Mother mammals can use three different methods to identify which child is their own, and likely often use a combination of them. Perhaps the most common method is smell, since most mammal species have a quite remarkable sense in this regard. If each child has a unique smell, or even if it just smells of you, that's a good way of identifying it. The other possibilities are identifying them by the sound of their voice, and, finally, by what they look like. For we primates, the latter may seem the most obvious, but facial recognition is something we're particularly good at.
Using sound does have advantages over the other two methods, though, in that it works over a distance. Even if, once you're up close, you're going to check you haven't made a mistake using one of the other methods, recognising a child's voice is a good place to start if you've got separated from them for some reason. And, indeed a lot of mammals (and birds) can do exactly that.
The extent to which a given animal needs to be able to do this, however, depends, in part, on how likely it is that they are going to get separated from their child in the first place. For many animals, the odds are likely quite good that this will happen on a regular basis. A mother needs, for example, to be able to get food for herself, and it's probably a good idea to leave the kids somewhere safe while they're doing this. Except... some actually don't need to get food.
Seals and sea lions are, on the whole, quite similar animals. Both are aquatic, with their feet formed into flippers, and both come ashore to give birth, and to raise their young until they are old enough to swim and forage for themselves. But they belong to two different, if closely related families. (Although I should note that, for these purposes, I am lumping fur seals in with the sea lions, since they belong to that family, rather than being 'true' seals). There are a number of anatomical differences between the two, but there are also distinct differences in how they raise their young.
Sea lions behave in the way you would expect most mammals to. Every now and then, mothers have to leave their young and head out to sea to catch food, suckling the children on their return. It takes several months to wean a young sea lion (or fur seal), and it's obviously important that the mother must be able to find the right child when she returns to land. In every species of the family that has been studied, it has been determined that mothers can easily identify their children by sound alone.
Indeed, they learn this pretty quickly. They have to, because they need to get the hang of it before they make that first foraging journey, and they're kind of hungry after giving birth. In fact, it has been shown that Australian sea lions can uniquely identify their children from the calls that they make within 48 hours of them being born.
But with seals, it's all rather different. Mother seals do not, as a rule, forage for food while they are suckling their young. Instead, they just starve themselves, going without food for however long it takes to complete weaning. While this does have an obvious downside, it does mean that they can afford to watch over their children 24 hours a day, seven days a week - they simply have no need to be anywhere else. This does, as one might expect, mean that young seals are weaned quite a bit sooner than young sea lions are, with about seven or eight weeks being the maximum, and some managing to do it in just a few days.
This does not necessarily mean that they don't need to identify their children at a distance. Elephant seals certainly do, on the grounds that they come ashore in huge groups where it's quite easy to get separated no matter how hard you're trying not to. The same goes for grey seals, which also give birth in large colonies. But in those seals that don't live in such large groups, it's far less of a problem, and it seems that the mothers call less frequently than sea lion mothers do, and that the calls of different pups are hard to tell apart.
But you may have noted that I said 'as a rule' when I said that mother seals don't forage. I said that because there are exceptions to the rule. One of those, as it turns out, is the harbour seal (Phoca vitulina), an animal sufficiently numerous and well-known that it is sometimes instead called the "common seal." These are found across the northern coasts of Europe, along pretty much the whole of both the Atlantic and Pacific coasts of the US and Canada, and along Asian coasts north of Japan.
It has been known for some time that harbour seals do not look after their young in the same way that most of their close relatives do. Mothers provide their young with milk for about four weeks, which is not unusual for seals, and certainly much less than for sea lions. For the first twelve days or so, they do without food, just as other seals do, but after that, they begin to travel out to sea, making dives to forage for food. It's possible that they do this because of their relatively small size, so that, as with sea lions (which are smaller than true seals), surviving on fat alone for weeks on end might be a bit of a problem.
Breeding in groups that are considerably smaller than those of elephant seals, the mother is unlikely to lose her pup for long during that first twelve day period, but, after that, she really does need to know which one to come back to. Moreover, young harbour seals begin swimming before they are fully weaned, so whatever method they're using had better work underwater, as well as on land. Indeed, we have known since at least the 1980s that harbour seal pups make a specific type of call that they stop using entirely once they are old enough to stop suckling. Mothers have been shown, unsurprisingly, to pay attention to these calls, which can be transmitted through the water, as well as through the air.
To find out whether there is anything in the calls that allows the mother to single out her own young, researchers caught and measured harbour seal pups swimming in the St Lawrence estuary in Canada. After attaching an identifying tag (it's designed to fall off the first time they moult), and listening to any sounds they made, they released them back into the water. At which point, the pup invariably swam away, calling as it did so - presumably trying to convey something to the general effect of "Help! A bad man just glued a pyramid to my head!"
When the sounds, both those made above water and those made while swimming, were analysed, they showed that each individual was, indeed unique. Some of that may be due to age, with older and larger pups being able to produce louder sounds then their smaller kin, among other differences. But, even taking that into account, it was possible, using computer software, to tell pups apart.
The differences between individual's calls vary across a range of different parameters. In general, the most distinctive aspects of the call were those that can be identified equally easily after transmission through either air or water, allowing the mother to hear her pup no matter what she's doing (so long as she's in range, of course). This would doubtless be particularly useful in a seal that starts swimming early, and may not always be on land itself.
Even so, the differences between individual calls were not so great as those measured in similar analyses performed on young sea lions. They are greater than in most other seals examined, however, suggesting that the harbour seal's ability to monitor its young is somewhere between that of sea lions on the one hand, and the majority of true seals on the other. (Again, we're ignoring elephant seals here). The differences that do exist start from very early on, being detectable even in pups that are only a few days old, so that the mother probably has time to 'learn' them and tell her own pup apart from others before she's likely to be away for any extended period.
Strictly speaking, we don't know that the mother actually pays any attention to these differences - that would require a different study - but its very hard to see why they'd exist otherwise. It seems that mother harbour seals, unlike kangaroos, know which children are theirs.
[Photo by Maximilian Narr, from Wikimedia Commons]
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