Sunday 23 January 2011

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]

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