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.


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.)

Caturday felid

August 8, 2009 • 7:51 am

by Greg Mayer

Having been thinking about the taxonomic distribution and adaptive significance (if any) of spots and stripes, I recalled that my cat, Peyton (see here and here), had some pattern elements quite reminiscent of tigers (beyond being a tiger tabby– the “tiger” stripes of tabby cats are not very like the stripes of tigers).  So I went to the Racine Zoo to look at the tiger.

The particular pattern element that seemed tiger-like was the striping on the inside of her legs, which is not well seen on the tiger here. I was more intrigued by the fact that along the back, some of the stripes of the tiger form rosette-like ovals (rosettes are found in leopards, jaguars, and young lions), which can be seen in this photo (blurred by the glass keeping me and the tiger separated).

Siberian tiger at Racine Zoo; note oval markings

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)?

How the tapir got his spots

August 4, 2009 • 7:11 am

by Greg Mayer

A while back Jerry posted a video of lion cubs at the Tulsa Zoo, and noted that they have spots, remarking

Many species of cats show this pattern in the cubs, even if the pattern disappears with growth.  It almost certainly reflects (as discussed in WEIT), an atavistic trait: the persistence in a descendant of traits that were adaptive only in an ancestor.  I suspect that the ancestor of lions had spots as adults, and that’s why they show up, briefly, in lion cubs.

I posted a comment to the effect that Hugh Cott, the great student of adaptive coloration, agreed with Jerry, although I wasn’t so sure:

Hugh Cott, in his classic “Adaptive Coloration in Animals” (Methuen, 1940) agreed with Jerry on this: “Among mammals and birds, first liveries acquired by the young– whether this happens before or after birth– often differ widely from the full dress of their parents. But it must not be assumed that such differences are necessarily adaptive. Lion cubs have spotted coats, and their tails are ringed…[Cott gives some more cat examples]… Since the kittens of all these animals…are born in sheltered dens or holes, carefully hidden or guarded by the mother, the spotted pattern can hardly be explained as protective.”(p. 21). I’m not so sure, though. Lions are not sheltered in dens or burrows, but rather are kept in thickets and kopjes, and may be on their own for a day at a time (George Schaller, “The Serengeti Lion” [Chicago, 1972], so the spots might be protective coloration for keeping the young hidden before they become formidable individually. (Protective coloration in the young is well known in mammals– whitetailed and mule deer, and tapirs, being good examples: their young bear dots and vermiculations that blend with sun- or moon-dappled forest floors.)

Since then, Jerry and I have conducted an off-blog discussion on this, and he has particularly challenged me with regard to tapirs.  While tapirs (and lions!) present many interesting aspects of natural history, the general question is one one of fundamental conceptual importance for evolutionary biology: how do you tell if a feature of an organism is an adaptation? So I’m going to pursue this question over a few posts.  To set the scene, let’s introduce tapirs. The best web source of info on them is the IUCN‘s Tapir Specialist Group.

Baird's Tapir at Franklin Park Zoo, Boston (from Wikipedia)
Baird's Tapir at Franklin Park Zoo, Boston (from Wikipedia)

Tapirs, along with horses and rhinoceroses, are odd-toed ungulates, members of the mammalian order Perissodactyla, which is the less species-rich of the two great extant orders of hoofed mammals. (Most hoofed mammals, such as deer, antelope, cattle, sheep, pigs, etc., are even-toed, members of the  Artiodactyla.) There are four species, all of which have short trunks.  Three are in the Neotropics (Tapirus bairdii, T. pinchaque, and T. terrestris), found from southern Mexico to northern Argentina. As adults, they are all more or less uniformly colored, brown to gray to black. The Malay tapir (T. indicus) of southeast Asia, however, is strikingly particolored.

Malay Tapir at Regents Park Zoo, London (from Wikipedia)

I’ll show some baby tapirs in the next post.

Coat color in wolves

March 6, 2009 • 11:08 am

by Greg Mayer

An alert reader has directed my attention to an interesting paper on coat color in wolves (abstract only without subscription) in today’s issue of Science by Tovi Anderson of Stanford and 14 colleagues from the US, Canada, Italy, and Sweden. Coat color in wolves is a polygenic trait affected by age, but Anderson and her colleagues show that black color in young wolves is associated with a 3 base-pair deletion in a gene called CBD103, and that there is a habitat correlation with the frequency of this color: black wolves live in the forests, gray (or white) wolves on the tundra.  This would be very interesting on its own, but Anderson et al. go further.  By careful phylogenetic analyses, they show that the gene for black color has entered North American wolves, and coyotes and Italian wolves as well, by hybridization with domestic dogs; and that the gene has been subject to recent positive natural selection (shown by low variability in the part of the chromosome immediately surrounding the gene, indicating what is known as a “selective sweep”).  Thus, a correlation between habitat and coat color that is suggestive of adaptation, is shown to be based on heritable variation undergoing natural selection.

The sort of combined field and lab study done by Anderson et al., using classical genetics (they have pedigrees of the wild wolves!), ecology, and now molecular genetics, is among the kind of work that first attracted me to evolutionary biology, and is known as ecological genetics. This field of study, pioneered by the great British geneticist E.B. Ford, was once characterized by the great American geneticist Dick Lewontin as carrying on the British “genteel upper-middle-class tradition of fascination with snails and butterflies”; I’m glad the fascination has moved to some of the colonies and beyond, and been extended to wolves, coyotes, and dogs.