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.
Mark Sturtevant has answered the call for photos with some lovely pictures of insects and plants. Mark’s notes are indented, and you can enlarge the photos by clicking on them.
We begin with the tail end of a trip to Ohio last summer.
There is a terrific bog at a park there which I shall always visit when “bugging” in that state. I don’t often photograph flowers, but these Showy Lady’s Slipper OrchidsCypripedium reginae were abundant, and they are rather special since this species of Lady’s Slipper is generally rare. Visitors are not allowed to stray off of the boardwalks in the park, so my long lens came in handy here. That rule was frequently broken by others, btw, and it really ticked me off:
Next up is a new species of spider, the Western Lynx Spider (Oxyopes scalaris). Lynx spiders are ambush predators that sit up high on plants. Despite its common name, this species is widespread in the U.S., although it was new to me:
At a prairie location, these Soldier Beetles were abundant on various flowers where they were feeding on pollen. Their bright colors are a warning that they are not palatable. I thought they were two species, but it turns out they are both Margined Leatherwings, Chauliognathus marginatus, a species that comes in different color morphs:
Back we go to my resident state of Michigan. At a park some hours to the south of me, there were these mini-swarms of beetles that were feeding and mating on low shrubbery. Another new species. It turns out they are Clay-colored Leaf Beetles, Anomoea laticlavia, and they have an interesting biology in that their larvae live underground where they are tended by ants:
While on the subject of beetles having sex, here are a pair of Asian Ladybird Beetles (Harmonia axyridis), a species that has a number of other common names. They are an introduced and hugely dominant species of “ladybug”, and I worry that they have displaced some native species:
But now we get into some very special items. Near where I live is a park that has several wetland areas with “fens”, or at least that is what our park service calls them. I am told they also have features for “bogs”, however. The different types of wetlands are based on chemistry and water movement, plus the presence of various indicator plants.
Anyway, I call my favorite one “Sturtevant’s Fen” since its location is well off any trail and no one else goes to it. So it is a great place and it is all mine. One of its best features is that it harbors a healthy population of our smallest dragonfly, called the Elfin Skimmer (Nannothemis bella), is a species that is very fussy about the wetland conditions upon which it depends. Ever since I’ve known of these amazing little dragonflies, I’ve had a vision to photograph them in hand in order to convey how incredibly small they are. Catching them with a net was super easy. First, here is a male. I promise he is not being harmed. Next is the very different looking female. She had recently emerged, and so was not inclined to fly. This picture is one of my favoritest pictures I’ve ever taken! Elfin Skimmers are the 2nd smallest dragonfly in the world, and the smallest is a close relative found in China:
Sturtevant’s Fen also has orchids. The most common are these lovely grass pink orchids (Calopogon tuberosus). I believe this is a bog and not a fen indicator, but they are still very nice. The strange yellow thingies up top are lures that are meant to fool bees into foraging upon them since they look like anthers. The weight of the bee then causes the petal to tip down to the central column below, where sticky pollen sacs await to attach onto the hapless bee. Darwin would have appreciated the contrivances of these orchids:
Today we have photos by Dean Graetz of Australia. His captions are indented, and you can enlarge his photos by clicking on them. Aussie backyards have some cool stuff, especially the birds!
A Southern Hemisphere Backyard
Here is a sample of the inhabitants of our backyard in Canberra, Australia. Mid-March, at latitude 35°S, is a time of rapidly shortening daylength, and of harvesting the fruits of a coolish Summer. Our non-native garden shrubs (Buddleia davidii, aka ‘Butterfly Bush’) are popular attracting this new and hard to identify, visitor. We think it is a ‘Brown’, or Heteronympha species:
A large butterfly with a 10 cm wingspan, this female Orchard Swallowtail (Papilio aegeus), is always eye-catching, and always harassed by ever-present Cabbage White butterflies:
The common Meadow Argus (Junonia villida) which, after enjoying a nectar feed, often unhurriedly suns itself on our warm garden pathways, adding colour in two places:
The also common, and charmingly named, an Australian Painted Lady (Vanessa kershawi) choosing feed on a desert wildflower (Xerochrysum sp.) which we also grow as another inducement for butterflies. All the butterfly photos were shot from a 3-5m distance with zoom lenses:
A pair of aged adult Crimson Rosellas (Platycercus elegans) feeding on our neighbour’s tall shrub. These parrots are everyday sightings in Canberra gardens that are not far from surrounding native woodlands where they breed as hollow nesters:
A juvenile Crimson Rosella in the process of changing its dull green plumage to the bright reds and blues of the sexually mature adult. The coloured feather contrasting patches are so sharp that these birds enjoy the common name of ‘Patchworks’:
An adult Satin Bowerbird (Ptilonorhyncus violaceus), sex not obvious, having enjoyed a vigorous bath now eyeing the photographer. At age 7 years, a male bird will change from this khaki plumage to a brilliant blue-black glossy version, build a bower in a grassy woodland, decorate it with blue objects (same colour as its eyes), such as flowers, clothes pegs, bottle tops. The purpose is to attract, court and mate with numerous females. Hard to believe? Go here to watch:
A juvenile Kookaburra (Dacelo novaeguineae) now regularly arrives and sits patiently surveying our back yard for any living food items, such as lizards, mice, or snakes. These birds readily habituate to hand feeding by the lonely to become a mendicant friend for life:
An adult male Australian King Parrot (Alisterus scapularis) enjoying the last of an unripe pomegranate in a neighbour’s tree. The dark lower beak is staining. These are frequent visitors to Canberra at this time of the year. Being predominantly fruit eaters – their favourite is cherries – has required nearby fruit growers to cover their entire orchards with parrot (and hail) proof tents:
Close by, and part of a family flock, was this juvenile female King-Parrot, elegantly holding an unripe olive with toe and beak. They skillfully rotate each olive with their blunt tongue to flense off all the edible flesh. To us, hard green olives are unappealing, but this female ate steadily for about 15 minutes before flying off with a noticeably full crop:
Thanks to the readers who sent in photos at my behest. And today we have one of most faithful contributors, Mark Sturtevant, with some lovely photos of arthropods. Mark’s captions and IDs are indented, and you can enlarge his photos by clicking on them.
Last summer I chose to go back to Ohio to spend a few days “bugging” the local parks with a camera. I had gone late the previous summer, but this trip was done much earlier. Here are some of the critters that I had found, beginning with moths.
Here is a Tulip Tree Beauty Caterpillar (Epimecis hortaria). This will become an intricately patterned Geometrid moth with variable color patterns, as shown in the link:
A Orange-patched Smoky Moth, Pyromorpha dimidiate. Larvae feed on decaying leaves in oak woods. The moth is clearly a mimic of one the toxic Net-winged Beetles, but I don’t know if this is a case of Batesian mimicry, where the beetle is the only one with a defense, or Müllerian mimicry, where both are unpalatable and so they mimic one another:
Deep in the woods, these boldly marked moths were quite common on the low vegetation, although they seldom allowed me to get close. It is one of the Haploa Moths (which is in the Tiger Moth family), but there are perhaps three species that are similar and I can’t be sure of the exact species. I can say that it is a dead ringer for Haploa lecontei:
Next up is a bumble-bee mimicking Robberfly Laphria sp. These robust predatory flies are always interesting to watch since they can swivel their heads around to look for prey. When I found this one, it had recently hauled in a Golden-backed Snipe Fly (Chrysopilus thoracicus), and it was still struggling.
Next up are a pair of Leaf-footed Bugs, Acanthocephala sp. The female is feeding on bird poo, which is a thing that these bugs often do:
I was quite happy to see this Cocklebur Weevil,Rhodobaenus quinquepunctatus. Larvae bore into cocklebur stems and in other members of the sunflower family. I presume it is a Batesian mimic of the toxic milkweed bug:
Here is a pair of black-headed Ash Sawfly larvae, Tethida barda. Although they resemble Lepidopteran caterpillars, sawfly larvae actually grow up into stingless wasps:
There were quite a few of these Stoneflies near a river. I cannot even begin to ID these further with any confidence. The immature stages of these archaic-looking insects are aquatic:
The terrain gets quite hilly farther south in the state, and so the park trails there would send me down deep gorges. Along these trails the rocks and trees were generously festooned with large millipedes (the size of pencils) that I think belong to the Narceus americanus/annularisspecies complex. The taxonomy in the group appears to be messy and someone needs to sort them out:
Lastly, here is an interesting spider, the Humpbacked OrbweaverEustela anastera with an unknown moth as prey. I don’t remember if I’ve ever seen one before:
Reader Athayde Tonhasca Júnior has stepped up to the plate with another words-and-video story. His captions are indented and you can enlarge the photos by clicking on them.
All in all, it’s just another brick in the wall
In 2022, the seaside resorts of Brighton and Hove in southern England came under international spotlight for making it mandatory to use ‘bee bricks’ in all new buildings higher than 5 m. These bricks are the size of standard house bricks but have holes of different diameters drilled into one side, which are intended to mimic natural cavities used as nesting sites by some solitary bees. The bricks’ purported objectives are to boost bee populations and their pollination services. The legal requirement may have stumped Brighton’s and Hove’s architects and builders, but serendipitously, a local company was on hand to sell them these bee-boosting devices.
Bee bricks caught people’s imagination, and other local authorities have been asked by their residents to adopt the initiative. Meanwhile, you can get in on the action right now by buying the product from a range of companies. One retailer offers a choice of yellow, grey or red bricks at £39.99 each (for comparison, a top of the range, handmade glazed brick costs £3). You want to join in but live in America? No problem: you can buy a brick imported from the UK for US$ 34, shipping not included (UK and America are ripe for an entrepreneur with a set of masonry drill bits).
One would expect that a mandatory planning condition – let alone a price tag of £39.99 for a chunk of concrete – would be backed by data. In other words, do bee bricks make a difference for bees and pollination? The answer is, at best, ‘we don’t know’.
Around 12 of the 270 or so species of bees in the UK are cavity-nesting: they occupy or expand naturally occurring spaces such as crevices under or between stones, cracks in a wall, the underside of peeling tree bark, holes in dead wood or hollow stems to build their nests. These species – mostly mason (Osmia spp.), leafcutter (Megachile spp.), and yellow-faced (Hylaeus spp.) bees – also make themselves at home in man-made structures such as bee houses or bug hotels, a feature that has helped farmers boost crop pollination with commercially reared bees, and has inspired the idea of bee bricks.
But no ordinary hole in the wall would do for cavity-nesting bees. A female selects a spot where she can fit in snugly; a too-wide hollow is an invitation to parasites to sneak in, and also requires extra work when she plugs the nest entrance with mud or leaves after finishing stocking the nest with pollen. Nest diameters for most bees are in the 4-10 mm range, so the 5 to 8 mm holes in bee bricks are adequate. But they fall short in depth. Their dimensions are the same as those of a standard house brick (21.5 x 10.5 x 6.5 cm), and several experiments with bee houses indicate that cavities must be at least 15 cm long; some studies suggest 20 or even 30 cm. We don’t know whether bees make do with bricks’ cramped spaces, and what the consequences are if they do. We know that small nests may affect the sex ratios of some species. That’s because eggs that originate female bees are laid in the inner brood cells; males are in the outer part of a nest cavity, so they can emerge first in the spring. If there is not enough space for all brood cells, bees of one sex may be produced in smaller numbers, with unknown consequences to the population.
A cross section of two cavities occupied by red mason bees. Eggs that will turn into male bees are on the left, near the nests’ entrances:
Location of the nest is crucial: homes of cavity-nesting bees must be exposed to sunshine so that the brood cells are sufficiently warm for the proper development of eggs and larvae. These bees also are not keen on heights. They prefer to nest ~30 to 50 cm above ground (Henry et al., 2023); the higher up the cavities, the lower their occupancy (MacIvor, 2016). And the neighbourhood matters a lot. After mating, a female bee spends her short adult life frantically gathering pollen and building brood cells; she will collect food as close as possible to her nest and can’t afford wasting time on long foraging trips. Maximum foraging distances are correlated with body sizes, but 150 to 600 m seems to be the range for the main species. To be on the safe side, nests should be no further than 150 m from a food source. The upshot is that a bee brick in a north-facing position, shaded by a tree, too high, or too far from abundant flowers, is not likely to be occupied.
The use of concrete does not seem to be a problem: Henry et al. (2023) recorded occupancy of holes drilled on concrete blocks increasing from 2.9% in the first year to 11.6 and 25.3% in the second and third year, respectively. These figures are promising, but the concrete blocks used in the experiment were placed in flower-rich spots in open areas under full insolation. And the possibility of concrete being insufficiently porous to prevent mould, a serious hazard to cavity-nesting bees, should not be neglected.
Because of the limitations described above or some other factor, the occupancy rates of bee brick holes are not particularly encouraging, ranging from 1.3 to 2.8% (Shaw et al., 2021); another two unpublished reports put the figure of inhabited bricks at 3.5% (Alton & Ratnieks, 2020). These numbers are considerably lower than the average occupation rate of 38.3% for a variety of artificial homes in urban environments (Rahimi et al., 2021). One possible explanation for such poor uptake is that cavity-nesting bees don’t need our help in finding suitable nesting sites: urban and semi-natural environments offer a range of perfectly habitable nooks and crevices to compete with bee bricks (MacIvor, 2016).
Bee bricks don’t seem to be living up to their hype, but there’s a silver lining here. High density nesting encourages the proliferation of pests and diseases, which are massive headaches to farmers who rely on commercially bred solitary bees. The impressive bee housing estate built under the auspices of Brighton and Hove Council may be mostly empty, but in all likelihood is not insalubrious.
More data may improve the perspective of bee bricks as tools for boosting bee populations. But based on the little we know, the initiative ended up in Alton & Ratnieks’ (2020) list of ineffective products sold to home owners keen to do their bit for conservation. Bumble bee nests (priced £34.95 for a humble wood unit or £161.20 for a fancy underground model) could be added to it, as they also do not perform as intended (Lye et al., 2011).
In Britain and probably elsewhere, conservation practices are based mostly on perceived ‘common sense’ and personal experience rather than evidence (Sutherland et al., 2004). The obvious shortcomings of such approaches are that decisions are often wrong, causing a waste of time and money, erosion of public trust, and possibly aggravating environmental problems. The haste in adopting untested bee bricks may have led Prince (now King) Charles to squander £55,000 on bee bricks for his housing development in Newquay – a 4,000% increase in brick costs. It may also raise suspicions of greenwashing – actions claimed to solve environmental problems but that are in fact futile public relations smokescreens.
To help bees and safeguard pollination services, local authorities and everybody else can take tried and tested measures such as creating, preserving and restoring flower-rich areas; reducing or banning the use of pesticides; reducing the frequency of mowing to give wild flowers a chance; planting pollinator-friendly trees and shrubs, that is, species that produce lots of pollen and nectar.
The familiar and run-of-the-mill don’t make a splash in newspapers and social media, but they more often than not give better results than the novel and untested. There’s security in the boring option.
Here’s a rare example of animal behavior being fossilized. In this case it’s in termites, whose modern representatives engage, as pairs, in a behavior called “tandem running”. This occurs after a group of reproductive termites who have left their natal nest fly away, a behavior certainly evolved as a way of staring new colonies. Unlike other social insects like bees, a termite colony contains both reproductive males and females, both of which have wings, eyes, and the capacity to mate and start new colonies (other workers lack wings and eyes). At mating time, a swarm of reproductive individuals fly away at random (they’re not good fliers), and then alight on the ground or, in the case at hand, on a tree trunk. After dropping their wings, they form mating pairs, each of which can start a new colony. To find that colony, a male and a female engage in “tandem running,” with (in the species below) the female running around with the male close behind, his head contacting her abdomen. Apparently some species can have either a male or a female as the leader in the tandem run. I can’t find out whether mating occurs before the tandem run or after the pair burrow into the ground to found their new colony.
When the female finds a site she likes, the pair digs in (most termites nest underground), and, after mating, the female becomes the “queen”, and the male the “king”. They remain monogamous, with the male continuing to fertilize the female throughout the life of the colony. This implies that all the termites in a colony are brothers and sisters. Since “kings” and “queens” can live for decades (25-50 years, according to one site, the colony can last a long time sending out reproductives to found new colonies.
At any rate, below you can see two examples of tandem running in reproductive alates (winged termites that have lost their wings). This is the behavior that appears to have been “fossilized”.
The YouTube notes:
When male and female termite alates (flying termites) pair up, they break off their wings and the male starts following the female around until she finds a suitable spot to start a new nest. This activity is called termite tandem running.
And so to the new paper in Proc. Nat. Acad. Sci. USA, which you can read by clicking on the title below or reading the pdf here.
The authors had a piece of 38-million-year-old Baltic amber, which is fossilized plant resin. (Baltic amber containing animal or plant inclusions like this can sell for a lot of money.) When resin or sap falls to the ground, it can, over long periods, be converted to amber by pressure and temperature of the sediments above. Eventually it becomes quite hard and can be mined.
In one pice of amber, the authors found two termites that looked as if they might have been tandem running when they got stuck in the sap and then preserved. Here’s a photo of their specimen, which is of the extinct species Electrotermis affinis. The caption to the partial figure below is “E. affinis pair in Baltic amber. (A and B) The dorsal and ventral sides of the tandem, respectively, with (B) an arrow pointing to the 15-articles antenna of the tandem leader.” The scale bars represent 0.5 mm.
This certainly looks like a tandem pair, but the problem is that they are not straight head-to-abdomen, but twisted a bit, so they are more side to side. Because it’s hard to get a good look at specimens in amber, and you can’t cut the amber open (that destroys the specimen), the authors used X-ray microtomography (a 3-D reconstruction using X rays) to show that the male is the one on the right in (A) and left in the ventral view (B); he’s smaller and the sexes can be told apart by the shape of the seventh “sternite”, or abdominal plate. They also saw that the female’s mouthparts were in contact with the tip of the male’s abdomen, which is what happens in tandem running. So we have a male and female in the right contact position, buttressing the idea that this is a tandem pair.
The authors then hypothesized that this was indeed a pair that was doing tandem running (probably on a tree) when they got stuck in sap, and the side-by-side position resulted from the pair trying to get unstuck. They failed, and eventually became part of a piece of amber.
To test this “position change” hypothesis, they put tandem-running termites of a living species, Coptotermes formosanus, in a sticky trap, a flat piece of cardboard covered with a sticky substance (I used them in the lab to catch cockroaches). This mimics a pair getting stuck in resin, and, as in resin, the pair could move around a bit after they got stuck. Would the tandem runners move more side by side?
Indeed they did. The stickiness led to the tandem pair shifting their positions as they tried to free themselves. In fact, they assumed a more side by side position once stuck. (I have to say that I find this experiment disturbing, as it involves killing insects for the sake of science. However, I killed cockroaches to keep my lab free of organisms other than fruit flies.)
Here’s what they found in 17 termites that didn’t escape the trap:
The spatial orientation of the leader and the follower after entrapment was significantly different than in natural tandem runs. The distance between the body centroids of the leader and the follower was smaller in trapped pairs than in natural tandems (Fig. 2 D–G and SI Appendix, Fig. S2, Exact Wilcoxon rank sum test, W = 599, P < 0.001). This is because partners of trapped pairs were often positioned side-by-side, differing from the linear positioning of natural tandems (Fig. 2 D–G). The shorter inter-individual distance could result from the two individuals entering the sticky surface together and becoming stuck near each other without the ability to move away, rather than their active behavioral interactions to maintain proximity.
And a picture of a living tandem pair (female in front) that wound up stuck more side by side, like the fossilized ones above:
(From paper): The relative position of females and males forming mating pairs. (A–C) Mating pairs of the termite C. formosanus in (A) a natural tandem run and (B and C) on a sticky surface. Females are marked in red and males in blue. The convoluted lines indicate the trajectories of a female and a male during 30 min after the pair entered the sticky trap.
They also concluded, from a complicated logistic regression, that the probability was 74% that the following individual was a female.
Finally, here’s a reconstruction in the paper of the original event that led to the fossil. Note that the “fossilized behavior” term is a bit incorrect, as what gets fossilized is not their normal behavior, but what seems to be the behavior of a tandemly running pair that’s gotten stuck. But given that there are individuals of both sexes in this pair, and that the antennae contact the abdomen, combined with what’s seen in the “resin mimicking” experiment, it’s seems likely that the authors are correct.
(from paper) Artistic reconstruction of E. affinis tandem pairs running freely on a tree bark and one tandem trapped by tree-resin.
What about other examples of fossilized behavior? I want to put in a paragraph about this from the paper, just for your delectation:
Some fossils preserve the “frozen” behavior of animals in actions at the moment of death (9, 10). However, our results demonstrate that animals on the sticky trap are not instantaneously immobilized and change their postures on the surface. These experiments imply that the spatial orientation of animals preserved in sticky matrices, such as in tree resin prior to fossilization into amber, is influenced by the process of entrapment. Therefore, the interpretation of fossilized behavior can be dramatically refined or even corrected by observing the behavior of living organisms under entrapment conditions. Some behaviors fossilized in amber may remain unaltered by the entrapment process. For example, the preservation of mating moths incopula (14) or hell ants grasping prey items (12) suggests that the inter-individual interactions of these behaviors are strong enough not to be disturbed by the movement on the sticky surface. However, entrapment in amber likely affects many other behaviors. For example, insects dispersing through phoresy [attachment to other insects as a way of moving around] can be preserved detached from the host insect, perhaps because the host struggled on the sticky surface before complete encasement (37). The consequence of different behavioral responses can be studied using extant relatives. Furthermore, animals have evolved behavioral responses to sticky objects. For example, recent studies have revealed that ants are not passively affected by sticky objects but actively modify them. Red imported fire ants cover sticky surfaces with soil particles to access food resources (38), and granivorous desert ants remove sticky spider webs from nestmates to rescue them (39). Scavenging insects can be attracted by large animals trapped on a sticky surface (11, 35), and the spatial distribution of these insects may have reflected their foraging behavior. Thus, future studies on behavioral responses to sticky objects by animals will increase our understanding of fossil records in amber, as well as shed light on the behavioral capacity of extant insects.
I found it really interesting that ants can get around the danger of sticky substrates by covering them with soil, and can even remove spider web stuck to other ants. Ants have brains about the size of a grain of sand, but this behavior is somehow coded in there (or else they learn to do this, which seems less likely).
Today’s instructive photo-and-text contribution comes from reader Athayde Tonhasca Júnior and deals with a topic we’ve discuss a lot: biological sex and its consequences. In this case, we learn about how organisms adaptively adjust the sex ratio of their offspring when conditions change.
Athayde’s captions and IDs are indented, and you can enlarge the photos by clicking on them.
Sometimes snips, snails and puppy-dogs’ tails, other times sugar and spice
As the story goes, during a tour of a government farm, American First Lady Grace Coolidge was being shown around by a farmer when she saw a cockerel and a hen romantically engaged. She asked her guide how often the cockerel would mate, to which he responded: ‘dozens of times a day.’ Good-humouredly, Mrs Coolidge retorted: ‘tell that to the President’. The farmer dutifully did so, and President Calvin Coolidge asked: ‘same hen every time?’, to which the farmer replied: ‘No, Mr President, a different hen every time.’ And the president: ‘tell that to Mrs Coolidge.’
Psychology Professor Frank A. Beach (1911-1988) saw this improbable anecdote as an ideal model to name a widespread phenomenon among animals: the Coolidge Effect, which is the enhanced sexual interest of males whenever a new female is accessible, regardless of the availability of previous sexual partners – a behaviour rarely reported for females. This shocking manifestation of male chauvinism has been offered a biological explanation.
The term ‘gonochorism’ makes us scramble for the dictionary, even though one of the first things we learned from our Birds and Bees lessons – or used to learn before ideological gangrene poisoned Facts and Reality – is that our species is gonochoric (or dioecious), that is, it has two sexes: the male sex produces or is geared up to produce gametes (reproductive cells) called sperm, while the female sex is equipped to produce gametes known as ova or egg cells. The lesson’s climax was the revelation that some types of frolicking could result in the fusion of these two types of gametes to produce babies.
Later in life, when we took biology courses, we were told that many plants and some animals are hermaphrodites (they produce male and female gametes), while other organisms don’t need sex to reproduce. But the overwhelmingly majority of animals, and all mammals and birds, are sexually binary: they either produce male gametes or female gametes – leaving aside the rare cases of individuals that don’t fit in either category. And, from humans to asparagus, that is, for virtually all multicellular organisms, the female gametes are larger – often much larger – than the male gametes; that’s to say they are anisogamous: the two types differ in size and shape. And anisogamy has much to do with the Coolidge Effect.
Because sperm are relatively small, energetically cheap gametes, males can afford to churn out and distribute lots of them. By mating with as many females as possible, males increase their chances of passing on their genes. If a male gamete ends up in an unsuitable female, it’s not a big deal: there are plenty more fish in the sea. It doesn’t work like that for females. They put a lot of energy into their eggs, which are gigantic when compared to sperm. So, a female can only make a few of them in her lifetime. Adding gestation and time spent nurturing their young, females have a much lower reproductive capacity. As they invest a great deal more in producing an embryo than males, they need to choose their mates well to maximize their chances of success; if their Romeos are weak and unfit, females may have wasted all their reproductive potential. For females, it’s a matter of quality, not quantity.
Together at last. A human male sex cell (spermatozoon) penetrating a human ovum. The spermatozoon is ~100,000 times smaller than the ovum. Image in the public domain, Wikimedia Commons.
These biological particularities are strong incentives to polygyny, the mating system where a male has multiple sexual partners while the female mates with one or a few males. Polygyny is the most common mating strategy for vertebrates; about 90% of mammal species are polygynous. These males are, like the Coolidges’ rooster, always ready for a new romantic adventure.
Angus John Bateman (1919–1996), a botanist who worked with fruit flies, found one important consequence of the Coolidge Effect. For most polygynous species, a small number of males monopolize the females and prevent other males from mating. That is, some males are highly successful in reproducing, while many more have no success at all. Things are more predicable for females: most of them will mate – the few successful males will make sure of that. The upshot is that males’ reproductive success is more variable than females’.
Enter evolutionary biologist Robert Trivers and computer scientist Dan Willard (1948-2023) to thicken the plot by proposing that differences in reproductive success can bias the production of male and female offspring. Trivers and Willard argued, reasonably, that sons and daughters of females in good condition (that is, well-fed, healthy, and not pressured by competitors) would also be in good condition, whereas sons and daughters of females in poor condition (malnourished or debilitated by parasites or competitors) would also be in bad condition. But, when the reproductive success of one sex – males, in the case of polygynous species – is more variable than the other, diverging strategies emerge. In an evolutionary sense, it pays for strong, healthy females to have many sons, who mate frequently and produce lots of grandchildren for their mother. Daughters on the other hand are a less promising investment because, despite being strong like mum, they are restricted by low reproductive rates. But if the mother is in poor condition, having daughters would be a better deal because despite being feeble like mum, those who survive to adulthood are likely to produce some offspring. Feeble sons on the other hand may never breed, as they would be no match for males in good condition (Trivers & Willard, 1973). In other words, when things get bad, it’s better to have more daughters than sons. This risk-spreading strategy is a form of biological bet-hedging to maximize fitness and applies beyond mammal polygyny. If females’ reproductive success is more variable, we should expect more sons than daughters when the going gets rough.
The Trivers–Willard hypothesis provides an explanation for a common occurrence among animals: sex ratios going astray. In theory, a species should produce about the same number of sons and daughters (1:1 ratio) to maintain long term stability. This is known as the Fisher’s principle – although it would be fairer to call it the ‘Cobb’s principle’ after the solicitor and amateur biologist John Cobb (1866-1920), who first proposed it (Gardner, 2023) (Cobb’s work is virtually unknown today, and academics from the Church of Woke would have conniptions at citing his paper, published in The Eugenics Review).
The Trivers–Willard hypothesis has had an enormous influence in evolutionary biology. Its predictions have been supported by studies with a range of species, although its universality has been debated and questioned. Nonetheless, the hypothesis has encouraged much theoretical and empirical research about sex allocation. This body of work has revealed that variation of reproductive success between sexes is not the only driver of sex ratio skewness. Food, mothers’ age, litter size, population density, the weather, or some other environmental or physiological factor may induce females to adjust the sex ratio of their offspring to maximise fitness.
It turns out that food availability is an important inducer of sex ratio fine-tuning for one group of animals of enormous ecological end economic importance: cavity-nesting solitary bees. Most of the 20,000 or so known species of bee build their nests in the ground, but about 30% of them took another path regarding housing. They occupy or expand naturally occurring cavities such as crevices under or between stones, cracks in a wall, holes in dead wood, hollow stems and tree bark, transforming them into cosy, safe environments in which to raise their young.
Like all solitary bees, cavity-nesting species are on the wing for a small portion of their lives, sometimes weeks. After mating, each female spends her short adult life tirelessly victualing her nest with pollen and nectar to provide for her brood. It’s a race against time and over hurdles such as bad weather, competitors, flower scarcity, pests and parasites. Reproductive success depends on the amount of food available for the young, and here their sex can be the decisive factor. Female bees – like most insects – are in general bigger than males, so they need more food. As these big eaters could be a survival risk, some tinkering may be in order.
A red mason bee (Osmia bicornis) man-made nest with brood cells well-stocked with pollen.
The orchard mason bee or blue orchard bee (Osmia lignaria), a cavity-nesting species from North America, is a valued pollinator of several fruit trees. During the early nesting season, when pollen and nectar are most abundant and mum is in top shape, her offspring comprise mostly females. As the season progresses, flowers become scarce, so she has to work harder to provision her nest. Now the sex ratio tilts towards the smaller males, who have better chances of survival because they need less food (Torchio & Tepedino, 1980).
The scenario is similar for the related red mason bee (Osma bicornis), a Eurasian species, but here parasites play a part. As the nesting season advances, females become less efficient and take more time to gather food, creating opportunities for nest-invading parasites. Females deal with the problem by reducing the amount of food stored, with a corresponding shift in the sex ratio towards the less demanding sons (Seidelmann, 2006). In the case of the Australian endemic banksia bee (Hylaeus alcyoneus), the growing food scarcity causes the reduction of the brood’s body mass and a shift in their sex ratios. But contrary to the prevailing pattern found in bees, male banksia bees are significantly larger than females. So unsurprisingly, the energetically cheaper daughters became more abundant late in the season (Paini & Bailey, 2002). Other cavity-nesting bees have also shown declines in foraging efficiency as the season progresses, and these changes have been linked to reduced size of their offspring and shifts in their sex ratios.
The facultative, condition-dependent shift of sex ratios is a remarkable survival tool. The power to quickly tilt the offspring’s sexual balance could make the difference for a species’ success. In the non-nonsense, unforgiving great outdoors, where long-term existence hangs on the ability to adapt to changes, boys and girls are not always equally valued: these are the times when a Sophie’s choice of sorts is necessary.
JAC note: Just to put an evolutionary-genetic gloss on this, the changes of sex ratio with environmental or other conditions are the result of evolution. That is, those individuals having genes enabling them to adjust the sex ratio in adaptive ways leave more copies of their genes than individuals who can’t adjust their offspring’s sex ratio. Or, to be even more accurate, genes that affect sex ratio in adaptive ways leave more copies of themselves than genes which can’t do that.
Today’s photos come from Borneo courtesy of reader Daniel Shoskes. His notes are indented, and you can enlarge the photos by clicking on them.
Just back from an incredible trip to Borneo. Just a smattering of the photos. Please forgive the lack of precision in species naming; I did my best to get the names from our guides.
First, a video of his whole trip can be seen here.
We are down to about five batches of wildlife photos, so please send in your good ones. Thanks!
Today’s batch of lovely insect photos comes from regular Mark Sturtevant, whose captions are indented (he also provided the links). You can click on the photos to enlarge them.
As I have gotten very far behind in post-processing of pictures, this set is hot off the press and scandalously has not been shared anywhere else except for here and on my main Flickr page. All were taken around the house or from area parks in eastern Michigan, where I live.
Beetles dominate this small batch. First up is a Rove Beetle, possibly Platydracus, from a staged focus stacking session on the dining room table. Rove beetles form a large family of very active predatory beetles (Staphylinidae), and they are easily identified by their short wing covers. They can be difficult to photograph since sitting still is not what they do, so I got this one to pause for a moment on a perch.
Next up is a pea-sized Dung Beetle, Canthon sp. There were several of these trundling around little balls of dung in the amazing place I call the Magic Field, where one can find critters that I see nowhere else. I tried hard to get pictures of them rolling their little treasures, but they would immediately stop and bury themselves in the soft soil on approach, refusing to come out and do what Dung Beetles do best. I hope for better luck this season. Dung Beetles are in another large family, the Scarabaeidae. I think most species have nothing to do with dung, but rather feed on roots, leaves, pollen, or fruits.
Here is a small beetle from another large family called the Leaf Beetles (Chrysomelidae). No matter the species, Leaf Beetles seem to always be bright and shiny, and they sit out in the open on vegetation. I somehow always know that I have a leaf beetle, even if the species is new to me as this one was. This one is the Sumac Flea Beetle, Blepharida rhois. Flea Beetles are Leaf Beetles that can jump.
Next up is an Assassin Bug, Zelus luridus. These common predators in the Hemipteran family Reduviidae can be found openly lurking on leaves along forest trails. Their extremely laconic nature makes me wonder how they ever catch anything.
The caterpillar shown in the next picture is the Bronzed Cutworm, Nephelodes minians. The larvae are generalist feeders on grasses, and are considered a pest on cultivated crops. No doubt I’ve seen many of the brown adult moths at the porch light, but there are so many species of “little brown jobbies” in their family (Noctuidae), that I doubt I would know them on sight. This larva was strangely inactive. Even moribund. It was either about to pupate, or it was terminally parasitized.
Back at the Magic Field, in the very early season one can find nymphs of my favorite grasshopper, the Coral-winged GrasshopperPardalophora apiculata. These spend the winter as nymphs (in fact I just got back from visiting this field in February during a freakish warm spell, and sure enough the wintering nymphs were revived and hopping everywhere). But come this spring they will quickly grow up to be a robust Band-Winged Grasshopper with pinkish-orange hind wings, as can be seen in the link. They are a delight to watch as they ponderously launch themselves to fly, but they never go far owing to their chonkyness. The Magic Field proudly hosts at least six different species of Band-winged grasshoppers alone. Grasshoppers in this group usually have brightly colored hind wings, which among other things are used as a kind of deception to fool predators into thinking that they are brightly colored, while in fact when at rest they are well camouflaged. Band-wings belong to the short-horned grasshopper family Acrididae.
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