Sunday, 28 August 2011

Our Jurassic Mother

Eomaia scansoria - no longer the oldest known eutherian
Today, almost all mammals fall into one of two major groups: the placentals and the marsupials. These two groups represent a fundamental split in the lineage of the mammals, a split that occurred long, long before the dinosaurs died out, when mammals were, for the most part, small shrew-like animals living in the shadow of the great reptiles. But how long ago, exactly?

Since 2002, the oldest known fossil of a possibly placental mammal was that of Eomaia. It is remarkably well preserved, even including impressions of the animal's fur, and dates from 125 million years ago. The oldest known marsupial fossil, Sinodelphys, dates from around the same time and place, and there had been reason to suppose that the two lines separated not long before that.

But just how long ago is that? I have recently been using charts to show the age of various fossils within the Age of Mammals. That entire era has lasted (so far) 65 million years, so to get back to Eomaia we need to head back almost as far again as the whole period covered by the chart. It was a time before such famous dinosaurs as Tyrannosaurus and Triceratops had evolved, and the best known animals of the time are perhaps Iguanodon, Kronosaurus, and Deinonychus (on which the 'raptors of the Jurassic Park films seem to have been based, although they decided to use the cooler sounding name of a rather smaller dinosaur instead).

Sunday, 21 August 2011

Secrets of the Mound-building Mouse

In temperate climates, such as we have in Europe, and in the northern parts of North America, winter can be a difficult time for animals. The weather is cold, there is often snow on the ground, and food is in short supply. Some animals hibernate through the winter, while others struggle on through the harsh weather. Smaller mammals, such as rodents, often store dry food in caches that they can return to when there is nothing fresh available. Squirrels, for example, hide nuts to bide them through hard times. But some rodents go to greater lengths.

Out of the over six hundred members of the mouse family, the steppe mouse (Mus spicilegus) is one of the three closest species to the familiar house mouse (Mus musculus). It is found from the easternmost border of Austria, through Hungary, Romania, and Serbia, and out into the steppe lands of Ukraine and western Russia. They are found primarily in and around agricultural land and orchards, and rather less in the wild grasslands where they presumably originated. Indeed, while the species seems to have diverged from the house mouse around three million years ago, long before modern humans were around, today it seems to rely upon us to create the unnatural habitats in which it thrives best. In this respect, it resembles the house mouse, which only rarely lives outdoors.

Steppe    Macedonian    Algerian     House
Mouse       Mouse        Mouse       Mouse
   |          |           |           |
   |          |           |           |         Asian
   ------------           |           |         Mice
         |                |           |           ^
         |                |           |           |
         ------------------           |           |
                 |                    |           |
                 |                    |           |
                 ----------------------           |
                           |                      |
                           |                      |

As a close relative, it also looks very similar to the house mouse, and the two are very hard to tell apart. So similar in fact, that this created a problem with the scientific name for the animal. The steppe mouse was first formally described by Alexander von Nordmann in 1840, under the name of Mus hortulanus. One of the key things you have to do when describing a new species is find a specimen of your animal, and use that as the holotype - the individual against which all other members of the species will be measured. When somebody else finds a new specimen, in theory what they do is check to make sure it belongs to the same species as the holotype specimen, and, if it is, they know what they've found. Nordmann already knew what a steppe mouse was, so he went out and found one to use as his holotype, wrote it up, and gave it its scientific name.

Only, unfortunately for him, it later turned out that he'd picked up a house mouse by mistake. His scientific name was associated with a specimen of the wrong animal, and had to be stricken from the record. A newer description, from 1882, was used instead, and that author's choice of name, Mus spicilegus, became the official one.

The steppe mouse is also called the "mound-building mouse", and that's because of an unusual habit that is quite different from those of its closest relatives. In the autumn, several mice gather together to collect seeds and other plant materials, and deposit them in a large mound that they then cover with soil. The mounds can be large - a foot or so high, and four or five feet across. Deep in the soil underneath the mound, the mice dig burrows centred around one or two nests, and there they spend the winter.

The burrows are shared between a group of mice. They aren't necessarily siblings, but more of an extended family, with related mothers, but unrelated fathers - effectively cousins related through their aunts. How many mice share each mound is not entirely clear, and may well vary from place to place. Estimates have proved difficult, because whenever researchers try to dig up the mounds to count the mice underneath, the mice tend to run away (who'd have thought?) A combination of having a few exit tunnels some distance away from the mound itself, and the habit of dashing into the tunnels of other nearby mounds while researchers are still chasing them does not help matters. But somewhere between six and twelve seems about typical.

Its natural to assume that the mounds are food caches to allow the mice to survive through the winter, so that they can emerge refreshed in the spring, and start breeding again. But just because they are giant larders doesn't necessarily mean they can't have any other function. Indeed, evolution often works like this, when a structure or habit evolved for one purpose turns out to be useful for something else as well. In this case, for example, it has been suggested that the mounds may also help protect the mice, whether from bad weather or from potential predators, or both.

Péter Szenczi and colleagues of Eötvös University in Budapest wanted to know how effective the mounds were as a shelter from the weather (since, after all, the mice are already digging burrows underground), and how useful they are as a winter food source. They looked at a large number of mounds in two different areas in Hungary, measuring them, analysing what they were made of, and checking to see how good they were at keeping the mice alive through the winter.

Their first finding is, perhaps, something of a surprise. The mounds are made of seeds and other parts from a number of different plants, depending on what was available nearby, with the main components (other than soil) being either barnyard grass or goosefoot, and the remainder being various other, mostly grassy, plants. But when they looked at the animals' dung pellets, right through the winter, that wasn't what they'd been eating. Instead, they preferred amaranth, ragweed, and cocksfoot.

The "obvious" explanation for the mounds, one you'll frequently see online or in books whenever the steppe mouse is discussed, apparently isn't true. The mice are not eating the stored seeds. That their burrows don't extend into the mounds, but remain quite separate in the soil underneath, one or two feet underground, also supports this idea. That's not to say that, if the winter got really bad, they wouldn't eat the seeds in the mounds at all - it would presumably be better than starving - but it doesn't seem to be something they normally do.

So, if the mounds aren't food caches, as most people have generally assumed, what are they for? Why bother to store a huge pile of food and then not eat any of it? Temperature probes and soil analysis provided what seems to be the answer: the soil below the mounds is warmer than that around them, even allowing for the height of the mounds themselves. Moreover, it's also drier, which helps to keep it more comfortable. By digging their burrows under a pile of insulating vegetation that soaks up rain and snow, the mice can huddle in cosy nests, protected from the worst of the winter weather.

It seems to work, too, because when the researchers counted the numbers of mice they found under mounds at the beginning and end of the winter, they came to about the same value. In other words, pretty much all the mice that went into shelter in the autumn were still alive in the spring. Whether the mounds also help protect against predators that might want to dig the mice up is less clear, and we can't really tell how much difference they make without disrupting them to see what happens, but that they go to the trouble of collecting around 140,000 cm3 (five cubic feet) of soil and plant matter would suggest its probably significant.

So, yes, rodents often do cache food to bide them through the winter. But that isn't necessarily their only reason for collecting plants. Because sometimes, the "obvious" answer isn't the right one.

[Image from the Carpathian Basin Digital Collection. Cladogram adapted from DeBry and Sashadri, 2001]

Sunday, 14 August 2011

Sex Without Pregnancy: how female macaques get what they want

Tibetan macaque (Macaca thibetana)
The females of most mammal species have some means of signalling to males that they are sexually fertile, and able to become pregnant. There is often some sort of breeding season, timed so that birth is likely to happen at a point in the year when food is abundant - spring, the rainy season, or whatever. But the cycles of female hormones mean that the female is only actually capable of becoming pregnant for periods of a few days, or even hours, during that season. Essentially, she comes into heat, and the males get excited.

That's the usual pattern. But there are some mammals that don't seem to do this. While they may not bother to mate outside of a mating season, they don't seem to be indicating to males when, within that season, they are likely to become pregnant. But is that really the case?

Among Old World monkeys - the type of monkeys to which we ourselves are most closely related - the females are often sexually promiscuous, mating with a number of different males. Although, in general, it tends to be males that sleep around, so that they can have as many children as possible, that's not relevant to females, who can have only one child (or litter) at a time. There are, however, some possible benefits for female promiscuity, which I have alluded to before, when discussing the common yellow-toothed cavy.

By mating with lots of males, you can increase the chance that the fittest one becomes the father of your children, whether that fitness is indicated by physical bulk or, as in the case of those cavies, sexual prowess. Even so, while they may want to give others a chance, just in case, female Old World monkeys normally have some way of indicating when they are most likely to become pregnant, allowing the largest male to drive off rivals around that time. For example, they may make specific calls, or have visible genital swellings during their peak of fertility.

The Assamese macaque (Macaca assamensis) is one of the exceptions - others include the Tibetan macaque, shown in the picture above. The macaques are a relatively large group of monkeys, found across southern and eastern Asia, and they include such well known species as the rhesus monkey. Perhaps more surprisingly, they also include the Barbary "ape" of north Africa and Gibraltar, although the Assamese macaque of south-east Asia is much more closely related to more typical species in southern India and China.

  Assamese         Rhesus       Lion-tailed
Macaque, etc.   Monkey, etc.   Macaque, etc.
     ^               ^               ^
     |               |               |        Barbary
     |               |               |        Macaque
     -----------------               |           |
             |                       |           |      Baboons,
             |                       |           |        etc.
             -------------------------           |         ^
                         |                       |         |
                         |                       |         |
                         -------------------------         |
                                     |                     |
                                 (Macaques)                |
                                     |                     |

Over the last fifteen to twenty years, there has been some debate over the relationships among the twenty-two species of macaque. However, there does seem to be broad agreement that the Barbary macaque is out on its own (as might be expected, given that it's the only one not found in Asia), and that all the others fall into at least three general groups. I have used here the most up-to-date classification I could find.

The breeding season for Assamese macaques lasts a full four months, from mid October to mid February, resulting in the birth of a single infant after a pregnancy of around 164 days. Although the females do show some degree of genital swelling during the breeding season, this is not noticeably different when they are actually fertile than at any other time during the four month period. Since they don't seem to behave any differently during these times, there does not seem to be any obvious way for the males to tell when the females are fertile.

Of course, just because its not obvious to us, doesn't mean that it isn't obvious to the monkeys concerned. Perhaps, for example, they smell different, or there's something subtle in the way that they act which we humans have missed. Ines Fürtbauer and colleagues, of Göttingen University, recently published a study that took a closer look at the sex lives of female Assamese macaques to find out what they're really doing, and why.

They did this by watching the sexual activity of wild Assamese macaques in Thailand throughout their breeding season, and analysing their dung to find out when the females were fertile - a completely non-invasive way of assessing their hormonal status. Perhaps the first thing that was obvious was that the monkeys were mating quite a lot - pretty much every day, even assuming they never managed to hide from the researchers. Nor did the females much care who they mated with; on average each one mated with at least 80% of the available males at one time or another.

What's more, on a whopping 94% of occasions, the females mated when they were not fertile. Indeed, they carried on mating for at least two months after they became pregnant. Now, like other macaques, these monkeys have a strong social hierarchy, with a few powerful alpha males and a number of younger, smaller, subordinate males. Sometimes these alpha males would monopolise particular females for weeks at a time, protecting them as male mammals often do when they want to ensure any resulting child is going to be their own. The thing is, they were just as likely to do this whether or not the female was fertile, and whether or not they had even the slightest chance of getting her pregnant.

Which would suggest that they really didn't know.

So, if the females aren't giving their partners any clues as to when they're fertile, why not? The answer probably lies in an ugly fact of life as a monkey: infanticide. Dominant males, in particular, are quite likely to kill the offspring of other males to ensure that their own are the ones that survive to adulthood. But you can't do that if you don't know which ones they are. If you've mated with all of the females, and can't tell which ones you got pregnant, any child might be your own.

Infanticide is obviously bad news for the infant, but its also bad news for the mother, who invests a lot of time and energy in raising her young. So it may be in her interest to keep the males in the dark, and sexual promiscuity is one way to do that. That's not to say that there aren't advantages to monogamy too, or to mating only with the fittest and strongest males, but, in this species, that doesn't seem to be the way that the balance has swung. This would be an advantage if, for example, alpha males don't last very long so that they won't be much good at protecting you in the future.

However, while they would mate with just about any male, the females did seem to have preferred partners. That is, 30-40% of their matings would be with just one individual, even if they weren't exactly faithful to them. That these partners were as likely to be subordinate as alpha males would suggest that it's probably the females doing the choosing, although there's no way to know for sure. Again, if the alpha males don't last long, today's subordinate will be tomorrow's alpha, so you aren't necessarily losing much.

Indeed, there may well be some advantage to having a particular "boyfriend", even if you don't stay faithful. Humans, after all, tend to have a preferred partner, and to mate with them even when they won't get pregnant. Aside from the obvious, in evolutionary terms, this creates a social bond that comes in useful when you need to raise your children, and it may be that the Assamese macaques are doing something similar. There is some evidence that males help the females to raise their young, and a strong social bond, engendered at least partly through regular sex, may make them more likely to do that.

Besides, while you might not want to risk killing off the young of other females you've mated with, that of your own partner has at least a fair chance of being your own. If you're going to put any effort into being a father at all, you might as well do it for that one.

When we see rutting stags or powerful bull seals, it's easy to think that, at least among non-human mammals, male sexuality is all that really matters. But, for a number of species, females sexuality is at least as important, and they can use it to get exactly what they want.

[Picture from Wikimedia Commons. Cladogram adapted from Li et al, 2009.]

Sunday, 7 August 2011

Why Cows Have Four Stomachs

(If all you want to know is what animals do, and what animals don't, have a four-chambered stomach, the shorter answer is here).

The stomach is an organ found in almost all vertebrates. Although there is considerable variation among the different groups, and there are some fish that don't have one at all, in general it has two functions. Firstly, it helps to store food for later digestion so that you don't constantly have to be eating, and secondly, it begins the digestion of food both by physically grinding it up and by mixing it with acid and digestive enzymes. Sometimes these functions are separated to some extent - for example, birds have a large crop for food storage, and a smaller true stomach below it that digests the food, and includes a muscular, grinding gizzard (a useful thing when you have no teeth). But the most complex stomachs of all are found in mammals.

A cow does, indeed, have four stomachs. Or, at least, it has a stomach divided into four separate chambers, which amounts to the same thing. Nor, of course, are cows alone in this. It's a feature found throughout the cattle family, which is a fairly large group consisting of over a hundred species - most of them antelopes, although it also includes the sheep and goats. The cattle were not the first family to evolve the feature, and we know that because its also found in all their close relatives, including such animals as deer and giraffes. In fact, the only truly cloven-hoofed animals that don't have four stomachs are the pigs and peccaries - which is why they aren't kosher.

In fact, we can group all the mammals that have this four-chambered stomach together. This group are called the "pecorans". Why not just "ruminants"? We'll get onto that later.