queen, her workers, and their useless drone brothers (produced by unfertilized eggs) makes the calculus especially complicated.
On the other hand, explaining the gene calculus to lay readers is hard, and it might be better to read the seminal work on how this all operates: Dawkins’s The Selfish Gene.
Here’s Schmidt’s short paper (click to read; if it’s paywalled, ask for a copy). He died just this February.
Fortuitously, when I hadn’t prepared any posts for today that require my neurons to work, reader Athayde Tonhasca Júnior came through with one of his patented text+photo stories, this time a fascinating one about how opportunistic natural selection can create predator/parasite niches within niches in completely unexpected and astonishing ways. This hierarchy was wonderfully expressed in the short poem “Siphonaptera” (the order in which fleas are placed) by British mathematician Augustus De Morgan:
Great fleas have little fleas upon their backs to bite ’em,
And little fleas have lesser fleas, and so ad infinitum.
And the great fleas themselves, in turn, have greater fleas to go on;
While these again have greater still, and greater still, and so on.
Athayde’s text is indented, and click on the photo to enlarge them.
The body snatchers
by Athayde Tonhasca Júnior
Family feuds abound in history and in the tabloids, but things got really out of hand with the offspring of Egyptian gods Geb (Earth) and Nut (sky). As the first-born, Osiris was naturally chosen to be the ruler of the world. But his brother Set didn’t care one bit for this undemocratic arrangement, so he decided to despatch Osiris to the Underworld. So he set out a murderous plan worthy of an Agatha Christie story. Set first commissioned a beautiful casket, tailored to fit a body with Osiris’ exact measurements. Set then organised a magnificent banquet, inviting heavenly celebrities and bro Osiris. When they were all done with the eating and drinking, Set announced a surprise. The casket was brought in, and the host told his guests that whoever could fit inside, could take it home (an odd gift to us, perhaps, but who are we to judge Egyptian gods?). One by one the guests climbed into the casket, which was too small or too big – until Osiris had a go at it. He laid down inside the casket, which, to his glee, fit him perfectly. Set’s trap was set; he slammed the casket’s lid shut and locked it, killing his sibling. Later Set retrieved Osiris’ body and chopped it into small pieces.
The Mummy (1932) escaped from his sarcophagus, but no such luck for Osiris. Art by Karoly Grosz, Wikimedia Commons:
Set’s shenanigans were the perfect inspiration for naming a new species from the genus Euderus, a small group of parasitic wasps in the family Eulophidae. Most Euderus species are moth and beetle parasitoids, but the wasp discovered by Egan et al. (2017) in Florida (USA) is peculiar, to say the least. Its host, Bassettia pallida, is itself a parasitic wasp, but of a different kind: this species is one of the many gall wasps or cynipids (family Cynipidae), which lay their eggs in oaks (Quercus spp.) and less commonly in related plants (family Fagaceae). The egg-laying induces the plant to produce a gall, which is an abnormal growth resulting from increased size or number of cells (galls can also be caused by tissue feeding or infections by bacteria, viruses, fungi and nematodes). Cynipids trigger their host plants to produce nutritious tissue inside their galls, which become ideal places for a larva to grow: there’s nothing better for one’s survival than a cosy, safe and nourishing nursery.
Oak galls or oak apples, growths resulting from chemicals injected by the larva of gall wasps © Maksim, Wikimedia Commons:
In the case of B. pallida, it induces the formation of galls inside stems of sand live oak (Q. geminata) and southern live oak (Q. virginiana). Each of these galls is called a ‘crypt’. So appropriately, B. pallida is known as the crypt gall wasp. When the adult wasp completes its development, it chews an exit hole from inside its woody quarters and flies away.
(a): a crypt gall wasp; (e): adults’ exit holes © Weinersmith et al., 2020:
Life looked good for the crypt gall wasp in the southeastern United States—until we learned about the machinations of its recently discovered enemy. The Eulophidae parasitoid locates a crypt and pierces it with its ovipositor, laying an egg inside the chamber, near or into the developing crypt gall wasp. We don’t know exactly what goes on inside the chamber, but the outcome is not good at all for the crypt gall wasp. When it tries to chew its way out, it’s no longer able to create a hole big enough to fit its body: the wasp becomes entrapped inside its crypt, Osiris-like. During its failed attempt to get out, its head blocks the exit hole. All the better for the parasitoid larva that hatched inside the crypt: it can feed at leisure on the host’s weakened body. On completing its development, the adult parasitoid wasp chews through the host’s head plug and comes out to the big wide world. So there was no better name for this species than Euderus set, the crypt-keeper wasp.
JAC: Isn’t that an amazing story? I’m sure we don’t know how the parasitoid disables the gall wasp in this way. Imagine the genetic changes involved in this complex evolution, involving the parasitoid’s egg-laying and multiple behaviors of its larval stage. But that’s a passing expression of amazement; let’s continue with Athayde’s tale:
(c): a crypt-keeper wasp pupa in a chamber made by a crypt gall wasp; (f): an exit hole plugged by the head capsule of a dead or dying crypt gall wasp; (g): a crypt gall wasp head capsule chewed through by an exiting crypt-keeper wasp © Weinersmith et al., 2020.
The relationships between oaks and these wasps are examples of host manipulation, which happens when a parasite influences the host’s behaviour or physiology to its (the parasite’s) advantage. The crypt gall wasp induces its host plants to produce galls for its benefit, and in turn the crypt-keeper wasp forces its host into becoming trapped and an easy meal for the parasitoid’s larva: the manipulation of a manipulator is known as a hyper-manipulation, an uncommon phenomenon.
A female crypt-keeper wasp, a hyper-manipulator © Egan et al., Wikimedia Commons.
There are many cases of host manipulation, and the zombie-ant fungus described by the co-author of the theory of evolution by natural selection Alfred Russel Wallace (1823-1913) is one of the better known. This fungus (Ophiocordyceps unilateralis) induces its host ants to climb up the vegetation and clamp their mandibles around a twig or leaf vein. An infected ant will stay put, rain or shine, while the fungus grows inside it. After 4-10 days the ant dies, the fungus grows a ‘stalk’ (stroma) from the ant’s head and releases spores that will infect ants walking about on the forest floor.
A dead Camponotus leonardi ant attached to a leaf vein. The stroma of a zombie-ant fungus emerges from the back of the ant’s head © Pontoppidan et al., 2009:
The more researchers look into it, the more they find cases of host manipulators such as the Darwin wasps Hymenoepimecis spp., which parasitize several species of orb-weaving spiders in the Neotropical region. A female wasp stings and temporarily paralyses her victim, laying an egg on its abdomen. The emerging larva bites through the spider’s cuticle and feeds on its ‘blood’ (haemolymph). The spider carries on with its life, building webs and catching prey, but the growing parasitoid takes its toll; eventually it kills its host.
L: A H. heidyae egg attached to a Kapogea cyrtophoroides. R: Third instar H. heidyae larva feeding on a recently killed spider; the inset shows details of the dorsal hooks used by the larva to cling to its host © Barrantes et al., 2008.
But shortly before the spider’s demise, somehow —probably by hormone injection—the larva takes command of the host’s behaviour. The spider builds a cocoon web made of thickly woven silk, which doesn’t look at all like a normal web. The spider dies, the larva enters the cocoon and completes its development. Some days later, the adult wasp emerges and flies away.
a. A normal K. cyrtophoroides web; b. The web’s hub; c. A cocoon web induced by the parasitoid; d. Central section of the cocoon web and the wasp’s cocoon © Barrantes et al., 2008.
Parasitic wasps are not deterred by the defences of hosts such as Anelosimus eximius. This is one of the few species of social spiders; they build massive tent-like nests that shelter hundreds or thousands of individuals, who hunt together in raiding packs and even cooperate in raising their young (click the next link to watch their comings and goings). But in the Amazon region, A. eximius can’t evade the Darwin wasp Zatypota sp. A parasitized spider leaves the colony and builds its own cocoon-like web. It then becomes immobilised, so that the wasp larva can unhurriedly consume it. When finished with its meal, the larva enters the cocoon to complete its development. The larger the spider colony, the more chances of being parasitized; up to 2% of individuals become hosts to the parasitoid (Fernandez-Fournier et al., 2018).
L: A group of A. eximius in a communal web © Bernard Dupont, Wikimedia Commons. R: A 5-m long, 3-m high colony of A. eximius; photo by A. Bernard © Krafft & Cookson, 2012:.
A fierce looking H. neotropica and its larva feeding on an Araneus omnicolor © Sobczak et al., 2012.
Host manipulation seems to be much more common than we thought, so we shouldn’t expect pollinators to be safe from it. And they are not. The conopid fly (family Conopidae) Physocephala tibialis forces bumblebee hosts to bury themselves in the soil just before dying. The nematode worm Sphaerularia bombi, found throughout the northern hemisphere and South America, infects queens of several bumble bee species, castrating its host. And at least for the buff-tailed bumble bee (Bombus terrestris), the nematode also alters the bee’s behaviour (Kadoya & Ishii, 2015). An infected queen feeds normally, but does not breed or build a nest. Instead, she keeps flying into the early summer months, and by doing so she unintentionally helps to spread the nematode. Certainly many other cases of pollinators’ manipulation by parasites wait to be discovered because their effects can be subtle and inconspicuous.
CSI Garden: a post-mortem examination of a buff-tailed bumble bee found dying on a roadside pavement in England revealed an infestation by the host-manipulating nematode S. bombi © The Encyclopedia of Life:
Host manipulation can be seen as a form of extended phenotype (Dawkins, 1982; phenotype refers to a species’ observable characteristics resulting from the expression of its genes). By changing the host’s behaviour for its own benefit, the parasitoid – ultimately, its genes – expresses its phenotype in the world at large. In Dawkins’ own words, ‘an animal’s behaviour tends to maximize the survival of the genes “for” that behaviour, whether or not those genes happen to be in the body of the particular animal performing it’. The phenomenon would have deep consequences for natural selection, but the extent of extended phenotypes has been debated since the publication of Dawkins’ book.
If you are smugly assuming that behavioural puppeteering is for lower animals such as insects, you’d better think again. Some studies suggest that rodents infected with the protozoan Toxoplasma gondii become more active but sluggish in reacting to alarm signals; worse, they may become attracted to the smell of cat’s urine. If so, an infected mouse has a good chance of prematurely ending its days in a moggie’s maw – which was T. gondii‘s ‘intention’ all along, since cats are its ultimate host. And the plot thickens: infected cats excrete T. gondii spores in their faeces, which can make their way into other mammals. A 26-year study with grey wolves (Canis lupus) from Yellowstone National Park, Wyoming, USA, revealed that infected individuals – probably the result of contact with pumas (Puma concolor) – are bolder, more likely to become pack leaders and have better chances of reproducing (Meyer et al., 2022). In humans, toxoplasmosis, the infection caused by T. gondii, is widespread but usually does not have any symptoms. Most people don’t even know they have it, but all sorts of behaviour and mental disorders such as heightened aggression and Parkinson’s disease have been linked to the infection. The effects of T. gondii on rodents and humans have been disputed because data often show weak, inconclusive or no effects (Johnson & Koshy, 2020). In any case, our invulnerability to the manipulative power of parasites should not be taken for granted. Rephrasing the quote misattributed to Margaret Mead, always remember that in biology, Homo sapiens is unique. Just like every other species.
Invasion of the Body Snatchers (1956). Art by Allied Artists Pictures Corporation. Wikimedia Commons.
JAC note: I don’t think that in any of these cases of host manipulation (or any others that I’ve heard of) do we know the chemical and developmental basis of the manipulation. What does a fungus do to an ant to make it climb a stalk of grass, grip it tightly with its mandibles, and then die? How does the Darwin wasp manipulate the spider’s behavior to cause it to weave a cocoon-like web instead of its normal web—something good for the wasp? These are incredibly sophisticated manipulations that have evolved in ways we don’t understand.
If this is the work of a creator, he must have been a sadist!
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