Tuesday, 29 May 2012

News in Brief #5

Mountain long-eared bat
Can Bats Fly Like a Bird?

This is a blog about mammals, so it's only right to point out, every now and then, that birds have no monopoly on animal flight. The only mammals capable of true flight are, of course, the bats - flying squirrels and their ilk can only glide, not truly fly. (Nor are mammals and birds alone, of course; there are many flying insects, and once upon a time there were also pterosaurs). Still, birds have been flying for much longer than bats have, so might they be better at it?

A recent study by Florian Muijres and colleagues reveals, perhaps surprisingly, that the answer is 'yes'. They put birds and bats in a wind tunnel, and measured their ability to generate lift, and the lift-to-drag ratio, which indicates how much energy they have to put into it. By both measures, birds outperformed bats of similar size. Why so? After all, while birds have been flying for three times as long as bats have, the oldest known bat fossil is still very old, and you'd think they'd be pretty good at it by now.

Indeed, by some measures, they were: the wake that bats created was, like that of birds, as perfect as it could be, given their low lift-to-drag ratio. Which suggests that the drag might be their only problem. And that, the researchers suggest, is because bats have big ears, and (at least in the bats they were looking at) a decidedly un-aerodynamic leaf-like structure on their nose. Birds, very obviously, don't have great big bat-like ears sticking out of their heads, and present a smoother, sleeker, aerodynamic profile. Their improved flight energetics may explain why insect-eating birds migrate south in the winter, when food is in short supply, while insect-eating bats hibernate instead.

The reason that bats have large ears is to listen in for their sonar pings. But what about the nose-leaf? Again, it's clear that it's to do with their ultrasound, in this case, emission, rather than reception. Those nose-leaves are often complex structures, with multiple baffles to diffract the outgoing sound waves - something that would, by its very nature, increase aerodynamic drag. Another study, by Lin Feng and co-workers, used laser Doppler vibrometry to see exactly how the nose-leaf of a greater horseshoe bat (Rhinolophus ferroequinum) works.

They showed that the forward part of the nose-leaf vibrates as the bat emits ultrasonic pulses, changing its geometry as it does so, and so potentially affecting the wavelength of the sound. The analysis wasn't detailed enough to say why that happens, but it might allow the bat to change the nature of the sound emissions, perhaps in response to changes in their environment.

Since we're on the subject of bats, there have been plenty of other papers on these intriguing animals over the last couple of months. Antton Alberdi and co-workers looked at the diet of mountain long-eared bats (Plecotus macrobullaris) in the Pyrenees using DNA barcoding technology on their dung. They subsist almost entirely on moths and, perhaps because they are the only moths they can find at the altitude where they hunt, mainly on ones that can detect ultrasound, and ought to be able to hear them coming. Presumably, they hunt very quietly...

Cristina McSwiney reports that Jamaican fruit bats (Artibeus jamaicensis) eat so much pollen in the spring that they turn yellow. And, finally, Sebastien Puechmaille reports that Natterer's bats (Myotis nattereri) from Corsica are sufficiently different from their mainland kin that they may represent a new species.

A Sweaty Chest is a Manly Chest

At least for Verreaux's sifakas (Propithecus verreauxi). The males of these lemur-like Madagascan animals sometimes smear their chests with smelly yellowish secretions while rubbing against trees and the like. Not all do, though, and some have relatively clean, white fur on their chests instead. According to Stefania Dall'Olio and co-workers, female sifakas do, as previously predicted, find stained chests sexy on their males.

While scent-marking in sifakas does, as in other animals, also serve a wider range of functions, they observed that males were more likely to stain their chests during the mating season than, for example, during the birthing season. These are animals where the females make the running in deciding whether or not to mate, and it was clear that males with stained chests did, indeed, get to have a lot more sex than their clean-chested counterparts.

They also had higher testosterone levels, something that presumably makes them want to stain their chests in the first place, and larger testicles, than their cleaner brothers. This hadn't been noticed before because, while the stain lasts year round, the testosterone levels and the enlarged testes don't - and previous researchers had previously only looked for such differences during the 'wrong' seasons, when everything except the stain is back to normal.

The cleaner chested, less dominant, males tried to compensate for their lack of manliness by grooming the females more often, in the hope (as it were) that being nice to them would make up for the lack of a well-oiled chest. By and large, it didn't.

Sifakas Are Not the Only Primates

Want more primates? Of course you do. Pygmy lorises (Nycticebus pygmaeus) are small, nocturnal, somewhat lemur-like animals living in the jungles of Southeast Asia. If you're nocturnal, moonlight isn't necessarily a good thing. That's because one of the main advantages of being nocturnal is that predators can't see you, something that doesn't work quite so well when the moon is full.

Carly Starr studied the behaviour of pygmy lorises in Cambodia to see how much of a difference moonlight made. As predicted, they were more likely to stay hidden (one thing lorises are very good at is not moving), and even sleep, when the moon was particularly bright. But they didn't mind so much in the rainy season, when the weather is hottest and vegetation they can hide among is at its densest. But lorises aren't comfortable in cooler weather, and the combination of both that and bright moonlight was enough to stop them going out at all - risking predators isn't a good idea at the best of times, and they certainly weren't going to do it when the weather didn't suit them.

Humans belong to the great ape family of primates, and it's not surprising that the other species in that family share a number of traits with us. One of those similarities might be the concept of culture, the idea that different 'ethnic' groups within the same species may have specific ways of behaving that differ for reasons unrelated to genetics, or to the vagaries of the physical environment in which they live. Serge Wich and colleagues listened to groups of orang utans (Pongo pygmaeus) to decide whether or not they had different dialects.

Previous studies had looked at orang utans accents - whether different groups have distinctive ways of making the same calls - but this one was looking at whether or not they make the same sounds to indicate the same thing. And it would seem that they do, indeed have such dialects. For example, this is an orang utan from one group saying something like 'I've just finished building my nest for the night', while this is an orang utan from a completely different group apparently saying the same thing.

Lastly, Douglas Makin and co-workers mapped out what they describe as a vervet monkey's (Chlorocebus aethiops) "three-dimensional landscape of fear" by demonstrating how they make the best use of branches and vegetation to avoid attack from both snakes on the ground and eagles in the air. As well they might.

Warm White Whales

The white whale family is one of the smaller, and stranger, families of mammals. Today, it contains just two species: the narwhal and the beluga. Belugas live throughout the Arctic Ocean, and by dint of just about sneaking through the Bering Straits, also in the very northern Pacific. Narwhals don't even travel that far south. Both are, as one might expect, well adapted to living in such chilly conditions. Given that fact, we might also expect that such Arctic adaptations help define the family, and explain why they are so different from other whales.

The fossil record for white whales is hardly spectacular. We know of some fossil belugas and narwhals, all from more or less where you'd expect. For instance, fossil belugas are found as far south as northern New York State, but since they date from the middle of the Ice Ages, that's not really surprising.

Until this year, we knew of just one other species, known only from fossils: Denebola brachycephala. The thing is, the fossil in question was actually found off the west coast of northern Mexico in 1984, and other fossils, of less certain classification, are known from the temperate zones of the northern hemisphere - either along the Pacific coast of North America, or around the North Sea of Europe. What were they doing that far south? Well, now we have a fourth species to add to the story.

The newly described species, Bohaskaia monodontoides, is not quite as old a fossil as Denebola. It was found in deposits around 3 to 4 million years old along the coast of Virginia, placing it during the mid Pliocene epoch, a few million years after its predecessor. The Pliocene was a time when the world climate was cooling as the Ice Ages of the next epoch approached, but it was by no means cold enough for Virginia to feel like Alaska, or, indeed, that much different from the way it does today.

The fossil consists only of a skull, albeit a fairly well preserved one, so we can't be certain of the full size of the extinct whale. Still, the skull is about the same size as the skull of living belugas and narwhals, so the rest of the animal was probably comparable - somewhere in the vicinity of twelve feet long, and weighing a ton and a half. The animal's snout has much in common with that of living belugas, but the top of the skull more closely resembles that of narwhals, indicating that it really is something different from either, yet related to both.

The skull was not on its own; other bones apparently belonging to white whales were found as well, although without enough detail to formally describe as new species. Indeed, there's a pretty good chance that at least some of them represent the hind end of Bohaskaia... we just can't tell which ones. But here's the thing: if they represent multiple different species, as they seem to, there must have been several different white whales living in temperate waters back in the Pliocene.

Which suggests that that was where they first evolved, and that the two surviving species represent some northern offshoot of the original stock. At the very least, white whales once lived further afield than they do today.

[Photo by A. Alberdi, released under Creative Commons licence]

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