Thursday, 26 January 2012

News in Brief #3

The Mandrill's Face

There are a number of colourful primates, but one of the more instantly recognisable is surely the mandrill (Mandrillus sphinx) whose adult males have strikingly blue and red faces. Under a strict definition, mandrills aren't really baboons, but they're very closely related to them, and the difference is pretty academic. Given that only the adult males have these extreme face-markings it's not surprising to learn that they are there mainly to advertise their masculinity and fitness as a potential father to female mandrills.

A study from 2005 confirmed that, yes, indeed, female mandrills find males with bright red noses to be particularly sexy. Oddly, though, until now nobody appears to have looked at what the bright blue patches on either side of the nose are for. In the new study, Julien Renoult and co-workers confirmed that the blue colour is more intense in dominant males, just as the red is. Indeed, it seems that what's really important is the contrast between the two. This suggests that, in the distant past, mandrills developed the red nose as a measure of their fitness, and the redder the nose, the more females liked them. So the noses of dominant males became ever redder... but there's only so red a nose can get. When some males began to develop a contrasting colour elsewhere on the snouts, the redness of their noses became more obvious, and, over the course of evolution, the bright blue/red contrast we see now developed.

Both colours seem to be under the control of testosterone, which would explain why they are most striking on the largest males, and it may also be relevant that both colours contrast strongly with the green of background foliage. There really isn't much else that you can mistake a male mandrill for...

Chimps and Children

Still on the subject of primates, our closest relatives are, of course, the chimpanzees and bonobos. Obviously, there are a number of differences between our species, but we also have a lot in common, and some of those commonalities can tell us something about how our own species evolved, and perhaps how it developed some of its unique traits.

A study by Giada Cordoni and Elisabetta Palagi examined how baby chimps begin to play, and how that compares to play in human children. It seems that, just as in humans, the way that young chimps play changes as they age. Like humans, chimps under the age of three are more likely to engage in solitary play, presumably to help hone their reflexes. However, very young chimps are just as likely to engage in more social play as their older fellows, whereas, in humans, it does become more common around the age of four.

In most other respects, there do seem to be striking similarities - for instance, young chimps do like to play with others of their own age, and not with other chimps that are slightly older or younger. Their games also become more complex as they age, and the way that they engage in more rough and tumble play also changes - until, that is, they become old enough to become genuinely aggressive, rather than merely playful. Just as human children are more likely to laugh when playing with others than on their own, young chimps are make more playful expressions when with engaging with others, possibly as a signal to indicate 'I don't really mean it' if play gets a little rough.

Where the Whales Hunt

Perhaps the least known and least well studied of all the families of large mammal is that of the beaked whales. Studying whales, of itself, presents problems that don't apply to most other groups of mammal, but beaked whales, in particular, are relatively rare, and don't like to visit shallow waters near land. Unlike animals such as right whales, beaked whales are true predators, feeding off relatively large prey, rather than plankton and shrimp. But, for an animal that's really large, living far out at sea, that presents the problem that most of what they want to eat is a very long way underneath them, where it's difficult to get at, or even to find.

A study by Patricia Arranz and co-workers used acoustic tags to not only follow Blainville's beaked whales (Mesoplodon densirostris) as they searched for food, but to record their sonar as they did so. That meant they could hear what the whales did, using the animals' own sonar to figure out things like the distance to the sea floor, just as the whales themselves do.

The study confirmed one thing we already knew: these whales are remarkably efficient divers. They dive to an average of about 830 metres (2700 feet) below the sea, with dives lasting around 48 minutes. Around half of that time is spent simply getting to the right depth, large because they don't seem to be able to dive straight down, as sperm whales do, but instead have to take a more gradual, sloping, descent. Considering that they can't breathe while they're diving, this must be fairly exhausting, and they have to spend spend two thirds of their day resting at the surface in between dives.

Once down there, they spend much of their time at the so-called "deep scattering layer" where most fish and other free-swimming organisms are found, with occasional forays down to the sea bed beneath. Here, they catch relatively slow-moving prey, and they apparently avoid hunting at shallower depths because the animals there tend to be more active and harder to catch. This tactic rewards them with about 30 catches per dive, which is evidently enough to keep them well fed and provides enough of a pay-off to make such difficult dives worthwhile.

Of Sea Lions and Marmots

There are a number of species of marmot, found mainly across Canada, Alaska, and Siberia. It's generally agreed that they first appeared in America - their closest relatives include, among other things, chipmunks - and only later reached Asia. By examining their genetics, we can get some insights into both how and when this expansion happened.

A phylogenetic study by Scott Steppan and co-workers examined the genetics of a range of different marmots to shed some light on their evolutionary history. They showed that the first Asian marmots appear to have separated from their relatives around 4.6 million years ago. Suddenly finding themselves in new and marmot-free lands, the colonists rapidly diversified, giving rise to several new species in a relatively short period of time. Today, each species lives in a different area of the Old World, including one species across in the Alps of Europe.

By contrast, the marmots left behind are still found in overlapping areas, with many places being home to more than one species. A particular puzzle that this study aimed to resolve was the relationship of the Olympic marmot (Marmota olympus) to its kin, with previous studies having produced contradictory results. Living only in one corner of Washington state, there had been some evidence that it was related to the nearby Vancouver Island marmot. But it seems it isn't, because that species appeared only 0.4 to 1.4 million years ago - presumably when Vancouver Island separated from the mainland - while Olympic marmots originated much earlier, 2.6 million years ago.

What seems to have happened is that Olympic marmots were once widespread across western Canada (although we do have to bear in mind that there are no fossils to prove this, so the story is still subject to revision). During the Ice Ages, some of the Olympic mountains were high enough to poke up through the ice sheets, leaving patches of land that were, ironically, more habitable than the lowlands round about. Here, the Olympic marmots clung on to life, while other Canadian marmots headed south. By the time the ice sheets melted, they were a new species, different enough from their relatives to inhabit the same lands.

A similar study looked at Steller sea lions (Eumetopias jubatus), another species found on both sides of the Bering Straits - although, in this case, along the coastlines from Japan to California. Using the methods of phylogeography, C.D. Phillips and co-workers showed that the species probably originated on the eastern side, and, indeed, the oldest fossil of a related animal does come from Japan, dating back around two million years. More significantly, perhaps, signs of past climate change had left their mark on the genetic record, showing how the species had struggled to survive at certain points. Now, that's largely cooling climate change, as the Ice Ages froze over their breeding rookeries, forcing them to move elsewhere. So it's not directly analogous to what we have today, but it does show, in general terms, how change in climate can affect the fate of species.

Sabre-tooth Kittens

Juvenile and adult Smilodon populator. Scale bar is 5 cm.
Not all fossils will be of adult animals, and its possible to tell when a skull belongs to a juvenile mammal because the bones will not have fully fused. Per Christansen of the University of Aalborg has taken advantage of this to examine how sabre-tooth cats (Smilodon spp.) changed as they aged, and compare them with how we know living big cats change.

The picture above shows how dramatic those changes were. The enlargement of the sabre-like canines is, perhaps, the most obvious change with age, but there are other alterations, such as the development of the high sagittal crest at the back of the skull, to which some of the jaw muscles would have attached. In general, this seems to mirror the changes in animals such as jaguars and leopards as they age, but they are far more dramatic. In fact, the skull of the juvenile cat does have some resemblance to that of early sabre-tooth species, long before the highly evolved Smilodon.

That's not to say that the changes during life simply echo the evolution of the animals. The changes in the skull are mostly associated with the development of increasing bite power, and some of those are visible even in the youngster. But not so many, not least because very young sabre-tooths won't have needed to bite hard; they'll have wanted to suckle from their mothers, just like any other cub.

[Mandrill image from Wikimedia Commons. Skulls from Christiansen 2012, released under the Creative Commons license].

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