Saturday 28 September 2013

Pleistocene (Pt 11): Sabretooth Smilodon

Smilodon fatalis
Over the  course of the last three parts of this series, I have looked at the various large herbivores that lived in North America during, and between, the Ice Ages. Naturally, there were also predators feeding on these animals, and none is more famous than Smilodon, the sabretooth cat.

However, Smilodon was by no means the only sabretooth cat, even in Pleistocene North America. Indeed, the trend for large cat-like animals to evolve huge, sabre-like canines, is one that arose many times during the Age of Mammals. So there are a wide range of, often quite unrelated, animals that look like "sabretooths." However, when we refer to "sabretooth cats" in particular, we're generally referring to the group technically called the machairodontines.

The machairodontines are one of three main subfamilies within the cat family, and the only one to have gone extinct; the other two are the big cats (lions, tigers, etc.), and the "true" felines (everything from cougars to house cats). They first appeared about 10 million years ago, long before the Pleistocene, and represent a genuine group of cats, descended from a single ancestor, and about equally related to the other two kinds, which arose slightly later. In fact, they don't all have enormous teeth - at least, no more so than a tiger does, which is quite large enough, really - but most of the later forms do.

Sunday 22 September 2013

Why Prairie Dogs Sleep Around

For the white-tailed prairie dog,
one male is quite enough, thank you
[Today is World Rhino Day. For past posts at Synapsida on the subject of rhinos, see here.]

The general pattern for mammals is that males seek to mate with as many females as possible, while females prefer to keep a single partner, at least in any one mating season. This is because females can't be pregnant with more than one litter at a time, so they may as well pick the fittest males around and get them to father all their children. Males, on the other hand, have no such investment, so the more females they mate with, the more children they have, and the better their genes survive.

But this is really a gross oversimplification. It's the basis of the polygynous mating system, where a powerful male drives off rivals and attracts a harem, or acquires multiple mates by some other means. This sort of thing is most apparent in animals like seals and deer, but it's by no means universal. For one thing, many mammals are more or less monogamous. This typically occurs where the male has to help to look after his children if they're to survive, thus putting him under the same pressure as the females.

But what are we to make of polyandry, where one female mates with multiple males in the same breeding season? It's surprisingly common, so there must be some reason for it. One, of course, is that they might not get much choice in the matter - if there's lots of randy males around, then it may be easier to give in than to put up a fight. It's hardly acceptable behaviour for humans, but, then, animals aren't human.

Sunday 15 September 2013

Caprines: The World's Largest Goats

Takin (Sichuan subspecies)
All of the various kinds of caprine that I have described so far in this series have looked, more or less, either goat-like or sheep-like. There are just two species left, and those form something of an exception. While most caprines are fairly medium-sized as hoofed animals go, these are much larger, more muscular, animals.

It used to be thought, on this basis, that they were closely related to one another, representing an early branch in the evolution of goat-like animals that split away from their relatives well before the appearance of actual goats or sheep. From modern genetic evidence, that no longer looks the case. They are, as we suspected, caprines, (although, despite the title of this post, not literally goats) but within that group, they are not particular closely related to one another. Instead, their apparent similarities are a coincidence, a case of parallel evolution where two animals, both fairly goat-like to start with, faced selective pressures to become larger.

The first of these animals is the takin (Budorcas taxicolor). Takins are very distinctive animals, quite hard to mistake for anything else, once you get a good look at them. The most obvious point, as I've already implied, is the size. A fully grown male takin stands over four feet high at the shoulder, and weighs upwards of 300 kg (660 lbs). The females are noticeably smaller, but still larger than even the males of any other caprine species (with, of course, one exception that I'll get to in a moment).

Sunday 8 September 2013

Island of the Giant Hedgehogs

Hoplitomeryx matthei
A well-documented phenomenon in evolution is that of insular dwarfism. What happens is that a population of large, usually herbivorous, animals become trapped by rising sea levels, finding themselves on an island where previously they had been able to roam free across a much wider region. In response, over the course of many, many generations, their descendants become smaller.

There are two main reasons why this happens. Firstly, there isn't so much food to eat on an island, so smaller animals that eat less will be at an advantage, and able to have more offspring that survive to have offspring of their own. But, while the problem of a limited food supply is more obvious on a smallish island, even on a large continent, the supply is never likely to be inexhaustible. In which case, why do large animals exist in the first place?

That, of course, is where the second reason comes in: predators. Being large is a protection against being eaten; lions and tigers, for instance, rarely eat elephants, rhinos, or hippos. Large predators, being at the precarious pinnacle of the food chain, and present in smaller numbers to start with, find it even harder to survive on small islands than herbivores, and they often simply die out. That removes the need for herbivores to avoid them: there's no real need to be too large for a lion to eat if the biggest thing you'll ever face is a fox, anyway.

Sunday 1 September 2013

That Funky Gibbon Song

A male agile gibbon
We humans learn to speak from our parents, with our mothers playing a particularly important role. It's because of this that we have different languages across the globe, rather than all speaking the same genetically-determined tongue, pre-programmed from birth. Yet, in other animals, communication seems to be primarily genetic, with social learning playing little, if any, role.

There are, of course, exceptions to this. We know, for example, that songbirds raised in isolation produce much simpler songs than those in a natural environment, able to hear, and respond to, other birds of their species. On the other hand, they do still sing, and the songs are much closer to normal ones than a similarly deprived human would be able to do with respect to English, French, or Gujarati.

Well, we're not songbirds, but we are mammals, and more specifically, anthropoid primates. Is there really such an absolute split between us and our close relatives? The calls of many primates can be quite complex, and this surely has something to do with why language became so important to us. Are these calls entirely genetically programmed, or are they, in some way, learned?