Cultural Traditions of the Spider Monkey

One of the distinguishing features of humanity is the great cultural variation that we exhibit across the globe. The traditions and practices of, say, the Masai of Africa, are quite different from those of western Europeans. In animals, by contrast, behaviour tends to be firmly dictated by genetics. Even where there is clear learning, with mothers teaching their offspring to hunt, for example, the end results of that learning are often fairly similar across a given species.

But, as is so often the case, the dividing line between humans and our closest relatives is not so clear as that broad generalisation would imply. Traditions have been observed in other animals, even including some that aren't primates, such as dolphins and killer whales. When we talk about 'traditions' in these animals, we are referring to behaviours exhibited by some populations of a species that are, for no obvious reason, not seen among other populations. The behaviours are obviously not genetic, or everyone would perform them, but on the other hand, neither are they simply the result of some lucky discovery by a single individual that was never passed on to their peers.

Most research on such traditions has been carried out on chimpanzees, and has tended to focus on the way that they use tools. This has obvious relevance to our own origins, as well as having the advantage of being easily identifiable. Chimpanzees do seem to learn at least some of their behaviour by copying others in their local cultural environment, and this sort of learning may extend quite a way back in our evolutionary family tree, since such tool-using cultural traditions have also been seen in orangutans.

But tools do nor represent the entirety of culture. To look further back into the origins of traditions, it may be useful to look at our more distant relatives, and to consider behaviour that is more relevant to the way they live their lives. Geoffroy's spider monkey (Ateles geoffroyi) is native to the Americas, placing it firmly on a branch of monkey evolution entirely separate from that which eventually gave rise to humans in Africa. They appear to be remarkably intelligent animals - perhaps even more so than gorillas - but they don't have opposable thumbs, and since the main thing they want to do in life is pick fruit off trees, they really don't need much in the way of tools, anyway.



 Spider        Woolly      Howler
 monkeys       monkeys     monkeys
    ^             ^           ^
    |             |           |
    |             |           |      Other American
    ---------------           |         monkeys
           |                  |            ^
           |                  |            |
           --------------------            |       Apes & Old
                    |                      |      World monkeys
                    |                      |            ^
                    ------------------------            |
                               |                        |
                               |                        |
                               --------------------------
                                           |
                                           |

They also have a relatively unusual social structure among mammals. They live in groups, but individuals often wander about different groups, and the groups themselves are always merging and splitting. In contrast, most group-living mammals tend to stick to herds where the females stay with their relatives and the males wander off to find and dominate new groups elsewhere, and then stick with them. The more complex arrangement among spider monkeys means that it will be relatively common for individuals to encounter past 'friends' that they haven't seen in a long time, and perhaps to develop their own greeting rituals for that purpose. It's worth noting that, although they belong to a different part of the primate family tree, chimpanzees are among the few other mammals to live like this (along with humans, of course).

It can be difficult to demonstrate that particular behaviours are not genetic in origin, since even within a species, there could well be genetic differences we are not aware of. However, the usual way of looking for cultural traditions, as used for chimpanzees and orang utans, is to examine the behaviour of animals in different locations, and see how much it differs. This was recently performed for an array of 62 different types of behaviour in Geoffroy's spider monkeys in Costa Rica, Panama, Belize, and Mexico.

Most of the behaviours turned out not to fit the definition of a 'tradition'. Around 20 were performed by monkeys in all the study sites, making it difficult to rule out these being genetically programmed habits. Others were very rare, suggesting that even if one animal had learned that this was a useful thing to do, it never (or at least, hadn't yet) passed the information on to its fellows. For a few, there were obvious reasons why the behaviour couldn't be carried out just anywhere - for example, its difficult to eat the fruit of cohune palms if there aren't any where you live.

But that still left 22 behaviours that just didn't fit the obvious pattern. Some of these were related to the choice of foods. In two sites, almost all the monkeys were seen happily eating the fruit of the elephant ear tree (which, despite being a tree, is also a really big legume). Yet, in the others, they never touched the plant, apparently not realising it was edible, even though there were plenty around. One Costa Rican population had even figured out that they could eat mushrooms, something not normally associated with this fruit-eating species.

Other behaviours, however, were more complex. In some areas, monkeys would climb onto the shoulders of their fellows in piggy-back style in order to appear larger and more threatening to an intruder, while in others it never seemed to occur to them to do this. The monkeys in Panama were often seen to walk upright on their hind legs, something that the monkeys in Belize never did, and was quite rare elsewhere.

As predicted, many of these more social behaviours were related with greeting outsiders, perhaps to determine whether they were long lost friends who shared the same traditions. These included various forms of scent marking, and whether or not they rubbed fig roots on their bodies (presumably to give themselves a distinctive smell). Although all of the groups greeted outsiders in the usual manner for their species - which includes a lot of embracing each other - some of them also kissed each other lightly on the cheeks. Yet the Panamanian monkeys, while they would blow kisses at each other from a distance, never actually kissed each other directly.

The same Costa Rican population that ate mushrooms also had an unusual habit of sometimes climbing to the tops of tall trees and facing into the wind with their mouths open. Presumably this felt pleasant, and probably helped them cool down, but again, its significant that none of the monkeys elsewhere did this, despite the obvious opportunity to do so.

All in all, there was significant variation among the different populations in terms of which practices they performed and which they didn't. Every population had at least some unusual practice that at least one other didn't possess. It's hard, as I mentioned earlier, to be sure that none of these are due to some minor genetic difference, especially since the communities are isolated from each other by the increasing fragmentation of Central American jungles (which has led, sadly, to the species becoming endangered). But that also reduces the chance for cultural communication across large distances, allowing particular traditions to develop and survive in isolated areas. That there was no pattern to which behaviours were exhibited where also tends to count against the genetic theory. If that were the case, then, since one might expect monkeys closer together geographically to be more related, one would also expect them to have more habits in common, and they don't.

Culture and learning are things that humans have developed far beyond any other species on Earth. But we are not entirely separate from the rest of the animal kingdom. The brains of our closest relatives are not simply hard-wired with everything they need to know, and sometimes animals can discover a way of doing something that other members of their species elsewhere have not. When they do, at least some of the time, they can pass this information on to their relatives and neighbours, something that requires the sort of cognitive powers that we tend to think are purely human.

Humans are animals, and the line between us and 'them' is sometimes a very blurred one.

[Picture from Wikimedia Commons]

The long-nosed herbivores of Argentina

Macrauchenia patagonica, the last member of its family

Today, there are two main groups of large, herbivorous hoofed mammals; one that includes those with cloven hooves, and one that includes the horses. But that hasn't always been the case.

Millions of years ago, South America was an island continent, as separated from the rest of the world as Australia is today. It separated from the other continents before either of the living groups of hoofed mammals had evolved, but the creatures that would give rise to those groups - a rather vaguely defined group of primitive herbivores called the condylarths - did exist, and were already present. In their isolation, these animals evolved into a whole array of large herbivores, creating a rich, and to modern eyes, rather strange-looking, mammalian fauna.

When North America finally hit its southern neighbour, it brought with it large mammalian carnivores, which had previously been absent. Every single one of these strange creatures has since died out - most of them not long after the arrival of sabretooths and jaguars, although at least one species probably survived long enough to be wiped out by humans. Yet, for millions of years, far longer than the paltry period of time since their disappearance, these were among the dominant mammals of their day.

There were many families, varying from each other at least as much as horses do from antelopes, but one of the more successful ones was that of the macraucheniids. A look at even this highly simplified version of their family tree reveals just how distant they were from living groups, for everything at all close to them is also extinct:

Macraucheniids    Adianthids     Prototheres

      ^               ^              ^

      |               |              |

      |               |              |

      -----------------              |

              |                      |            Notoungulates

              |                      |                 etc.

              ------------------------                  ^

                          |                             |

                          |                             |

                          -------------------------------

                                         |

                                         | 

The macrauchaeniids looked much like llamas in their bodily proportions, which makes sense, as they lived in a similar environment. (The llamas themselves, of course, arrived only when North America did). Like the living hoofed mammals, they had a reduced number of toes. In their case, they had three, with the central one being the largest and bearing the most weight. This is somewhat similar to modern rhinoceroses, although the feet of most macrauchaeniids would have been more slender, closer to those three-toed animals from which horses evolved. On the other hand, while all the living species of large herbivore have lost many of their teeth, keeping only those best suited to chewing plants, some of the macrauchaeniids had the full set of 44, although these were clearly adapted for a herbivorous diet.

One of the more distinctive features of the group is the position of their nostrils, which are far up on their skull, not at the tip of the snout, where one would expect to find them. This position became more extreme as the family evolved, so that the bones that would normally form the top of the snout (and in humans form the bridge of the nose) are remarkably short. It has generally been assumed that this means they had a short, flexible trunk, running forward from the nostrils over the face. A very similar pattern is, after all, seen in the skulls of living tapirs, which do have a proboscis of exactly this sort.

Because such a trunk, not being made of bone, has never been found in any fossils, its hard to know for sure that it really existed. It seems likely, but there are at least some other possibilities. For example, nostrils placed far back on the head make it easier for an animal to swim without drowning, which would be useful if they lived in a particularly wet environment.

Although the oldest fossil macrauchaeniids go back further still, it seems that the family really began to take off and diversify between about 29 and 21 million years ago. Yet most of what we know about them comes from more recent specimens, typically less than about 20 million years old, and in particular, from Macrauchaenia itself, for which the group is named. Macrauchaenia is undoubtedly a fascinating animal, and, at around five feet high at the shoulder, one of the larger members of its family. But, having died out just 20,000 years ago, one thing it doesn't tell us is how the family came to be in the first place.

The earlier forms, from the little we know of them, were less specialised than their descendants later became. For example, the nostrils were in a less extreme position, which suggests that the trunk, if present, would have been very short, and resembling more of an enlarged nose than anything else. As is often the case, however, the fossils that we do have are frustratingly incomplete. Its actually quite unusual to have anything approaching a complete skeleton in any fossil except the most recent ones, and the macrauchaeniids are no exception.

In this context, last November an Argentinian fossil was described that, for the first time, includes both a complete skull and some other parts of the skeleton belonging to a macrauchaeniid that dates from the time that we think the family was really beginning to diversify. Skulls are good for identifying fossil mammals, since they tend to be much more distinctive than other bits of the skeleton, although at least one early macrauchaeniid species has been described based on the discovery of part of a foot and the associated ankle. (This may not sound like much, but its frequently all there is to work with, and, at least in this case, the peculiar three-toed feet are fairly identifiable).

This new fossil belongs to a species that's already known, Cramauchenia normalis, although it hadn't been previously known to have existed so early. When I say that there are some other parts of the skeleton that can be matched with the skull, its still not a lot - the right humerus, a small bit of the left foot, and one toe. But that's actually quite a lot more than you often get, and its worth noting that when the species was originally described by Florentino Ameghino back in 1902, all he had to work on was half a crushed skull.

This skull, however, is relatively well preserved. We can tell, for example, that there is nowhere for sizeable muscles to attach to the area around the nostrils. Which means that, whatever the case might be for later species, this one probably didn't have a trunk. The shape of the skull and teeth also tell us that the animal was a browser, feeding on relatively soft leaves and shrubs, rather than tough grasses. While we may not have a complete limb, the shape of the bones we do have indicates that the animal would probably have been good at running, enabling it to escape quickly from predators.

What predators, you might wonder, given that I've already mentioned this animal lived long before the arrival of large mammalian carnivores. Its true that it would never have had to face large cats, wolves, bears, or any of the other creatures that we normally think of as preying on, for example, deer or antelopes - and, with a skull 27cm long, that's roughly the size range we're thinking of. But, strange as some of the mammals of this time might have been, the dominant South American predators were perhaps stranger still.

For it was not lions or sabretooths that Cramauchenia would have needed to flee from, but giant, flightless, terror birds. South America has changed a lot since those days.

[Picture from Wikimedia Commons]

Sons or daughters?

Tammar wallaby, with a joey

In many animal species, there are more females born than males, or vice versa. Indeed, at the extremes, there are species that have done away with males altogether. Things don't tend to be quite so stark when we get to mammals, but its still the case that one sex can be noticeably more common than the other. With adults, it could be the case that one sex dies off more quickly than the other, leaving the other sex more common in the population. But the fact is, we see this uneven distribution of sexes even among young animals. This isn't necessarily hard-wired into the species; it can change depending on the environment. At some times, mothers consistently produce more sons, and at other times, more daughters - indicating that they have some sort of "choice" in the matter.

There are several reasons why this might be useful, but, for the moment, lets just look at two of the more popular ones. The Trivers-Willard hypothesis applies to species where males normally mate with several females, and states that healthy mothers should tend to have more sons than unhealthy ones. The assumption is that less fit mothers will also tend to have less fit offspring, and that they want to have as many grandchildren as possible. For female offspring that's not much of an issue; they're going to mate with somebody at some point anyway.

But for males offspring, it makes quite a difference - if male offspring aren't fit, they won't get the chance to mate, because the bigger males grab all the females first. So, if you're healthy, have lots of sons, because they will give you lots of grandchildren, but if you're not so fit, concentrate on daughters, because your sons won't have much luck. Its worth noting that 'healthy' in this context, doesn't necessarily refer to disease (although it could), but to anything that affects the chances of males mating - genes for larger antlers, perhaps, or for a more sexually dominant attitude.

The "local resource competition" hypothesis, on the other hand, suggests that mothers living in areas of dense population should have more male offspring than those living elsewhere. This is because, in most mammal species, males tend to wander off in search of females when they reach adulthood, while the females stay around in the area they were born - and, as a result, ensure they don't end up mating with their brothers. (There are a few species where the opposite happens, in which case this theory predicts that they will want female offspring when the population is high). The reason for this is that there is only so much food to go round - if you have lots of daughters, they will hang around and leave less food for you. If the population is high and food is scarce, you want sons that will wander off and find less inhabited places where they aren't pestering you, but, if not, you can afford lots of daughters.

There's no particular reason, of course, why both of these explanations can't be true at the same time, which can make it very difficult to tell what's going on in a particular species.

But all this raises the question of exactly how you ensure you have more children of one sex than the other. Perhaps the most obvious reason why you wouldn't expect this in mammals is that whether your child is male or female depends on their genetics. Males have the XY chromosome pattern, and females the XX, and the two should be produced in reasonably equal numbers. (It's worth noting, incidentally, that the XX/XY system is a peculiarly mammalian thing and isn't true, for example, of reptiles). There is some evidence that rhinoceroses, among others, might be able to influence the survival of embryos in the womb, although the details are not fully clear.

Another problem is that, compared with other animals, mammals spend a lot of time looking after their offspring. Even by the time you've finished being pregnant, and discovered that your offspring isn't the sex you wanted, you've already put a lot of biological resources into raising them, and its a bit of a waste to abandon them now. On the other hand, mammals do at least have the option that they could do that if they really wanted, by giving the offspring less milk. That won't work for mammals that give birth in litters, but it might for those that only have one at a time. In fact, a study conducted on tammar wallabies last year did seem to demonstrate this. The researchers swapped baby wallabies between different mothers, and found that mothers who had given birth to sons did a better job of raising their foster children, even when the foster children were (unbeknownst to the wallaby) female.

Indeed, it has recently been suggested that marsupials are exactly where we should look for sex selection in mammals. Although the authors give a number of reasons for this, perhaps the most apparent is that pregnancy is really very brief in marsupials. That means that, compared with placental mammals, putting less effort into raising offspring after birth wouldn't be quite such a waste of energy. The authors point out that strong biases, mostly in favour of males, do occur in many marsupial species, to the extent that this can be a nuisance for anyone trying to breed them in captivity. A particularly interesting study from 2009, supporting the Trivers-Willard hypothesis, showed that Tasmanian devils infected with a nasty face-eating disease had more daughters than those that were healthy.

Most mammal species do seem to produce roughly equal numbers of sons and daughters, as humans do. But with those that don't, being able to understand how they manage it could have important implications for preserving endangered populations.

[Picture from Wikimedia Commons]

Wolf or Jackal?

This one really is a golden jackal

A species, you may have been told, is a group of animals that can breed with each other, but cannot breed with other species to produce fertile offspring. So wolves and domestic dogs are the same species, because they can interbreed, while horses and donkeys are not, because mules are infertile. There are a lot of problems with this idea, because it turns out that nature isn't really all that neat and tidy. For example, polar bears are quite clearly not the same thing as grizzly bears, but the two can produce fertile offspring - something that is happening increasingly as the polar ice caps melt and the natural habitat of the polar bears shrinks.

Its also not at all obvious how you're supposed to apply this definition to animals that don't reproduce sexually (though that's obviously not a problem with mammals). And what about those cases where two species have separated so recently in the evolutionary past that some individuals can crossbreed, but others can't? The classic example here are the herring gulls and lesser black-backed gulls, where the Siberian populations of these two species can interbreed, but the British ones can't. It really does get messy very quickly.

Even if it wasn't for all this, if we really did use the fertile interbreeding definition, how useful would it be? Well, we would have to accept that polar bears are really just odd-looking brown bears, for a start. But more than that, what about other species? Are we really going to try and cross-breed every plausible combination to see what happens? It can be hard enough to get some animals to breed in captivity at the best of times, making the whole idea completely impractical.

Inevitably, this means you sometimes have to end up applying some common sense, and describing something as a separate species largely because it's useful to do so. So polar bears, for example, are considered a species, because they are different in so many ways from brown bears. At the opposite end of the scale, there are many species that don't generally interbreed, but are so closely related that they look almost exactly the same, making it really hard to tell where one species starts and another ends. A lot of these are mice of various kinds, but it's also true of dogs.

The wolf (Canis lupus) is a very widespread species. Before humans drove it off from many parts of its range, it was found throughout Europe, and in most parts of Asia and North America. Human intervention brought them to Australia, in the form of dingos, and the domestic dog is also considered a subspecies of wolf. They once used to inhabit the southern coast of the Mediterranean, but now wild wolves are completely absent from Africa. Or are they?

The closest living relative of the wolf is the golden jackal (Canis aureus). Golden jackals are found in the Balkans, and across Arabia and southern Asia, but they are also found across northern and eastern Africa as far south as Nigeria. It would seem, therefore, that the golden jackal is more suited to these hot climates than its more northerly cousin, the wolf.

Wolf      Golden     Coyote    Ethiopian
          Jackal                 Wolf
  |          |         |           |
  |          |         |           |
  |          |         |           |         Other
  ------------         -------------        Jackals
       |                     |                 ^
       |                     |                 |
       -----------------------                 |          Dhole
                 |                             |            |
                 |                             |            |
                 -------------------------------            |
                                |                           |
                                |                           |
                                -----------------------------
                                              |
                                              |

Like the wolves, golden jackals are a varied bunch, with several different subspecies recognised. Indeed, one subspecies, found in Egypt, is quite hard to tell apart from a wolf - were it not for the fact that, officially, there are no wolves in Egypt. There had been some dispute in the past as to the exact status of these Egyptian animals, but, more recently, some very wolf-like jackals were also spotted much further south, in Ethiopia. Were they really jackals, as the textbooks said they had to be, or were they something else?

You can't just capture them, breed them with wolves, and see what happens. Not least because we know that wolves can breed with coyotes without any difficulty, and they're certainly a different species, so fertile offspring produced in captivity wouldn't prove anything either way. To see what they really are, we need to look at their genetics. If two groups of animals regularly crossbreed in the wild, their genetics should be very similar, and we have as good a reason as any we can come up with to say that they're really the same species.

So that's what the researchers did. They collected samples of the animals' dung, sequenced their DNA, and compared it with samples taken from jackals in Egypt, with wolves and golden jackals from a range of locations, and, for good measure, from seven other species of the dog family. (Including foxes - when you're doing this sort of test, you need to include something that's sort-of-similar yet manifestly not the same, just to make sure it's working properly).

Of course, they didn't analyse the entire genome, since modern technology isn't up to making that at all routine. Instead, they looked at a particular gene, cytochrome b, which is used as a standard in these sorts of studies. It's presumably not perfect, but with everyone using the same gene, at least it is possible to compare results with those from other workers, and this particular gene has done a good job so far at figuring which mammals are related to which other mammals.

The results showed, firstly, that these Ethiopian jackals were almost identical, genetically, to the ones seen in Egypt. This alone is significant, since it means that this subspecies is found much further south than previously thought. But it also showed that these animals were more closely related to wolves than to other golden jackals. Indeed, in some respects, they were more similar to wolves from places like Canada and Sweden than they were to wolves from the Himalayas.

So, whatever these animals are, they can't possibly be golden jackals. It appears that they diverged from the main group of wolves after the Himalayan wolves did, which means they have to be wolves, too. All the textbooks say there are no wild wolves in Africa, but we now know that there are - at least in Egypt and Ethiopia, where they have been living alongside golden jackals, and looking so similar that few people could tell them apart.

This is exactly the sort of problem that occurs when you have very similar looking species living side by side. Indeed, this sort of finding crops up quite often with mice, with nobody much noticing (or caring, frankly). But even then, is this animal actually a subspecies of wolf, or something else entirely? Just a few years ago, it was argued that Himalayan wolves were themselves a separate species. If that's right, then the Ethiopian animals might well be a species of their own, too - one we didn't previously know about. It's debatable either way, since they do seem to be much closer to northern wolves genetically than we'd expect a new species to be, yet different enough that a blood test will tell them apart.

Does it matter, though? Do we care whether this is a species or a subspecies? Well, funnily, enough, yes it does make quite a difference. Wolves are not an endangered species (and neither are golden jackals), so if these animals are a special sort of wolf, they will get relatively little protection from the various international conventions about such things. But if they're a species, then there's pretty good reason to suppose they might be endangered - there certainly aren't very many of them, and the local farmers do have a habit of killing them to protect their livestock. That's not a problem (from a species conservation perspective) if they're also found elsewhere in the world, but if this is the only place the species is found, then they can get protection under international law.

Considering that we can't come up with a good definition of what a species is, it makes a heck of a lot of difference whether a group of animals is one or not.

[Picture from Wikimedia Commons]

Have Children and Die

For many invertebrates, reproduction is an "all or nothing" affair. They mate once, produce a huge burst of offspring, and then die. Mayflies are a particularly extreme example, but there are a great many others. Vertebrates are almost always different; assuming they don't die for some other reason, most females will give birth more than once during their lifetime. Perhaps the best known exception is the salmon, which struggles its way upriver to its spawning grounds just once before it dies.

The almost complete absence of an "all or nothing" approach to reproduction among mammals means that, for them, raising a litter is always going to be something of a trade-off. The mother has to invest a lot of effort on her young; first she has to nourish them in her womb, and then provide them with milk, before we even consider teaching them to hunt, or whatever else might be needed before they can travel off on their own. But, on the other hand, she can't invest too much energy in them, because then she herself will suffer. A female mammal has to ensure not only that her children reach the point when they can survive on their own, but also that she survives to raise further litters. There's inevitably a balance to be reached between those demands.

But... well, what when there isn't? Animals don't live forever, and the more litters you have, the less likely that you're going to survive to have another one anyway. Something called "terminal selection theory" therefore suggests that mothers should invest more energy in looking after later litters than earlier ones. From an evolutionary perspective, once you've had all the litters you're going to, there isn't much to lose by investing everything you can in the last one.

There doesn't really seem to be a great amount of evidence that mammals actually do this, though. That's partly because it's a fairly complex process, with a lot of confounding factors. It's true that the first litter of many mammal species tends not to do so well as the later ones, but that can often be put down to parental inexperience, or that younger mothers may be smaller and not so fit as they become in later years. Long-lived species, which tends to mean the larger ones, may have several litters throughout their life, making it unclear in advance which one is likely to be the last one. It may be difficult for a mother to control the amount of energy she expends on her developing offspring while she's still pregnant, in contrast to animals that look after eggs. In some social animals, there may even be advantages to surviving into post-reproductive life, passing on your knowledge of the environment to your grandchildren (humans are the most obvious example here, but it also seems to be true, for instance, of elephants).

With all of these reasons to muddy the waters, its not surprising that terminal selection either doesn't happen in mammals, or at least, is very difficult to spot. The best place to find it would be in some animal that doesn't live very long, doesn't have to do anything much once its finished weaning its offspring, and, ideally, has more control than usual over how much energy it expends on its young. Enter the brown antechinus (Antechinus stuartii).

You might think, from the picture, that this is some sort of mouse, but it's not. Instead, its one of several species of small marsupial that happen to look remarkably mouse-like. It feeds on insects and other small invertebrates, and belongs to the same family as most other marsupial carnivores.

The fact that antechinuses are marsupials is important, because marsupials spend very little time actually pregnant. Instead, most of the development of the young takes place in the pouch, with the mother supplying them with milk, something that may well be easier for her to control than nutrition in the womb. The brown antechinus (there are also nine other species) is unusual among mammals in that it doesn't give birth to many litters during its life. Indeed, most individuals really do breed only once. Unlike salmon, however, it is at least possible for them to survive to breed again, and a significant minority of females do manage a second litter. The males, incidentally, seem to have no such luck; they get to enjoy a single three week mating season, and then simply die off, their life's ambition completed.

Even in placental animals, the maximum size of a litter is usually dictated by the number of available teats, and this is even more so in marsupials, where the young are physically attached to the teat for much of their development. In the case of the brown antechinus, that maximum is eight (or occasionally nine - mammals of a given species don't always have the exact same number of teats). Unsurprisingly, in an animal that only breeds once or twice, the litters are usually pretty close to that maximum size, and that's a significant cost for the mother. By the time the young are weaned, they're already over half the weight of the mother - which means, when you consider how many of them there are, that, for at least some time, the mother is providing milk for offspring whose combined weight may be four or five times her own. A fairly considerable commitment by anyone's standards, so its perhaps unsurprising that most mothers die (presumably from exhaustion) as soon as the young start fending for themselves.

A study published last week in PLoS One compared the litters of brown antechinuses that bred only once with the first and second litters of those that bred twice. One of the first things that was apparent was that the young of mothers that bred only once grew significantly faster than those in the first litter of mothers who later went on to breed again. That could be due to the second group of mothers being less fit, and not able to provide as much milk for their offspring, but that seems unlikely, since it wasn't possible to predict in advance, just from the mother's weight which group she would fall into. Instead, it seems that those mothers who went on to have two litters had exhausted themselves less with the first one, increasing their own chances of survival.

As one might expect, all the mothers put on weight as they became pregnant, and generally kept this weight up as long as the young were in the pouch. Those mothers having a second litter put on less weight, but only because, being older, they started out rather heavier. Once the young really began to grow, however, but before they were fully weaned, the mothers all dramatically lost weight, evidently diverting all of their resources to milk production. The "terminal selection" theory would predict that mothers having their second litter would expend far more energy the second time around, there being no chance for a third anyway, and this was exactly what happened. These mothers lost something like a third of their body weight in a little over a month, and then promptly died once their young were weaned.

That that sudden death wasn't due to old age was demonstrated by one individual that failed to breed in her second year. Reaching the grand old age of three, she did eventually manage to breed the following year, and then demonstrated the same pattern of putting dramatic effort into raising her second litter, and dying once they were safe.

Perhaps surprisingly, the mothers that had two litters had no more children reach adulthood than the great majority, who had only one. The relative lack of investment in their first litter meant that those offspring grew more slowly, and were more likely to die, while the second litters tended to be slightly smaller. All in all, it doesn't seem that, in this species, there is much advantage in having a second litter, which is probably why around 80% of mothers don't bother.

Its worth noting that one of the few other studies to show this effect in a mammal was also on a marsupial - in that case, a brushtail possum. Marsupials, it seems, find it easier to control the amount of milk they give to their offspring than placental mammals. Additionally, young placental mammals that are still in the womb have a surprising amount of control over their mother's metabolism, demanding nutrients for themselves without her being able to do much about it. That's a problem marsupials don't have to face, and may explain why the very few mammals that do have an "all or nothing" approach to raising young are all short-lived marsupials.


[Picture from Wikimedia Commons]

The Origin of the Cloven Hoof

Foot of a giraffe

The shape of the feet in mammals falls into one of three categories (ignoring those that don't really have feet, such as dolphins). Plantigrade animals, such as humans, walk on the soles of the feet, with both the heel and the toes touching the ground. This might seem the obvious way of doing things, but its actually fairly unusual among large mammals. More common is the digitigrade stance, which we can see in, for example, cats and dogs. Here, the animal effectively walks on tip-toe, with the ankle held well clear of the ground. The part of the foot in between the ankle and the balls of the feet is generally elongated and narrow, so that the ankle looks rather like a backward-pointing knee.

This pattern is thought to allow the animals to move more quickly, making the limbs longer and more flexible, with an extra joint that can bend in the direction of motion. This is usually more important in the hindlimbs than the forelimbs, because they are the ones that push against the ground to propel the animal forwards, while the forelimbs are more important for braking. As a result, there are a few animals, such as raccoons, that have evolved digitigrade hindlimbs, but never got around to doing the same with the forelimbs.

The third possible stance is called unguligrade. These animals go even further than the digitigrade ones, standing only on the very tips of their toes, like a balancing ballerina. Such animals are usually hoofed, and an obvious example is the horse. Yet, while the evolution of horses and their single hooves has been described in many places, the origin of the cloven hoof is perhaps less well known. This, despite the fact that are far more animals with cloven hooves than there are species of modern horse. But then, for the same reason, the story is also rather more compliacted.

The group that these animals belong to (the "even-toed ungulates") includes ten families, although, admittedly, not all of them actually have hooves. These families can be placed into five broader groups, as follows:

  Other cloven    Chevrotains     Hippos
 hoofed animals                   etc.
      ^               ^            ^         Pigs
      |               |            |         etc.
      2               |            |          ^       Camels
      -----------------            |          |        etc.
              |                    |          |         ^
              4                    |          |         |
              ----------------------          |         |
                         |                    4         2
                         |                    |         |
                         ----------------------         |
                                    |                   4
                                    |                   |
                                    ---------------------
                                              |
                                              |

When it comes to fossil animals, it's often difficult to determine exactly how they walked, since skeletons are rarely found in a perfectly articulated form. When they are mounted for museums, a fair degree of guesswork has to go into how the bones all fit together, and sometimes those guesses are going to be wrong. Fortunately, with mammals, unlike older and arguably stranger, fossils, like those of dinosaurs, its possible to compare the animals with living forms that look fairly similar. Comparing such things as the proportions and shapes of the limbs can reveal a lot of information about how mammals long vanished from the world moved and lived when they were still alive. An article recently published in the Journal of Vertebrate Palaeontology used this approach to examine some of the details of how the cloven hoof evolved.

Like the ancestors of horses, the ancestors of cloven-hoofed animals originally had five toes. Among their living relatives today, this is only true of the two species of hippopotamus, an animal whose large size makes it very different from its distant ancestors. Indeed, the living animals whose feet most resemble those of these early creatures appear to be the dogs.

For the most part, their feet were fairly typical of other early mammals, although the thumb/big toe was already quite small, and some of the bones do have a similar shape to those found in dogs (and, to a lesser extent, cats). It seems likely that, just as dogs are fast running animals because they want to catch their dinner, these early ancestors of the cloven hoofed animals would have been fast running because they wanted to avoid becoming dinner.

If this is right, then the first animals of this group would have lacked many of the distinctive features that the group has today. Their toes probably weren't strong enough to support their body weight without some assistance from the balls of the feet; in other words, they would have been digitigrade rather than resembling modern hoofed animals. They may even have had pads on the soles of their feet, as dogs do, although that's the sort of thing that can be difficult to know for certain.

These animals, then, would already have been quite good at running. The next step was the loss of the thumb/big toe. This was clearly a useful adaptation, because it appears to have happened more than once. We can tell this, because hippos, which still have all five toes, are not the most primitive members of the group. This means that the evolution of a four-toed pattern must have happened at all of the points marked '4' on the diagram above - and this ignores some extinct groups that don't appear to be the direct ancestors of anything around today.

In addition to the disappearance of the thumb, the index finger/2nd toe and little finger/5th toe also become much shorter. At the same time, the other two toes become stronger, able to take more of the weight of the animal, and the muscles that, in most mammals, move the toes apart are replaced by a tough ligament. There are also changes in the structure of the wrist or ankle to accommodate the changing arrangement of muscles.

The most obvious modern example here is the pig, which has a true cloven hoof, but also has two additional toes on each foot that do not reach the ground. So it should be at this stage that fossil mammals evolved from a more typical digitigrade stance to something at least much closer to the unguligrade one. At least one group of extinct mammals, one thought to be related to the ancestors of camels, does quite closely fit this description.

Some other extinct four-toed animals in the group, however, are a little different. Their feet look more like those of their primitive ancestors, with only some of the changes found in their living four-toed counterparts. Their joints are not quite the hinge-like shape found in pigs, although they are certainly more so than in dogs, and some of the other changes also seem less extreme. It would seem that this represents the actual change from digitigrade to an unguligrade stance, a stage reached with the three-toed Mesohippus in the evolution of horses. It seems likely - although far from certain - that these animals still had a foot pad, and not hooves, and that (unlike pigs) they used all four toes to walk.

Its obviously tempting to assume that these animals were the transition between the 5-toed and "more evolved" animals, such as pigs. There's probably some truth in that, but it can't be the full story, because at least some of these groups never evolved into anything else. Perhaps the best known such group, the Protoceratids, survived with more or less this arrangement for around 40 million years - about two thirds of the total timespan since the extinction of the dinosaurs. So its obviously a pretty effective way of moving about, something that works well in its own right, without being just a necessary transition to anything 'better'.

Nonetheless, in three groups, the outer two toes became more radically reduced, leaving only two true toes - the middle finger/3rd toe and the ring finger/4th toe - on each foot. Two of these groups are shown in the tree above, marked with a '2', but the other is extinct, and exactly where they belong really isn't clear.

Quite what happened to the other two toes varies quite a bit between different groups, but in all cases at least some of the bones disappear altogether. For example, in goats, only the very last bone in each toe survives, as a sort of small dewclaw at the back of the foot, while in camels, the toes vanish entirely. At the same time, the limbs became longer, especially between the ankle ("hock") and the foot, allowing a further increase in running speed. The shape of the joints also changed, to allow for the newly vertical posture of the foot, and to create hinge joints more suitable for fast running.

Again, the pattern isn't entirely neat. While there is a lot of similarity between the way that the cloven hoof formed in the three groups, there are also some differences, reflecting the fact that this happened more than once, instead of being a simple linear progression. In the case of camels, for example, the animals lost the unguligrade stance of their ancestors, and went back to being digitigrade, with a foot pad instead of a hoof. Flat splayed feet are, after all, more use in a sandy desert than sharp hooves.


[Picture from Wikimedia Commons]

Bat Swarms of Colorado

When we think of bats, the environment that first springs to mind is that of a cave. While there are a number of bats that roost in trees during the day, a great many are, indeed, cave-dwellers. Because caves aren't all that common in most places, bat colonies within them can be huge, which creates something of a problem when they all try to fly out in the evening. But, if you watched a bat cave throughout a year, in most cases, the activity level would change radically as the seasons progressed.

This is largely because most bats eat insects. In temperate climates, insects aren't around much during the winter; in many cases they've died off and left their eggs to hatch the following spring when the weather has improved. Any animal that relies heavily on eating insects to survive has to deal with this scarcity somehow. Insect eating birds, such as swallows and thrushes, cope with the problem by migrating in the winter, heading to warmer climes where insects are still common. There are some bats that do exactly this - for example, the hoary bat migrates annually between Canada and the southern US - but more commonly, they deal with the absence of food by hibernating.