News in Brief #6

Japanese macaques

Baby Monkeys are Cute

For we humans, it's fairly easy to estimate how old another human is just by looking at them. It may not work perfectly, but we can instinctively assign someone to at least to a broad age class - young adult, middle-aged, and so on. There are plenty of clues to help us to do this, and it's a useful ability for shaping our interactions with one another, so it's not too surprising that other social animals can do the same thing. Perhaps one of the more striking examples is our reaction to the faces of our young.

Very young humans have a constellation of features that highlight their age: rounded face, large eyes in comparison to the face, and so on. The same set of features are found in other baby mammals, and were described by the great behavioural biologist Konrad Lorenz as "kindenschema". Perhaps a more everyday term for the same phenomenon is "cuteness". Cuteness triggers positive, maternal and protective reactions in humans, and its by no means restricted to our own species. Kittens and puppy dogs are undeniably also "cute" to the vast majority of humans and its for the same reasons. We can see the same phenomenon in the looks of toys such as teddy bears, the kawaii appearance of characters in Japanese cartoons, and so on.

If humans find puppies cute, do other species? Well, they didn't test it with puppies, but Anna Sato and co-workers, from the Primate Research Institute in Kyoto, did try something fairly similar. Their test subjects were Japanese macaques (Macaca fuscata), and they used a technique that's also common in research into the behaviour and thought processes of human babies. Essentially, they show the monkey (or baby) a picture of something, get them used to it and then show them a picture of something else. If the second image is of a different kind of thing than the first, the subject stares at it for longer, and otherwise reacts to the change more than they would if it was of the same kind of thing - without actually being the same. This enables us to tell how the subject understands the concept of 'kind of thing'.

In this study, the macaques were shown images of other members of their species, some of them old, and some of them young. They were, as expected, able to tell the difference, reacting more when a picture of an adult was replaced with one of a youngster than when it was replaced with one of a different adult. So far, so good. But then, they replaced the Japanese macaques with Campbell's monkeys (Cercopithecus campbelli), showing them the same pictures - that is, the ones of Japanese macaques. This time, they didn't react in the same way, and it seems that they just lumped all the pictures together as "strange-looking monkey", not really thinking of them as "old" or "young".

But here's the thing: while they didn't react in the same way to the change of photographs, given the choice, they preferred to look at the pictures of baby macaques. The researchers interpret this as meaning that, even if they just saw the macaques as "other species", they still found the baby ones "cute". They could tell they weren't looking at a baby of their own species, but still, it was a baby, and, like all baby monkeys, it was cute, and who doesn't like to look at cute things?

Don't Stare at the Angry Monkey!

In another paper, Emily Bethel and co-workers used a similar technique to gauge how rhesus macaques (Macaca mulatta) reacted to pictures of members of their own species looking angry. Previous studies have shown that we can reasonably expect the monkeys to stare at the angry faces more than they would at peaceful ones, presumably on the grounds that they want to be sure nobody's about to try and beat them up. But, in this study - and, apparently, it's the first time this has been tried - they tried the same test on monkeys that were stressed out because they'd just had a medical check, and on monkeys that were happy because they'd been living in a fun new environment recently, with lots of things to play with and explore.

The stressed monkeys were still wary of the aggressive-looking pictures, but they reacted in quite a different way. Instead of staring at the image, they looked away as soon as they could, apparently trying to appear submissive and not draw attention to themselves ("don't stare at the angry monkey - he won't like it"). That this happens shows that there's a lot more going on inside the heads of monkeys than simple behavioural theories might suggest, and it may well have relevance for how monkeys are looked after, as well as the possible origins of things like anxiety disorder in humans.

Brainy Carnivores

On the subject of brains, one of the reasons we humans evolved especially large ones is thought to be due to our complex social lives. Humans are capable of understanding far more complex social relationships than any other species, and the drive to do so may have led to the evolution of our brains. Similarly, chimpanzees, for example, are capable of more complex social understanding than, say, lemurs, and also have larger brains than they do.

There are, of course, all sorts of other reasons one mammal might have a larger brain than another. For a start, a rhino is obviously going to have a larger brain than a mouse, but even once one adjusts for overall body size, there may be factors such as the complexity of the environment that they live in to take into account. Brain size may not even be directly related to the need for greater 'intelligence'; there may be other evolutionary factors at play. Nonetheless, studies on monkeys do seem to show that social complexity is very important as a predictor of brain size (and vice versa - it inevitably cuts both ways).

A number of studies back this up. Thing is, they're almost all on primates. Does the same hold true for other mammals?

Eli Swanson and colleagues recently published the results of a similar study, not on primates, but on carnivorans. They used CT scans to examine the shape and size of the brain cavities in the skulls of 36 different species of carnivore, from raccoons and meerkats to lions and tigers and bears (oh my). It's not really the first study on this, especially where size alone, rather than shape, is considered, but it may well be the most thorough to date. Do social carnivores, such as lions, have larger brains than solitary ones, such as wolverines?

In short, "no". But it's actually a bit more complicated than that.

The authors didn't find a correlation between brain size and complex social lives. Bears, for example, which mostly live on their own, have quite large brains for their size, and so do members of the weasel family. However, social animals did tend to have a larger cerebrum, relative to the rest of their brains. The cerebrum is the part of the brain that's involved with more complex behaviour and sensory analysis, rather than, say, running the body's internal workings, and it's exceptionally large in humans - the main reason why our brains are larger than those of chimps. So, while the brain itself may not be bigger overall, one particular part of it is.

However, overall brain size did correlate with what they ate. Meat-eating carnivorans did have larger brains compared with those with more omnivorous diets. Probably it's harder to catch a fleeing antelope than it is to eat some berries off a tree, and you may get more calories that way too, giving you energy that you can use to fuel a larger brain. The size of the frontal cortex - the part of the brain that, in humans, is responsible for thought and higher cognition - was also larger in animals that regularly travelled over larger territories, and in animals that had to use their paws more to manipulate food.

For a mammalian carnivore then, while social life is relevant to brain size, what, where, and how you eat is at least as important.

New Species Reported

While the most dramatic discovery of a new mammalian species recently is that of the lesula, a new kind of guenon monkey, it's far from the only one in the last few months. Peter Taylor and co-workers, for example, have reported the discovery of four new species of bat in southern Africa.

They had been looking at an animal called Hildebrandt's horseshoe bat (Rhinolophus hildebrandti), a relatively large bat that lives from Ethiopia, across eastern Africa, and down all the way to South Africa. They had wondered why it was that different populations of this species, although apparently identical in other respects, used noticeably different echolocation calls. Genetic analysis and detailed physical examination of specimens revealed the answer: the bats represent at least two different species.

In fact, by the author's estimation, there are five. The differences between the other three are slight, though, and it's always possible that they won't stand the test of time, and will eventually be demoted to subspecies status. One of the species is, of course, the one we already knew about, which leaves up to four new ones to be described. The one that's most clearly a species is now the Mozambiquan horseshoe bat (Rhinolophus mossambicus), and seems to have diverged from its relatives during the late Pliocene. The other three are much closer to the genuine Hildebrandt's bat, and probably diverged during the Pleistocene, as the colder climate broke up the previously monotonous savannah habitat.

The authors note that there may even be some examples of insular gigantism going on here, with individuals isolated in higher altitude mountains becoming larger, in the absence of competition, than their lowland kin. This may have been what drove the evolution of the different species, and the different sizes alone may help to explain why the echolocation calls are different - larger bats make deeper calls.

It's not just bats, either. In 2006, Shin-Ichiro Kawada and colleagues were researching moles in Vietnam when a local guide informed them of a particularly small kind of mole living at the foot of the Tam Dao mountains that they might be interested in. It was another two years before they had the chance to catch any, and another four before their results could be formally published.

In short, they had found an entirely new species, a relatively small and slender mole with a tail so short that the authors describe it as "resembling a wart". It lives, we now know, in deciduous forest among the limestone foothills of northern Vietnam. It has been named as Euroscaptor subanura, but does not yet have a common name in English. Moles are the sort of animals that you might well overlook, since they spend so much time underground, but it's interesting to note that the tail alone makes this one distinguishable from it kin once you manage to get a close look at it.

[Photo by Tony Hisgett, from Flickr]