Elephants are not the only mammals to have an elongated, flexible proboscis; so do elephant seals, saiga, and tapirs. The latter have long been favorite items of discussion here at WEIT, Jerry and I having debated the extent to which the spotted patterns of young tapirs are adaptive camouflage. I hope to get back to that discussion, but in the meantime, in the spirit of one good puggle deserves another, here’s a baby Malay tapir born at the Denver Zoo last month that needed help to start breathing.
According to the Zoo:
On September 3, Denver Zoo’s female tapir, Rinny, gave birth to calf, Dumadi, inside the rhino/tapir building of Toyota Elephant Passage, but was he was stuck and unresposive in his amniotic sac. After watching Rinny unsuccessfully attempt to free him, zookeepers safely separated mother and calf then freed the newborn from the sac and began providing mouth to snout rescue breaths and manually stimulated the baby for regular breathing and in order to expel liquid from his lungs. After a few minutes of rescue efforts, the infant successfully began to breathe on his own.
The above, short clip is from ITN. The next video, from the Zoo’s Youtube channel, also shows the birth and the post-resuscitation Dumadi swimming and walking around. Note the strong contrast between the dappled stripe and spot pattern of the baby, and the particolored adult.
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 neatexperiment 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.)
Although far from the longest chapter in WEIT, I find the chapter on biogeography the single most persuasive one for showing why evolution is true. I think Jerry finds it compelling as well. This might seem surprising since he’s a geneticist: one might think he would find some of the genetic evidence most compelling. But I don’t think it is surprising, given that it was the biogeographic evidence, that, as the great zoogeographer P.J. Darlington put it, showed Darwin evolution.
The first thing you might think needs explication is the disjunct distribution. But before tackling this, a mis-impression must be corrected: although we tend to think of tapirs as typically South American, from a historical perspective, they are recent interlopers. Along with many other animals we consider typically South American (jaguars, llamas, peccaries), they entered South America from the north about 3 million years ago when the Panamanian portal became the Panamanian isthmus during the Great American Interchange.
What, then about the disjunction: how did they get from Central America to Malaya? They didn’t. Tapirs are a northern group. They and their relatives date back to the lower Eocene (ca. 50 mya). The modern genus, Tapirus, dates back to the Oligocene (ca. 30 mya), and was found in Europe, Asia, and North America. They have gone extinct in Europe, most of Asia, and most of North America. Tapirs thus have a relict distribution, being still found at two endpoints of their historical distribution. Geology, paleontology, and systematics thus combine to give a most satisfying account.
I promised baby tapirs, so here are baby tapirs! (From Zooborns.)
Adult Malay tapirs, as you’ll recall, are particolored:
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.
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)?
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.
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.