A Time to Breed and a Time to Die

Grey slender opossum

Looking after the children can be a stressful and exhausting activity. Other issues aside, offspring need feeding and looking after, which can put a drain on the mother's own energy reserves, never mind the need to feed the extra mouths in the first place. For mammals, one particular energetic stress is the need to provide milk, and this seems to be particularly true for marsupials, who carry their young around, permanently clamped onto teats inside their pouches, for the earliest part of their life.

Given these extra energy requirements, it's unsurprising that most mammals time their births for times when the greatest amount of food is likely to be available in their environment. Certainly there are species that breed pretty much year round, at least in the absence of unexpected drought or the like, but most have at least some kind of 'breeding season', arranged so that the young are born at the best time of the year - often the spring, in temperate climes, but more likely the rainy season near the tropics, where there isn't a winter to avoid. In the case of marsupials, the breeding season and the birthing season are pretty much the same thing, since pregnancy is an extremely brief affair, and it's the time spent in the pouch that's really crucial.

But how to determine when the best time is? Perhaps the most obvious cue is the changing length of the day, which should give some idea of what time of year it is. This is known to be the case, for example, with at least some Australian marsupials. But not all marsupials are Australian, and rather less study has been made of the life cycles of those elsewhere. Some of these animals are influenced in other aspects of their yearly lives by the temperature, entering a state of near-hibernation during the coolest parts of the year, say, or even just when it's driest. So it's at least possible that these rather more direct cues also influence when they choose to breed.

One reason why there might be a difference is that, for the most part, American marsupials are not particularly related to their Australian kin. This is because marsupials lived in South America before they reached Australia. The one branch of the family that emigrated to the latter continent thrived, with few placental mammals to compete with, but those that were left behind also managed to survive, albeit in smaller numbers, and they remain there today, with an evolutionary history stretching back as far - and in some cases further - as anything in Australia.

The largest family of American marsupials is the opossum family, the Didelphidae. (The word "opossum" is, incidentally, often pronounced without the initial 'o' in parts of America, but biologists tend not to do that because "possums" are, in fact, a different sort of animal, found only in and around Australia). Physically, opossums have changed less from their ancestral appearance than have many Australian species, never having had the opportunity to move into new habitats as the ancestors of, for example, kangaroos and wombats, have done. They are pretty much all forest-dwelling species, many of them spending a lot their time up trees, and they tend to be omnivorous, often with a lot of insects and the like in their diet.

A few species of opossum are descended from ancestors that left their home continent and headed to North America. The best known is, of course, the Virginia opossum, native to much of the eastern United States (and since introduced to the Pacific coast, as well), which is likely the animal most Americans immediately think of when they think "opossum". However, there are two more species in Mexico, and at least a hundred others south of the Panamanian isthmus.

Which is rather a lot, demonstrating that marsupials are by no means a purely Australasian phenomenon. To return to our discussion of breeding seasons, though, this gives us the opportunity to test different species against each other. If they're all following the same cue, and are eating more or less the same food, all the species in a given area should breed at round about the same time. Furthermore, if this period isn't very long, then, given that many of these species have short life spans, they aren't going to get many opportunities to breed in their lifetime. Which means that, compared with other species, they may put more effort into getting it right when they have the opportunity.

A recent analysis of the populations of five species of small opossum in the Morro Grande Forest Reserve in southern Brazil was only able to find juveniles between the months of December and May, which represents the second half of the rainy season and the start of the dry season. Now, these are free-living juveniles, the sort that can actually be caught and examined before release, which means that, by definition, they have already left the pouch. The period we're really interested in is the one before that, but we know that these species spend one to two months in the pouch before being weaned, so we can knock a month or two off that to figure out when the mother is spending most of her effort in looking after them.

Which, indeed, is basically the rainy season, when we'd expect it to be. For myself, just eye-balling the data, I don't think there's enough to prove that all five species breed at the same time - there just aren't enough samples of some of the rarer ones to know for sure - but they're at least within a few months of one another. A more detailed statistical analysis in the report itself apparently showed that day length was a better predictor of breeding time than the weather, showing that they're probably using the same method as the Australian sort. (Although, to know for certain, you'd probably want to see what happens when you put captive ones under conditions where you can alter the 'day' length experimentally).

What's perhaps more interesting, however, is just how much effort mothers put into looking after their young. The most common species in the area was the grey slender opossum (Marmosops incanus), with numerous individuals captured over the two years of the study. Except that is, in the early part of the rainy season, around November, when they weren't able to capture any at all during either year.

Okay, so the females are probably staying in their nests, nursing. But where were the males? And even after that, once the researchers started finding juveniles... they couldn't find any adults until March. None at all. They clearly weren't still looking after the young at that point, because the juveniles had already left home. So where had they gone?

A clue comes from another piece of information: the number of times they caught the same individual twice in different years. Which was: zero. This fits with a something scientists had previously suspected about this species, namely that they only live for about a year. Which is pretty short for mammals, even small ones, and is particularly significant when the animal in question has a defined breeding season, rather than just producing fast-growing litters whenever it has the time.

Because that means that the animal gets to breed just once in its entire life. When that's the case, the mother may as well throw absolutely everything she'd got into raising her young until the point that they can fend for themselves. So what if she puts so much effort into the task that she literally dies of exhaustion afterwards? She was going to die anyway, so she has nothing to lose.

Which is why there didn't seem to be any adults after the young had left home: they were all dead by that point. And then 'adults' start turning up around March, because that's how long it takes the juveniles to grow up. The population turnover might not literally be 100% - you'd need a much bigger study to prove that - but it seems to be fairly close.

Except, oh yes... what about the males? They don't help look after the young, so why weren't there any of them around? In fact, in their case, the problem is even worse, because the females do at least have to live long enough for their young to be safe. The males, on the other hand, only have to live long enough to mate, and then... well, their continued existence is kind of pointless. So the suggestion here is that the males have so much sex during the breeding season that they just drop dead from over-exertion.

This is a pretty weird thing for a mammal to do, but it isn't unique. In fact, I blogged about it four years ago. That was with regard to a small Australian marsupial called the brown antechinus, a mammal that is, to use the technical term, "semelparous" - which is to say, it reproduces once and then dies. Other antechinuses are similar, and the same pattern has been observed in their close relatives, the phasocgales.

It's not been clearly demonstrated in South American marsupials before, but, as I noted above, it has been suspected. One reason for this is that males of at least one species closely related to the grey slender opossum, caught after the breeding season, seem to be in very poor shape. They shed their hair, lose weight, and even begin to suffer from serious parasitic diseases, most likely as their immune system collapses under the strain.

It's not entirely clear, though, why this should be an advantage to the opossums. It makes sense if the animal really can't live longer than a year, but there's no obvious reason why it couldn't do that. True, we can't say for certain that having children is a guaranteed death sentence for them - even female brown antechinuses sometimes survive for a second attempt. But that seems to be a rare event, which makes more sense in an arid environment like Australia, where you have to make the most of things when the rains come, than it does in the relatively lush Atlantic Forest of Brazil.

Perhaps it's a holdover from their evolutionary past, something that made more sense then than it does now, and that there's never been sufficient reason to change. But either way, two groups of small marsupials, on opposite sides of the Pacific Ocean, and not particularly closely related to one another, have, over a long period of time, evolved the same, rather unusual, method of breeding.

[Photo by Ramon Campos, from Wikimedia Commons]