Pronghorn populations

The antelopes are not a true group of animals; generally speaking, the term just refers to any member of the cattle family that isn't a cow, sheep, or goat. There are many different kinds of antelope, found not only across Africa, but also in Asia.

Under this definition, however, the pronghorn (Antilocapra americana) is not really an antelope. At first glance, this North American animal certainly looks like one, with cloven hooves, horns, and a four-chambered stomach. But a closer look reveals some differences. A look at the cloven hooves of any member of the cattle family reveals that they have four toes: two that form the hoof itself, and two smaller, vestigial ones that don't reach the ground. In the pronghorn, these evolutionary remnants have vanished altogether (although there are still cannon bones within the leg), so that they only have two toes on each foot.

The horns are different, too. In members of the cattle family they are never branched - unlike the antlers of deer - but the male pronghorn, as its name suggests, has a forward-pointing prong on each horn. Furthermore, they shed the outer layer of skin from the horns each year, like deer do, but quite unlike cattle. (Unlike deer, however, they don't shed the horn itself).

All of this places the pronghorn in a family all of its own. While the pronghorn family today consists of just one species, in prehistoric times, it was much larger, representing a branch away from the line that led to true antelopes that crossed over into the Americas, and developed on its own.

Pronghorn     Cattle Family     Deer Family
                                    etc.
    |              ^                 ^
    |              |                 |
    |              |                 |         Camel Family
    ----------------                 |              ^
           |                         |              |
           |                         |              |
           ---------------------------              |
                         |                          |
                         |                          |
                         ----------------------------
                                      |
                                      |


The pronghorn is not an endangered species; it is present in large numbers across a wide swathe of the western parts of the USA, and also into Canada and Mexico in places. This doesn't mean that it's had everything its own way; as with many animals in North America, and elsewhere, it used to be present in much larger numbers in the nineteenth century than it is now. But, if anything, it is more numerous now than it was at the end of that century, due in part to the introduction of such factors as regulated hunting seasons.

However, it can still be useful to look at the various factors that affect the pronghorn population, some of which can be extended to other, similar, animals. One study looked at the population of pronghorn in western Nebraska from 1953 to 1993, to see what conditions favoured the animals, and which didn't. It turns out to be quite a complex story, indicating that conservation isn't always a simple matter of fixing one or two problems and getting the same result everywhere.

One does, incidentally, have to be a bit careful with this sort of retrospective study. Because it's looking back over a long period, there might be all sorts of differences in the way that the individual studies that comprise it were conducted. Indeed, in this case, the 1993 cut-off was chosen because population studies after that date were just too different to be compared with the older ones. But, bearing that in mind, some trends do appear to be visible.

For many animals, the advance of agriculture is generally a bad thing. It disrupts the natural habitat, and leads to competition with livestock if you're a herbivore, or being shot by farmers if you're a carnivore. Pronghorns, however, don't really seem to have this problem. There is no doubt that they are more numerous in the wilder parts of their range, but, at the same time, the presence of nearby agriculture seems to help protect their population from such problems as unexpectedly harsh winters.

There are probably a couple of reasons for this. Importantly, pronghorns and domestic cattle tend to eat different kinds of plants, so that the cows aren't eating all of the pronghorns' food. Indeed, if anything, cattle grazing promotes the growth of herbs and grasses that the pronghorns can then feed on, so that in years when the cattle suffer, the pronghorns also struggle. Agriculture also increases the presence of winter wheat, and of cattle feed such as alfalfa during the harsh winter months, both of which are available for the pronghorns as well as their intended recipients.

However, an interesting thing happens when populations get very small, as happened at times for the pronghorns over the forty years examined by the study. Normally, when populations in a given area get larger, each animal produces less offspring that survive to adulthood. There is, after all, only a certain amount of food to go around, and animals are effectively competing with members of their own herd for survival. As the population shrinks, there is less stress on individual animals to find food, shelter, or other resources, and they produce more offspring.

But if the population shrinks enough, something called the Allee Effect often kicks in. Here, the usual trend of increased reproduction for reduced population density reverses, making the population dip still further. Continue this for long enough and the animal is wiped out in that area - and, if the area affected is big enough, that may even mean extinction. Perhaps the major reason behind the Allee Effect is inbreeding, due to a limited choice of mates.

There may, however, be other reasons as well. In the case of herd animals like the pronghorn, for instance, because the herds themselves will get smaller, less of the animals will be available to keep guard against coyotes while the others are feeding. We can also see this effect in play in one of the world's rarest large mammals, Przewalski's gazelle (Procapra przewalskii), which now lives only in a small area around a single lake north of Tibet (and, which, incidentally, very clearly is threatened by advancing agriculture). Now present in numbers too small to properly guard against the local wolves, they are increasingly seen sidling up to herds of the closely related, and more numerous, Tibetan gazelle, so that at least somebody is on the lookout, even if it isn't a member of their own species.

In the case of the pronghorns of western Nebraska, the population densities did, at times, dip low enough for the Allee Effect to kick in. But, in every case, the population recovered as conditions improved in subsequent years, or the hunting season was temporarily halted. So they're hardly in any danger of extinction yet. But even here, exactly how far a population has to fall before problems arise varies considerably from area to area. In this case, where agriculture is more common, the pronghorn herds could reach far lower population densities before the Allee Effect began to take its toll, but for other animals, it could be quite the reverse. And even where it isn't, it may mean that you have to take conservation action sooner in wild areas than in better farmed ones - quite the opposite of what you'd normally expect.

[Picture from Wikimedia Commons]

Squirrel Masturbation

Male masturbation appears, from a biological point of view, to be a rather bad idea: you're simply wasting sperm that you could use to fertilise mates. For that matter, you're losing some nutrients and water, as well. But, when we look at the admittedly small number of studies into this kind of thing among mammals, we find that it's not just humans who do it, or even just primates. So, why is that?

The obvious answer, from a human perspective, is that they enjoy it. Or, to put it more biological terms, that the animals have a sex drive that compels them to mate with females, and, in the absence of females, they'll take the next best option. It's worth noting that humans are unusual among mammals (although not unique) in that they don't come into heat, and are willing to have sex at pretty much any time. This means that, when it comes to non-human mammals, we would expect them to masturbate more frequently at times when the female is in heat, since that's the sort of thing they find sexually exciting. Secondly, they will do it more often when they can't get at any females, perhaps because bigger males are getting there first.

But is this what really happens? A recent study looked at this, and a number of other possible explanations, to find out just why squirrels masturbate.

Gaming and World Peace: Lemurs Lead the Way

Violence towards outsiders is something observed in most primates. It can range from individual targeted aggression, through cooperate gang attacks in monkeys, to, in the case of one rather extreme species, inventing the machine gun. But, at the same time, primates are social animals, who gather together, and are pretty good at cooperating even when it doesn't involve beating the crap out of somebody else. Where does that come from?

At its very base, the primate evolutionary tree divided into two lineages; one leading to the monkeys (and us), and another consisting of what might be termed "lower primates". That's not a terribly good term, since they have, of course, been evolving for just as long as we have, and are perfectly well adapted to their environment... but it's easier to spell than "strepsirrhine". To understand what the first primates may have been like, its useful to see what this group have in common with our own lineage.

A recent study took the unusual step of analysing playful behaviour in Verreaux's sifaka (Propithecus verreauxi), one of nine species of sifaka. They belong to the woolly lemur family, and, like all lemurs, are found only in Madagascar. Madagascar has been separated from Africa for a long time, and just as Australia preserved its marsupials, this isolation preserved a number of interesting animals no longer found elsewhere, of which the lemurs are just one of the more obvious examples. Indeed, the other members of the woolly lemur family are not themselves particularly well known.

Sifakas      Indri      Other Woolly     "True"
                          Lemurs         Lemurs
   ^           |             ^              ^
   |           |             |              |       All Other
   |           |             |              |        Lemurs
   -------------             |              |           ^
         |                   |              |           |
         |                   |              |           |
         ---------------------              |           |
                   |                        |           |
                   |                        |           |
                   --------------------------           |
                               |                        |
                               |                        |
                               --------------------------
                                           |
                                           | 

Young primates, like those of many other mammals, are often seen playing. Of course, their games don't have defined rules in the ways that many of ours do, but are made up on the spot, as those of young human children often are. Their purpose seems to be to help develop both motor and social skills, and to practice things they will have to do for real as adults. Baby sifakas, unsurprisingly, play quite a lot.

We're not talking about that sort of playing.

No, what was relevant to this study - and has rarely been looked at before in any non-human species - is play among adults. Adult sifakas don't play as much as the young, but they certainly do play with each other from time to time. They're already skilled enough at what they do, so we can't assume that they're doing it for the same reasons as the young. So what is the reason?

The researchers looked at how often the adults played, who they played with, and what sort of games they played. Sifakas were particularly useful for this study, not just because they are good example of a non-simian primate, but because they live in small groups of up to twelve individuals with a somewhat fluid composition. Many other lemurs are either solitary, or live in tight-knit xenophobic groups that rigorously chase off intruders.

The core of the sifaka groups are the females, who, as in many mammal species, tend to stay with their families. Males, on the other hand, wander between groups, enabling them to find mates to whom they are not related. Finding a group at just the right time is important to the males, because the females only come into heat once a year, and then only for three days at a time. Under those circumstances, you'd better make friends with a new female pretty quickly, and hopefully avoid being beaten up by any male friends she already has.

While the presence of males is pretty useful to the females, it's rather a double-edged sword for any males currently in her group. Although having rivals for the affections of a female is obviously a bad thing, the more males are around, the better able the group is to watch out for danger, or for other animals trying to pinch their food supply. And it was the reaction of males to outsiders that turned out to be key.

It seems that adult male sifakas by and large don't play with individuals they already know. Instead, when they play, they do so with males from other groups who come nosing around. This is very different from grooming behaviour, where the adults pet each other and cement their social bonds. Males generally don't groom other males that they don't know, and are noticeably more likely to groom their friends if they think a stranger is watching. Grooming seems to be saying to outsiders "he's my friend, and you're not"; a means of social exclusion, and is therefore obviously different from playing.

The females, on the other hand, didn't seem to care much which males they played with. This was actually a little surprising, since in some primate species, play seems to form part of courtship. But female sifakas, it seems, would much rather their males are good at grooming and scent marking than at playing games.

What was significant however, was what happened after play between males. Whereas normally they chase of intruders, after playing with them for a little while, the males seemed to accept the strangers, and stopped fighting with them. It didn't really matter who started the game - and both strangers and those already in the group were equally likely to do so - once they'd played together, males could accept strangers as "one of them", at least temporarily.

In effect, the games are a sort of "ice-breaker" to get the male sifakas used to each other.

There was also a difference in the type of games they played. On the rare occasions when males chose to play with individuals from their own group, the games consisted of pushing, pulling, gentle wrestling, dangling from branches, and pretending to bite each others' genitals. (This last one may not sound much fun, but apparently it's really entertaining if you're a sifaka). But when they met strangers, the games tended to get rougher, with mock fighting, more vigorous wrestling and so on.

[Although they're rather slow to download, you can view videos of "rough play" here, and of gentler play here]

These mock fights were almost always short, and certainly differed from the outright aggression that males display when driving each other off. By keeping it short, they were probably able to emphasise "this is just a game". If anything, when they did play roughly with individuals they knew, the games went on for longer, presumably because they already trusted each other. But by playing such games with strangers, they were able to get a better idea of the capabilities of the outsider, and perhaps also to tell how committed they could be to future friendship.

So, in short, playing games is a useful social glue among adults, especially as a way of getting to know people you aren't very familiar with. It promotes trust, and reduces costly and pointless aggression. Which has to be a good thing.

[Picture from Wikimedia Commons]

Ground Sloths and the Size of Fossils

The sloths are among the stranger groups of mammals, belonging to a lineage that split off from all other placental mammals probably even before the dinosaurs went extinct. They originated in South America, a continent that was, for a long time, as isolated as Australia is today. Just as marsupials survived in Australia, so a number of strange and early placentals survived in South America (as did some marsupials, come to that). Of course, South America eventually joined itself up to North America, and a great number of odd animals went extinct as more familiar forms crossed the Panama land bridge heading south. The sloths, however, survived - or, at least, some of them did.

Today, there are only six living species of sloth, all of which are tree-dwelling animals. Their ancestors lived not in trees, but on the ground, and many of them were much, much, bigger than those that live today. The last of these giant ground sloths died out remarkably recently, perhaps around 9000 BC, meaning that they must have lived alongside humans for thousands of years.

Perhaps the best known ground sloth is the largest one, Megatherium, which reached the size of an elephant. But there were a number of others, many of which were still quite substantial. In zoological terms, the ground sloths aren't even a real group; this is because they just represent those sloths that didn't happen to head up into the trees, and they aren't all directly related. Instead, the ground sloths form a number of different families, with at least some being more closely related to one or other of the families of living tree sloth than to each other.

Bats and the Cocktail Party Nightmare

After the rodents, bats are the second largest order of mammals. In fact, around one fifth of all mammal species are bats. They inhabit every continent except Antarctica (the presence of encircling seas not having deterred them from reaching Australia as it has most other placental groups), and all but the coldest of habitats. Unless you happen to live in, say, Iceland, there are probably bats of some sort living not too far away from you.

And, yet, we don't know as much about them as we do most other broad types of mammal. They're mostly nocturnal, they fly about where it's difficult to spot them, and they tend to sleep in some pretty inaccessible places. That most people don't find them very cute and cuddly probably doesn't help much.

But, of course, they are pretty cool animals, when you think about it. The whole flight business is fairly remarkable, and has only been achieved on three other occasions in the entire evolutionary history of the animal kingdom. And, of course, there's the whole sonar business.

Seals, Sex, and Sickness

Fur seals are members of the sea lion family, and not, as their name might suggest, the seal family. One of the easiest ways to tell the difference is to check the hind flippers. In sea lions and fur seals, the flippers stick out to the sides (at least when on land), enabling the animal to waddle about on all fours, or even rear up on its hind limbs. True seals, however, are more thoroughly adapted to life in the water, and their hind flippers, while great for swimming, stick out to the rear, making them pretty much useless on land - the animal has to drag itself about using only its front flippers, and cannot walk.

Of course, the two families are related, and share a number of features in common. Some of these are related to the difficulty of breeding in an air-breathing animal that spends almost all of its time in the water. They come ashore just once a year, during which time the females give birth to their pups, and then almost immediately mate again before retreating back to the sea. As a result, gestation almost always lasts just under twelve months, regardless of species, and the animals show a great ability to synchronise their births to the same time of year.

In both groups, the males tend to be much larger than the females, aggressively defending patches of shoreline and dominating a harem of females, once the latter have finished raising their pups and are ready to mate. Their larger size, visible differences from the female (such as a sea lion's mane), and aggressive attitude all require a lot of energy, and are controlled, as in other mammals, by the hormone testosterone.

One might think, therefore, that having more testosterone is an undoubtedly good thing, if you happen to be a male fur seal - you're bigger, sexier, and more likely to be a hit with the females, and overall, end up with more children. But there could be a downside as well - you're more likely to get sick.

A couple of recent studies, published in PLoS ONE and the Australian Journal of Zoology, looked at the breeding tactics of New Zealand fur seals (Arctocephalus forsteri) at a breeding colony near Kaikoura. These are one of eight species of fur seal found in the southern hemisphere, and are actually more closely related to the sea lions than to the single northern hemisphere species.

Southern   Sea Lions    Northern     Walrus    True Seals
Fur Seals               Fur Seal
   ^           ^           |            |          ^
   |           |           |            |          |
   |           |           |            |          |
   -------------           |            |          |
          |                |            |          |
          |                |            |          |
          ------------------            |          |
                  |                     |          |
                  |                     |          |
                  -----------------------          |
                             |                     |
                             |                     |
                             -----------------------
                                        |
                                        |

The breeding males fell into two main groups. Around half of them followed what is, perhaps, the more obvious breeding tactic. They established patches of ground, vigorously defending them from other males, and gathering harems of up to fifteen females each. New Zealand fur seals are, compared with some of their relatives, not especially aggressive - they are more likely to posture, shout, and threaten than to physically attack their rivals, but even so they spent a lot of time showing off their masculinity. These were, as might be expected, the bigger, more muscular males - presumably the sort that the females tend to fancy. And, as a result, they got to have a lot of sex.

However, nearly as many adopted a quite different tactic. They tended to spend only a couple of days at the breeding site, wandering about and looking for a good opportunity to have a quickie while the territorial males weren't looking. They were generally smaller than their counterparts, with less pronounced masculine features, and they spent a lot of time running away. A third, much smaller group, were somewhere in between, sticking to a specific area of the shoreline for a few days at a time, but not establishing long-term territories. Nonetheless, the differences in behaviour between the two main groups were quite clearly defined, rather than just being points at either end of a spectrum.

Interestingly, when it came to doing paternity tests on the pups born the following year, it turned out that about as many of them were fathered by the wandering transients as by the big, masculine males. Certainly (so far as could be determined), the transients had had sex far less often than the territorial males, but it didn't seem to make much difference to their eventual chances of fathering offspring. Which, from an evolutionary perspective, is all that matters.

New Zealand fur seals, it seems, depend on a balance between two different tactics to father offspring, with each tactic having its own strengths and weaknesses.

The researchers also studied the urine and dung of the various males. This has to be collected fresh, which isn't terribly easy when you have large and aggressive males stomping about (they weigh up to 185 kg / 410 lbs). It doesn't help that, since they are spending so much time posturing and so little time hunting at this time of year, they really aren't eating much, and consequently, don't poo very often, either. But, with due Antipodean diligence, the researchers managed to collect enough of the stuff to analyse.

Perhaps unsurprisingly, the big territorial males had much higher levels of testosterone than their wimpier cousins. But all this male hormone sloshing about and making them macho had apparently come at a price - because their dung also contained many more parasites than that of the transients. Roundworms, tapeworms, and flukes were all present, often in relatively high numbers. The price of so much masculinity, it seemed, was that they were also more likely to get sick.

Why would this be? One possibility is that, in order to bulk up their muscle, they had eaten more food before arriving at the beaches than the transients had. Since most of these parasites are passed on in food, that would mean they were more likely to get infected. But another possibility is that it's the testosterone itself that's the problem. Even assuming the parasites don't literally thrive on the hormone, its possible that because the seal's body is diverting energy reserves to building up mass, a thick and manly mane, and so on - not to mention all that exhausting posturing and fighting - they have relatively little left over to run their immune system properly.

The smaller males may not get to have sex as often, but they are healthier for it, and they seem to have just as many kids in the end, anyway. Their tactics only work because the other males are busy defending their harems, so both approaches are needed... but the big males don't get everything their own way, as casual inspection might lead one to think.

[Picture from Wikimedia Commons]

The Intelligence of Chimps

Our closest living relatives are, of course, the chimpanzees. There are, in fact, two species of chimp: the common chimpanzee (Pan troglodytes), and the bonobo (Pan paniscus). The latter is sometimes called a "pygmy chimpanzee", although, frankly, there's not a lot of difference in size between the two species. Both species are equally related to humans, having diverged from a common ancestor less than two million years ago, long after that common ancestor diverged from the line that eventually led to us.

Common
Chimp    Bonobo   Human   Gorillas
  |         |       |
  |         |       |
  -----------       |        ^
       |            |        |
       |            |        |
       --------------        |
             |               |
             |               |
             -----------------
                    |
                    |

Both species live in fairly similar environments in the jungles of tropical Africa, and eat more or less the same kinds of food, so we might expect that their behaviour would also be similar. But that's clearly not the case; common chimps are significantly more aggressive than bonobos, while the latter are renowned for their frequent sexual exploits. Common chimps are also more likely than bonobos to use simple tools to extract food from difficult to reach places.

Do these differences in behaviour reflect real differences in intelligence between the two species? A recent large-scale analysis, published in PLoS ONE, aimed to find out. "Intelligence" is a fairly tricky thing to pin down, even in humans, so "which species is the more intelligent?" wasn't the sort of question that the study could answer. Rather, the researchers tested members of both species on a range of tasks designed to look at different aspects of intelligence. Would they perform the same, or would one species prove better at some tasks than the other?

Of course, the study was not conducted in the wild; the animals in question were orphans raised by humans in ape sanctuaries - their parents, in most cases, presumably having been killed by bushmeat poachers. Nonetheless, they had not previously experienced these kinds of tests, so they would have to solve the problems on their own.

For the most part, there wasn't a great difference. The researchers tested spatial awareness by placing food under cups and then rotating the table, moving the cups about, and so on, to see if the chimps could figure out where the food had ended up. Members of both species got this right a little over two thirds of the time. To test their ability to count and perform simple addition, the experimenters placed differing amounts of food under covers and watched to see which one the chimp went for first. Again, both species got this right about two thirds of the time. (This does not, incidentally, imply actual arithmetic - just that the animals could recognise that, say, six peanuts is more than three).

In tests of communication - could the chimps either understand an experimenter trying to tell them where the food was, or could they themselves indicate to the experimenter where hidden food was located - they still managed to get it right over half the time, but again, there was no difference between the two species.

The most difficult test was one in which the food was placed inside a container that required a relatively complex method to open. The researchers showed the chimp how to get the food, and saw if the animals could successfully copy them. In most cases, they couldn't - and, in previous trials elsewhere, no chimp had ever solved these particular problems on their own - although, interestingly, in both species, the females were far more likely to succeed than the males.

However, in tests designed to see whether the chimps could use clues to find hidden food, or could use tools to obtain food, although both species got it right more often than not, the common chimps were significantly better than the bonobos. Tool use, in particular, is something we particularly associated with human intelligence, so we might think that this makes common chimps the more "human-like" of the two species in this respect.

But, of course, using tools isn't really the be-all and end-all of human intelligence. Our ability to work together as a species relies, in part, on our ability to understand the minds of others and to bond together socially. The final set of tests evaluated the chimps' abilities to grasp concepts such as attention and intentionality. For example, a piece of food would be hidden beneath one of two cups; the chimp could not see which cup it was, but a second human could. When the second human tried (and failed) to grab one of the cups, would the chimp work out that this was because that was where the food was hidden? On this test, the bonobos did better, succeeding just over half the time.

So, given that they scored similarly on all the other tests, it seems plausible that the basic intelligence of the two species isn't all that different. Yet the way that they use that intelligence, or the particular skills that they possess, is different. Common chimps are better at physical tasks and understanding the operation of the physical world. But the peaceful and relatively shy bonobos scored higher on tests of social awareness and the ability to understand the minds of others. Both of these are important aspects of human intelligence. Our closest relatives have evolved along paths parallel to our own, and each, perhaps can show something of how we rose to our present position of power over our planet.

[Pictures from Wikimedia Commons - upper image is a common chimp, lower image is a bonobo (both males)]

Cat Eats Pikachu

We're all familiar with the various species of Big Cat - the lion, tiger, jaguar, and so forth. However, with a few notable exceptions, the smaller wild cats are less well known. This is hardly surprising, since they are pretty good at hiding, and tend to do so at the first sign of an approaching zoologist or natural history camera team. When it comes to those that are about the size of a house cat, the words "shy" and "secretive" often spring to mind when describing them.

In fact, there are actually a large number of such cat species, found in every continent except Australia and Antarctica. Although a lot of them tend to prefer woodland - with undergrowth from which they can pounce on unsuspecting dinner - they have adapted to a surprisingly wide range of habitats, such as mountains, deserts, and swamps. In total, there are 23 recognised species that are at least approximately the size of a house cat.

The one in this picture is Pallas's cat, also called the manul (Octocolobus manul). It lives in central Asia, and can be found from northern Iran as far across as north-eastern China, taking in places like Afghanistan and Turkmenistan on the way. It likes hills and high grasslands rather than forests, using rocks and low bushes as cover while hunting.

Welcome

Welcome to Synapsida. As its title suggests, this blog will be a random collection of musings on the subject of mammalian zoology. It largely consists of whatever scientific paper catches my eye in a particular week, written up so as to be a little more accessible than it might otherwise be. It doesn't claim to be anything original, and I'm not a professional zoologist, so you'll have to take it as it is - a layman's view of the latest mammalogical news.

Pretty much everything mammalian will be on topic here, and I certainly couldn't tell you from one week to the next what I'll be writing about. Having said that, I'll try not to repeat myself too much, and, while there are no guarantees, I'll hope that whatever interests me happens to be something that interests someone else as well. Which may not work, but we'll see.

To begin with, here are some old posts from my other blog, with news from August and September.