Thursday 22 November 2012

News in Brief #7

Geoffroy's spider monkey
Monkey Sleep, Monkey Poo

Poo is useful stuff to zoologists: a lot of field work involves examining the stuff. It can tell you what an animal has been eating, how it behaves, and maybe something about it's genetics. And, of course, it can also tell you where it's been. Take Geoffroy's spider monkey (Ateles geoffroyi), for example.

Spider monkeys are moderately sized monkeys, noted for their prehensile tails and remarkable agility. I saw some (although not Geoffroy's species) at London Zoo on Monday, and, despite the poor weather, they really were a delight to watch, full of enthusiasm and energy. Seeing them at play, it's hard to escape the impression that they have five arms: their feet are effectively an extra pair of hands, and their tail is so flexible and has such a good grip that it might as well be a fifth one.

They live, unsurprisingly enough, up in the trees - specifically ones in Central America - sleeping in them at night and travelling between them to feed during the day. They have particular favoured trees to sleep in, and deposit a lot of dung at the base over the course of the night. Such 'latrine' use is common among many mammals, and just finding where they are provides a good means of establishing what the monkeys are up to without having to disturb them.

A new study has compared the sleeping and latrine sites of a number of these monkeys at sites in untouched forest and in forests more fragmented by human logging. In the latter sites, the monkeys were forced to clump together rather more, not spreading out as much as they would apparently like. In all the sites, however, they seemed to prefer particular sorts of trees, generally those with lots of long horizontal branches to swing from. That they picked particular tree types to sleep in, and then defecated around the base may be ecologically important, since particular kinds of seeds will germinate in the dung, so that certain combinations of trees will often be found growing nearby. In so doing, monkeys may help shape the very forests that they live in.

Of Singing Mice and Bellowing Koalas

The primary mode of communication between humans is speech, so it's hardly surprising that vocal communication is also common
among other mammals. Two recent studies help to illustrate just how fine-tuned the vocal and hearing abilities of some mammals are.

Male mice sing to attract mates, but their songs are so high-pitched that we humans simply can't hear them - that is, they are ultrasonic. Compared with, say, songbirds, their songs aren't very complicated, and they don't vary too much between individuals. That may well be because the areas of brain circuitry that control the sophisticated calls and learning abilities in songbirds (and possibly also in humans) just aren't there in mice.

Or are they? A recent study by researchers in South Carolina showed that, not only do they have similar brain structures to songbirds, but these areas are active when they are singing. Moreover, the mice were able to modify their songs based on what they were hearing, using feedback from the sound of their own songs and those of their neighbours. This is not what we'd expect in an animal whose songs are entirely instinctive (see, for example, this work on a non-song bird). Granted, the mice aren't quite as good at it as songbirds are, and the brain regions aren't quite as well developed. But it's a lot closer to birds than we had previously thought they were.

It's not just mice that sing to attract mates. Male koalas also make a deep grunting bellow during the mating season, apparently to show off their manliness and virility. (Seriously. Take a listen). An international team of researchers have now analysed these sounds to see what information they contain. They conclude that the calls of each koala are distinctive enough that it should be possible for koalas to distinguish specific individuals at up to 50 metres (165 feet) away. That is, they have clearly distinguishable 'voices' that could enable other koalas to find a favoured mate or avoid a tough rival.

Even at three times this distance, it should be possible for koalas to at least assess the size (and hence, hunky masculinity) of calling males. Technically, we don't know whether or not they actually can do this, but the information is there, and its hard to imagine they don't use it.

More Secrets of the Mound-building Mouse

You may recall that, back in August of last year, I wrote about the steppe mouse (Mus spicilegus), a species of mouse from eastern Europe with the unusual habit of constructing large mounds of plant material over their burrows in winter. It had previously been assumed that the mounds were food supplies, giant caches and larders to tide the mice through hard times. But, in the study I was reporting on then, Peter Szenczi and colleagues had shown that the burrows didn't extend into the mounds, so that the mice weren't, at least most of the time, eating the piles they had constructed. Instead, it seemed that the mounds were there mainly to keep the burrows underneath warm.

But why make the mounds out of a potential food supply when you could just make them out of soil? In a follow-up study, the same researchers constructed their own mounds, some from earth, some from plant matter, and some from a mixture, to see what the differences were. It turns out that the piles of seeds and straw that they built were better at insulating the ground underneath than simply covering it with more soil. Not only that, but they were also quite a lot better at absorbing rain, keeping any potential tunnels underneath the mounds not just warm, but dry and cosy. It probably takes the mice more time to build the mounds the way they do, but there's a good reason for doing so.

Why Herds Are Good for You

Herbivorous animals such as deer and cattle live in herds because there is safety in numbers. Some animals can eat while others are keeping a look out, and, so long as they take it in turns, everybody benefits. This is clearly a tactic that works, and we do, indeed, find that herd-dwelling herbivores tend to live longer than their solitary kin. But how much of an advantage is this?

If the advantages of being in a herd simply offset the fact that there's lots of predators out there who might eat you (and, indeed, might find you easier to locate if you're with a huge bunch of friends) then that's useful, but it might not make a great difference in your life compared with related animals that live on their own, but aren't attacked as often. But if it provides a larger advantage, it may make enough of a difference that the animal's natural lifespan can increase. In other words, even if there aren't predators around, herd-living animals may live longer, because, under more natural conditions, they're more likely to get the chance to enjoy the benefit of doing so.

That, at least, was the theory behind a survey of members of the cattle family - most of them antelopes - by Jakob Bro-Jørgensen of the University of Liverpool. He found that, on average, herd-living cattle species do naturally have longer lifespans than those that instead rely on hiding. (Hiding, on the other hand, is going to be a more effective strategy than herding if you don't live, for example, in vast treeless plains where there isn't anything to hide behind). 

On the other hand, another common habit of cattle species had the opposite effect. In many such species, the males are larger than the females, putting all their calories into making themselves larger, muscular, and more noticeable. This has the obvious advantage that they're more likely to mate and have calves, but it's costly and potentially puts them at risk. The survey showed that, in species where the male is much larger than the female, males have shorter natural lifespans - presumably, the fact that they're more likely to wear themselves out and end up being eaten means that there's little point in evolving a naturally long life. Females of the same species, however, had no such problem, and end up living much longer than their partners.

How Fast Does a Whale Grow?

Speaking of growing to large sizes, a study by Sarah Fortune and co-workers, from various Canadian and American universities, examined the growth rates of North Atlantic right whales (Eubalaena glacialis), an endangered species of huge, krill-eating whale. It turns out that they grow remarkably fast, increasing their weight by a factor of over twelve in their first year of life. Given that they're pretty big to start with, this amounts to an average of putting on 34 kg (75 lb) every single day.

After their first year, they slow down, having reached about three-quarters of their adult size. It's notable that this is also about the time that they are weaned, suggesting that a lot of this incredible growth spurt is down to the amount of their mother's milk that they're drinking. This must put quite a strain on the mother, and is probably why they only give birth about once every three years - they need the other two to recover. 

There are three possible reasons why they might do this. It may be that rapid growth of the whale calf means that they can quickly reach the size where nothing can eat them, increasing their odds of survival. Another possibility is that it might help them reproduce, since they're large enough to start breeding much sooner than they would otherwise be. A third possibility is that it's to do with the growth of the mouth specifically since this is, of course, exceptionally large in these animals compared with the rest of the body. In turn, that would mean that they're far better at scooping up huge amounts of krill by the time they have to stop relying on milk.
It could, of course, always be a bit of all three.

New Species Reported

Once again, top of the list of species discovered in the last couple of months or so are some bats. Mouse-eared bats are fairly typical insect-eating bats living across most of the world. There are already over sixty species of them known, and many of them are pretty hard to tell apart. Fifteen of those species are said to live in South America. Now a new survey has looked at the genetics of a number of specimens collected from across that continent.

Their analysis shows that there are at least eighteen different species represented in the samples they have looked at - and, obviously those are just the ones that have been caught and preserved. And, moreover, caught and preserved by people who didn't think they'd found anything special - who knows how many people who thought they had nothing special didn't bother to keep a sample of it?

In particular, the black myotis (Myotis nigricans), which is said to live everywhere from central Mexico to southern Brazil, appears to actually several different species. It seems as if every time somebody caught a small, dark-furred bat of the right shape, they assumed that they had a black myotis. Sometimes that will have been, in retrospect, a clear mistake, and the bat belonged to some other species already known, such as the rather similar looking riparian myotis. But, in others, it seems clear that they had an entirely new species that looks so similar that nobody could tell them apart just by looking at them.

According to their analysis, the black myotis seems to represent at least two - and probably more - different species. The real ones, far from living right across the continent, may only be found in south-eastern Brazil. The authors of the paper stop short of naming or describing these new species, because there isn't really enough data to do that, but there look to be solid grounds for assuming that they're there.

South America isn't a bad place to look for new species of small mammal, and our other one for today comes from there, too. This is a new species of tuco-tuco (Ctenomys ibicuiensis), found in southern Brazil. They live in a small area where the native habitat is rapidly being destroyed by the expansion of plantations, and may already be endangered.

If, incidentally, you were unaware that there was even one species of tuco-tuco in the first place, I should explain that they are moderately sized rodents (this one is about eight inches long) that look rather like wrinkly, yet furry, long-tailed hamsters. They spend much of their lives underground in burrows, which, as I've mentioned before, is a really great place to be if you don't want scientists to notice you. There are something like sixty different species of the beasts, all of them in South America, and, like the bats, they can be hard to tell apart even once you've got them out of their burrows.

There's doubtless more of them to be found.

[Picture by Lea Maimone, from Wikimedia Commons]

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