Sunday 26 January 2014

Mini-Monkeys: Marmosets of the Northern Atlantic Forest

Common marmoset
The official name of every family of animals is derived from the name of the 'type genus' - a smaller group of closely related animals within it that are deemed to be 'typical', and which are used to define that family. Of course, what's 'typical' is a subjective opinion, and will only reflect what somebody thought at the time the family was named. So, really, it's more of a convenience than a truly meaningful statement. But, still, it's a convenience that we continue to use.

The type genus of the marmoset family is Callithrix, and the scientific name of the family is therefore 'Callitrichidae'. If we're honest, they're not really any more 'typical' than most other species in the family, but they're as good a place to start my survey of the world's marmosets and tamarins as anywhere else.

The Callithrix monkeys are true marmosets, a group that is distinguished from other members of the family by a heavier reliance on tree gum as a source of food. All callitrichids eat gum, but for true marmosets, it's a vital part of their diet that they eat all year round. Having evolved to do this, they have a number of physical adaptations that suit them for the lifestyle. Unlike their relatives the tamarins, they have no tusks in their lower jaws, meaning that the teeth at the front of their mouths form a relatively straight line. They feed by clamping the teeth of their upper jaw onto tree bark, and then scraping upwards with that flat row of teeth on the lower jaw.

Although they aren't born that way, over time, these teeth are honed to a sharp chisel-like edge. However, for this to work, they also need a remarkably wide gape, and their jaw and its muscles are adapted accordingly. Nor does it end there, because there's the business of digesting the gum.

Sunday 19 January 2014

Fast-Running Hyenas of Tibet

Spotted hyena
The hyena family is one of the smallest families amongst the living Carnivorans. Only four species survive today, of which by far the best known is the spotted or "laughing" hyena (Crocuta crocuta). Hyenas are, oddly enough, closer in their evolutionary origins to cats than they are to dogs, despite their appearance, and, to be honest, much of their behaviour. Having said that, of course, they're hardly very close relatives of cats (they're closer to mongooses), but they do happen to be on that half of the family tree.

Today, hyenas fall into one of two basic body types. There's the one everyone knows, typified by the spotted hyena, which involves a relatively heavy build and adaptations to crunching up bone. These hyenas have a reputation for eating carrion, and they certainly do that, although spotted hyenas in particular are pretty good at hunting. Still, they'd rather jump out on their prey from a short distance away than have to chase it for long distances, as a wolf would. If their prey isn't going anywhere, because it's already dead, that's even better, and spotted hyenas are strong enough to chase leopards off their kills (the fact that they hunt in packs, and leopards don't, is doubtless helpful here).

Sunday 12 January 2014

The Puzzling History of the Clymene Dolphin

The taxonomy of dolphins is, as I've mentioned on a couple of previous occasions, a complicated issue. The problem is that at least some dolphins underwent a very rapid diversification in the not-so-distant past. New species appeared so shortly after one another that, from the perspective of the present day, it's very difficult to figure out in what order it happened, or which dolphins are most closely related to which others. This particularly affects the genera Tursiops (the bottlenose dolphins) and Stenella (the spinner dolphin and its relatives), and likely has some effect on one or two others as well.

If you look back at those two posts (sorry, this won't work if you're reading this on a mobile) you'll see that I drew up a plausible relationship tree based on the research of Kate Charlton-Robb et al, who identified the existence of the Burrunan dolphin as a distinct species in 2011. One of the animals you'll notice that I mentioned was the Clymene dolphin (Stenella clymene), and that I said in one post that "the spinner, spotted, and Clymene dolphins were all assumed to be related, on the not unreasonable grounds that they look much the same as one another..." Emphasis on the past tense, because the study seemed to show that, despite their visual similarity, they weren't all that closely related after all.

Sunday 5 January 2014

How Long to Make an Elephant?

Biology, on the whole, is not a great one for following mathematical laws. Physics has no such problem, and is replete with mathematical formulae for calculating this and that, which never fail, so long as you've understood them properly. Biology, not so much. It's squishy, and variable, and while it does follow rules at least some of the time, there are so many confounding factors that they're really more like guidelines. (Barring, of course, those imposed directly by physics, such as the minimum wing area needed to get airbourne).

Still, it does have some, and a lot of them are to do with the mass of an organism. We can say, for instance, just to pick an obvious example, that a bigger animal needs to eat more food. Or, more accurately, more calories, since otherwise you get into all sorts of difficulties depending on how nutritious their diet happens to be.

It's not simply a matter of twice the mass, twice the calories, either, because there are economies of scale here. Talking about mammals, as we do here, these economies have a lot to do with heat loss. A bigger animal retains more heat than a smaller one, and, since mammals are warm-blooded, that means a bigger animal doesn't need to work quite so hard to keep that body temperature at the right level.