Category: Adaptations

March is the cruelest month…

March 21, 2010 • 2:48 pm

by Greg Mayer

…breeding mulleins out of the dead land.

Mullein along railroad track, Arlington, Virginia, March 12, 2010.

During a visit to the Washington, DC, area last weekend I made a point of looking at how advanced the spring greening was. Despite  the east coast’s hard winter, and the mild winter in the midwest (and 2010 in general is starting off quite warm), things are much more alive (as I expected) in the east, but still pretty gray-brown. In addition to the mulleins above (the greenish ground rosettes, with last year’s meter high flowering stalks still standing in many), a number of flowers (all in cultivation) had also emerged.

Washington, DC, March 17, 2010.
Washington, DC, March 17, 2010.

Suitland, Maryland, March 16, 2010.

Early emergence is a key adaptation for many plants, especially those of the forest understory. Later in the season, the forest floor will be in deep shade from the trees, so many herbaceous and small woody plants take advantage of the photosynthetic opportunities provided by the early spring. The animals were also active– fish crows and mockingbirds on the Mall in DC–, but most noticeably large and loud choruses of spring peepers (Pseudacris crucifer, a tree frog) in many places in Prince William and northern Stafford Counties, Virginia. I attempted to record a chorus in Dumfries, Virginia, and thought I had, but the file came up blank, so you’ll just have to imagine hundreds of little frogs, all saying “peep” at the same time (or listen here). When I lived in the DC area, peepers began calling around the same time redwinged blackbirds did, February 15, so they’ve probably been calling for around a month already. No frog calls yet in Wisconsin.

How old are mammalian pheromones?

November 13, 2009 • 12:40 am

by Matthew Cobb

Sex pheromones are widely used by mammals to communicate and detect the sexual status of a potential mate. This is particularly the case with female mammals, whose pheromones are primarily detected by a structure known as the vomeronasal organ (VNO), which is in the base of the nose/roof of the mouth. (And no, humans don’t have a functional VNO, although it does appear briefly during embryogenesis).

There are two kinds of smell receptor molecules in the VNO –  V1Rs look pretty much like an ordinary smell receptor, and the neurons that house them send their axons into the part of the brain that deals with food and so on. But the other kind of receptor – V2Rs – look very different and project to a different part of the brain. The assumption is that key parts of mammalian pheromones are detected by the V2Rs, but that pheromones often contain a blend of compounds, some of which may be detected by V1Rs and by a specialised receptors called TAARs in the main part of the nose.

The really interesting thing is quite how far back these receptors go. The recent sequencing of the Platypus genome showed that there were V1R and V2R genes, strongly suggesting that this form of communication goes back at least 165 MY:

Mammal evolutionMammalian evolution – Nature 453, 175-183 (8 May 2008)

So what does a male mammal do when he detects a pheromone? Anyone with a horse – or a cat! – will know. He produces what is known as “Flehmen”, a characteristic curling back of the lip, with the mouth held open. Cats do this when they smell the urine sprayed by a male, and get a faraway, stoned look in the eyes while they’re about it. Here’s a picture of a tapir showing Flehmen:

So do marsurpials show Flehmen? You betcha! Here’s a  video of a male kangaroo testing the reproductive status of a female, by tasting her urine. Note the “flehmen” response he makes with his mouth, just like a placental mammal. Note the way he shakes his head afterwards… Who can blame him? DON’T TRY THIS AT HOME!

Sharks with head claspers (sort of)

September 30, 2009 • 9:00 am

by Greg Mayer

In my post on the genitalia of ratfish (which are shark relatives), I noted that although no extant sharks had similar structures, some fossil ones did, so here are what two species of these sharks looked like. Both are members of the family Stethacanthidae, known for its sexual dimorphism.Falcatus falcatus, by Smokeybjb, from WikipediaFig. 1. Female (above) and male Falcatus falcatus (Carboniferous of North America). Note the pelvic claspers on the male, and the roughened denticles atop the head, as well as on the head clasper

How exactly the male ratfish uses his head clasper during mating is obscure (at least to me), and the use of the head clasper (the spine of the first dorsal fin) in the shark Falcatus would also be obscure, except that a pair has been found fossilized in flagrante delicto, the female grasping the head clasper in her mouth, her body parallel to and above the male’s.  There would have to be more to their mating than this to bring the male’s pelvic claspers in to position, but it does provide at least a partial picture of mating and courtship in this fossil species. A nice photo of the fossil pair is at the fine website on Fossil Fishes of Bear Gulch maintained by Richard Lund and Eileen Grogan. They also have a photo of the rather similar Damocles serratus (presumably so named because its own sword [clasper] was always hanging over its head).

Equally bizarre is Stethacanthus, with a brush-like set of denticles atop the first dorsal fin, a large first dorsal fin spine, and roughened denticles atop the head. It’s not clear exactly how, or for what, this structure was used, but the fact that it occurs only in males, and that the related Falcatus (and almost certainly Damocles as well) used a similar structure in mating, suggests some sort of sexual behavior function.

Stethacanthus by Dmitry Bogdanov, from Wikipedia

Fig. 2. Male (to left) and female Stethacanthus altonensis (Carboniferous of North America).

Lund and Grogan provide further discussion and illustration at their website, and one of Lund’s papers is in the American Museum of Natural History’s digital library of its scientific publications. Matt Celeskey at the Hairy Museum of Natural History has reconstructions of both Falcatus and Stethacanthus.

Filling a demand that I didn’t know existed, in the mid 1990’s two excellent and well-illustrated popular accounts of the history of fishes were published, both emphasizing the fossil record. WEIT readers should enjoy both; Long has more on these odd sharks and ratfish.

Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Johns Hopkins University Press, Baltimore.

Maisey, J.G. 1996. Discovering Fossil Fishes. Henry Holt, New York.

That’s not ratfish genitalia. That’s ratfish genitalia.

September 28, 2009 • 8:58 am

by Greg Mayer

Over at Pharyngula, PZ has linked to a story at Deep Sea News about the description of a new species of ratfish with “forehead genitals”. While it’s a great concept, the tentaculum, or cephalic claspers, of ratfish are not genitals.

Male chimaeridFig. 1. That’s not ratfish genitalia. A male ratfish (family Chimaeridae) showing the tentaculum.

The genitalia of male ratfish are the pelvic claspers, modifications of the medial side of the pelvic fins used as intromittent organs for the introduction of sperm into the female’s reproductive tract. The ratfish’s pelvic claspers are bifid and especially spectacular.

Claspers of male chimaerid.Fig. 2. That’s ratfish genitalia. A male ratfish’s (family Chimaeridae) pelvic claspers; note the bifid structure, giving the appearance distally of four claspers. The anterior of the fish is to the bottom of this photo. The medial lump anterior to the pelvic fins is the rectum, prolapsed.

For comparison, here’s a female ratfish with unmodified pelvic fins.

Female chimaeridFig. 3. A female ratfish (family Chimaeridae), showing unmodified pelvic fins.

Ratfish comprise the Holocephala, one of the two major subdivisions of the cartilaginous fishes, the Chondrichthyes, the other major subdivision being the sharks and rays (elasmobranchs). Pelvic claspers are found throughout the modern cartilaginous fishes, which therefore have internal fertilization (most bony fish have external fertilization). Although living species of sharks do not have tentacula, some fossil ones (e.g. Falcatus) did, and others had other sorts of spine encrusted bits on their front ends which may have been involved in courtship and mating.

Whether or not tentacula are genitals is a matter of the definition of genitals, of course, but the term is, to my knowledge, reserved for structures involved in the transfer and reception of gametes. If parts of the body used in courtship are considered genitals, then the throat fans of anoles and the long fingernails of turtles would have to be considered genitals, too; indeed, so would the entire human body.  Many commenters at Pharyngula  have remarked about ratfish having penises on their heads (or something to that effect), which, of course, they don’t: their genitals are in the normal place (for cartilaginous fish), alongside their pelvic fins.Female (left) and male chimaerids.

Fig. 4. A ratfish couple.

(I tried to post a short comment to this effect at Pharyngula last night, but found I wasn’t registered to do so, and then I thought, “Why talk about it, when you can show pictures”, so I waited to take some photos this morning and posted here.)

What frogs go through for their tadpoles

September 4, 2009 • 2:30 pm

by Greg Mayer

Matthew has once again given me a hot tip: the BBC has a video of a mountain chicken, Leptodactylus fallax, feeding its developing tadpoles with unfertilized eggs. (I am unable to embed the video– do click through to watch). The mountain chicken is actually a large frog (so named because they are good eating– many years ago an attempt was made to establish the species in Puerto Rico as a food source) that is found on the eastern Caribbean islands of Montserrat and Dominica, and formerly Martinique.

Leptodactylus melanonotus, a smaller relative of the mountain chicken

Feeding unfertilized eggs to tadpoles may seem like a bizarre and exotic way to take care of your offspring (the BBC labeled it an “alien scene”), but it’s actually sort of mundane in the world of amphibians. Amphibians have the most diverse set of modes of reproduction and nutrition of juveniles of all the land vertebrates. Laurie Vitt and Janalee Caldwell, in their superb herpetology textbook, list 40 different reproductive modes for frogs. Its hard to pick a strangest amphibian reproductive mode, but I’d go with either the gastric-brooding frog, Rheobatrachus silus, in which the female swallows her eggs, which develop in her stomach, or histophagy, practiced by several amphibians, in which the fetuses feed upon the mother’s hypertrophied oviductal lining (i.e. while still inside of her).

Dendrobates auratus with tadpole, Est. Biol. La Suerte, Costa Rica

The poison dart frog, Dendrobates auratus, above is a male, carrying a single tadpole in the lower middle of its back, which it will transport to some wet spot, such as a puddle or tree hole. The female of its relative Dendrobatus pumilio (featured in a previous post) places its tadpoles in the axils of bromeliads or other arboreal plants, and then returns, like the mountain chicken, to feed them with unfertilized eggs.

DSCN3789The pair of tree frogs, Hyla phlebodes, above was found in amplexus (that’s what mating is called in frogs); the male is the smaller one. Unlike the other frogs mentioned in this post, this species has the typical, un-exotic reproductive mode for frogs: eggs are laid in the water, which hatch into tadpoles, which metamorphose into froglets. It’s a pretty wondrous way of life, too, it just seems a bit un-exotic because we’ve been jaded by its familiarity.

Many amphibians have recently undergone significant population declines, and a number of species, including the gastric-brooding frog are now extinct. A fungal disease called chytridiomycosis has been implicated in causing the decline of many species, including the mountain chicken. The BBC video was made as part of a captive breeding conservation effort involving the Jersey Zoo, founded by the late naturalist and author Gerald Durrell,and the London Zoo.

How the tapir got his spots III

August 13, 2009 • 12:49 pm

by Greg Mayer

The two great classes of phenomena that Darwin set out to explain were those of adaptation– the fit between an organism’s features (structure, behavior, etc.) and its conditions of existence; and unity of type — the similarities of basic structure among organisms in diverse conditions of existence (e.g., the one bone-two bones-many bones pattern of tetrapod forelimbs, whether they be burrowers, swimmers, climbers, runners, etc.). The unified explanation that Darwin provided for these phenomena was descent with modification: the similarities were due to inheritance from a common ancestor (i.e. descent), while adaptation arose from the process of modification (i.e. natural selection).

The methods of studying adaptation are thus crucial for biology.  How can we tell what (if anything) the spots of the baby tapir are adaptive for?

There are three basic ways of studying adaptation, in the sense of determining what a trait is an adaptation to. The first is engineering: does the feature conform to what we would expect if it is performing some adaptive function?  Study of hydrodynamics enables us to understand the shapes of the bodies, flippers, and fins in fish, dolphins, icthyosaurs, etc. as adaptations to movement within a fluid environment.  The dorsal fin of an ichthyosaur, for example, stabilizes the reptile in its forward movement through water, preventing unwanted roll (for recent discussions of ichthyosaur aquatic adaptations, see here, here, and here). For another example of the engineering approach, see Richard Dawkins’ delightful account of bat sonar in chap. 2 of The Blind Watchmaker.

Second, there is the method of correlation (also called the comparative method): does the feature evolve repeatedly in particular environmental circumstances? Thus even if we were wholly ignorant of hydrodynamics, the repeated evolution of dorsal fins in aquatic fish, reptiles, and mammals provides evidence that dorsal fins are adaptations to an aquatic existence.

250px-Tigershark3800px-Ickthyosaur_MNHOrca_dorsalfin_NOAA

Third, we can study the effects on survival and reproduction of variations in the trait of interest.  This can be done either by altering the features of the character experimentally (as in this neat experiment on sexual selection in widowbirds) or by studying naturally occurring variants (as was done with peppered moths by  H.B.D. Kettlewell).

The evidence for the adaptiveness of spotting/striping in mammals is primarily of the first sort (Hugh B. Cott, in his classic Adaptive Coloration in Animals, has a lot about optical principles, and what makes things hard to see), the second sort (pacas, bongos, deer, tapirs all have spots and/or stripes [and note that pacas are rodents, and that tapirs, which are perissodactyls, are not at all closely related to the artiodactyl deer and bongo, so it would be hard to argue it’s a retained ancestral feature]), and very little of the third sort– no one’s painted baby tapirs’ spots over to see what happens to them (at least as far as I know). I’ll touch on all three sorts as they relate to tapirs in later posts.

(For other examples of camouflage, see Matthew Cobb’s earlier post on the subject.)

How the tapir got his spots II

August 5, 2009 • 11:03 am

by Greg Mayer

I promised baby tapirs, so here are baby tapirs! (From Zooborns.)

Baby Malay tapir
Baby Malay Tapir (Tapirus indicus; from Zooborns)

Adult Malay tapirs, as you’ll recall, are particolored:

Malay Tapir with baby
Malay Tapir with baby (from Zooborns)

The three other species of tapir, all from the Americas, also have spotted/striped young. Here’s a lowland tapir, found throughout much of cis-Andean tropical South America; the others are very similar in appearance.

Brazilian tapir with Baby
Lowland Tapir (Tapirus terrestris) with baby (from Zooborns)

We can thus see that all baby tapirs look much alike, and quite different from adults.  Adults are either self-colored (the American species) or particolored (the Malay tapir). (It’s interesting that both young and adults have white edges to their ears.)  The question is, is this coloration of the juveniles an adaptation? Or is it an ancestral feature of no current utility, which makes a brief appearance in the young, but is then lost (like the coat of hair that human babies have in utero)?