The mother may, of course, receive assistance in this sometimes arduous task. In monogamous species, the father also helps in raising his young, and this is one of the main reasons for evolving monogamy in the first place - if the young require care by two parents in order to stand a decent chance of survival, then the father had really better stay around. But this is not the only source of such assistance. In many mammals that live in social groups, we see the phenomenon of alloparenting. This means that individuals other than the parents take up some of the burden of looking after the young. Usually these are younger, non-breeding, females, cooperating to raise infants communally, although it doesn't have to be.
At least 120 mammal species, and an even greater number of birds, engage in this to at least some extent. The benefit to the infants is obvious, especially where they require a significant investment in time and effort in order to reach maturity. But what's in it for the alloparent?
The most common reason is probably that the alloparent is related in some way to the infant. A common social structure among group-living mammals is that the females are related to one another, while the males come in from outside. This means that, if a particular female has no young of her own, she is still likely to be the aunt, or other close relative, of any young that do exist. While looking after nieces and nephews isn't quite as beneficial, genetically speaking, as raising your own children, it will still pass at least some of your genes on to the next generation.
Another possibility, and one that could well work alongside the one just mentioned, is that non-breeding alloparents are practising for the future. If they can gain experience in looking after infants of other mothers for at least some of the time, perhaps they will be better at it when they have children of their own, and have to take the lion's share of the work.
Or perhaps, if the alloparent is a mother herself (of some other infant), she's just made a mistake, and doesn't realise that she's looking after the wrong child. This can happen, for example, in bat colonies, where the potential number of children and nursing mothers is so huge that it may just not be worth the trouble to put too much effort into finding the right infant. Indeed, there is some evidence that young mammals of some species may benefit from stealing milk, in that they are exposed to a wider range of antibodies in the process, and are better able to resist disease as they grow older.
More extreme than simple alloparenting though, is full-on adoption. By this term, I'm referring to a mother taking sole care of a child that is not her own, entirely supplanting the role of the biological mother. This has been observed in more species than you might think, and for much the same reasons as more typical alloparenting - close relationship, practice for the future, or simple cock-up.
Elephant seals, for example, raise their young in busy, noisy colonies, where it is entirely possible for a baby seal to become separated from its mother. If a young mother loses her own child, and comes across one of these abandoned orphans, there is a good chance that she may adopt it as her own, apparently gaining child-rearing experience that she can put to good use the following year.
In elephant seals, this tends to occur with especially young infants, which may also be the ones most likely to be in trouble. This is often the case with other mammals, too, but one instance where it can't be is among marsupials. Marsupials, of course, have a very short pregnancy, after which the newborn young crawl into the mother's pouch, where they remain, clamped to the teats, before growing old enough to venture forth. Clearly, a mother marsupial can't nurse somebody else's child during this stage.
Yet, it is known that eastern grey kangaroos (Macropus giganteus), for example, are at least able to adopt young after they have left the pouch. These are one of four species of "true" kangaroo, and are found throughout roughly the eastern third of Australia, including parts of Tasmania. They are the second-largest species of marsupial, after the red kangaroo, and, given that the eastern end of Australia is the bit with the largest human population, are probably the most commonly encountered kangaroos outside of zoos.
Pregnancy in eastern greys lasts about 36 days, with the single young remaining permanently in the pouch for at least 200 days. Even then, they try not to leave if they can help it, and jump straight back in again if they somehow fall out. They begin to leave on purpose at about nine months, spending increasing amounts of time making forays outside over the next month, before leaving for good. After this, the infants are known as "young-at-foot", and, while too large to actually climb back into the pouch, still stick their head inside to suckle. They are eventually weaned at about 18 months of age.
A recent study looking at over 300 kangaroos and their infants identified 11 cases of adoption - a rate of around 3%. The phenomenon had previously only been observed in zoos, where it can be deliberately manipulated to save young. So, while 3% may not sound like a lot, it is significant here for the fact that it happened entirely naturally in wild animals.
Why are they doing it? Well, for a start, it isn't for the usual reason of protecting close kin that might otherwise have died. The social structure of kangaroos is not particularly based on sisterhood, and, when tested, it turned out that the adopted young were, for all intents and purposes, entirely unrelated to their foster parents.
In fact, it's worse than that. On one occasion, a mother adopted a child that wasn't her own, only for her real infant to return later and try to climb into her pouch. She kicked it away, and would not allow it to follow her. Another kangaroo mother found with two young in her pouch, only one of them her own, when eventually forced to reject one (kangaroos cannot rear more than one child at a time), happened to pick her biological child, keeping the adopted orphan instead.
What about gaining experience? Well, for that to make sense then, as with elephant seals, we'd expect that most foster parents would be recently-bereaved first time mothers. That did, indeed, happen once, and the mother in question went on to successfully rear her own child the following year. Clearly it worked for her, but it's one instance out of eleven, with all the others being mothers who already had experience at successfully rearing young in the past. So, most of the time, that can't be what's going on, either.
Do the young gain some benefit from suckling from different mothers? On two occasions, a pair of mothers directly swapped their offspring (so that's four out of the eleven adoptions), which might suggest this as a possible reason... except that, so far as we can tell, adopted young fared no better in later life than the 97% that stayed with their mothers. So, if there is a benefit, it's too small to detect.
So what are we left with? Cock-up.
Which rather implies that eastern grey kangaroos can't recognise their own children. But is that really the case?
It's not, after all, as if they don't check. A baby kangaroo trying to climb into a pouch will first be sniffed by the pouch's owner, and is only let inside if she likes what she finds. This makes sense; a lot of mammals identify their young by their scent, and there is no reason to suspect kangaroos would be any different.
Now, it is true that the adopted children tended to be about the same age as the mother's real child had been, and they were almost always the same sex (the one exception was, as it happens, the first-time mother mentioned above). So it's not as if there's no similarity, but you'd think the scent would be more accurate than that. And, indeed, with only 3% adoption rates, presumably it is.
A clue as to what may be going on comes from the fact that when young kangaroos were definitely orphaned - that is, they had actually lost their mother - they were never adopted. Instead, adoptions seemingly only occurred among infants whose mother was perfectly fine, but had presumably been somewhere else at the time.
We can't say for sure, but what it looks like has happened is that, after having left the pouch, but before they were old enough to do so permanently, at some point the herd of kangaroos had been disturbed. Everyone panicked, and ran for it, as herbivores are inclined to do. The young dived for the safety of the pouch, and, having other things on their mind at the time, the mothers just hadn't taken the time to check them. Some of the young ended up in the wrong pouch.
And the mothers never noticed. After that, the adopted children presumably smelled of the foster parent's pouch, which must be "good enough" so far as the mother is concerned.
There are times, it seems, when kangaroo mothers really don't know their own children.
[Photo by Danielle Langlois, from Wikimedia Commons]
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