Baby's First Dive

Perhaps the most distinctive feature of mammals is that they provide their young with milk. This allows the young mammal to continue growing after it has been born, but before it is ready to fend for itself. The downside, of course, is that it leaves the infant entirely reliant on its mother for survival, something that we don't see, for example, in most reptile species. 

At some point, however, the infant has to survive on its own. Before it can truly leave home it needs to be able to find its own food, so there's going to be a period between it feeding entirely on milk and it becoming fully independent during which it's learning the necessary techniques of foraging or hunting. We see something similar in birds; while they obviously don't produce mammalian milk, young birds do necessarily rely on their parents for food until, as a minimum, they get the hang of flying. (It seems plausible that the same was true of pterosaurs, although there is evidence that at least some species learned to fly very quickly and that the period of dependence was correspondingly short).

While the reliance on suckling means that all mammals do this to some extent, the closest obvious analogy to birds would be bats - and, indeed, it does take them time to learn how to fly. But another group of mammals that has to learn how to do something nearly as dramatic before becoming self-sufficient are the pinnipeds (seals and their relatives).

Unlike, say, dolphins, baby seals are not born underwater. Typically (although not always) born with fluffy white fur that's great for camouflage but rubbish for waterproofing, like other mammals they spend their early days suckling from their mother. Eventually, they shed the baby fur (technically 'lanugo') and venture into the water to learn how to hunt. How long this takes varies between species, but it's usually quite short, with the baby seal guzzling down huge quantities of extra-high-fat milk to bulk up its body at quite remarkable speeds for mammals of their size.

The most extreme example of this is the hooded seal, which is weaned within just four days of birth, thought to be the shortest period of dependency of any living mammal species. Like some other seals with relatively short periods between birth and weaning, hooded seals don't enter the water as soon as they stop suckling, spending some time surviving on their fat reserves until they're ready to take the plunge. This, it seems, gives them time to develop the ability to hold their breath for sufficiently extended periods.

Many seals, however, take the opposite approach, starting to dive before they are weaned, so that they are ready to go as soon as their mother stops feeding them. This allows suckling to go on for longer so that the pup doesn't have to grow at such an insane rate - and the mother can spread the effort of producing the milk out over a longer period of time. In addition to giving them time to learn to fend for themselves, the fact that the young are relatively active also means that they don't need quite the same level of attention and protection. 

This is the pattern we see, for example, in the bearded seal, which is quite a close relative of the hooded seal but takes 24 days to wean its young. Which, admittedly, is still quite short by human standards but is a good deal more relaxed than providing a full supply of milk in just four days.

One of the other species that uses this latter strategy is the Weddell seal (Leptonychotes weddellii). This lives very far south, essentially hugging the coasts of Antarctica and with only a few, apparently lost, individuals being spotted as far north as, say, the Falkland Islands. It may well be the extreme cold of their preferred habitat and of the ice floes on which they breed that means their young need time to get ready for independent life. (Although hooded seals live off Greenland, which isn't much warmer, so maybe not).

Whatever the reason, Weddell seals do have one of the longest periods of pup dependency of any seal - about six weeks. (They're also not pure white, as many other newborn seals are). For at least some of this time, the pups are honing their diving skills, apparently taught by their mothers, or at least closely watched by them. Although it's not unique, the most detailed study of how the pups go about doing this has recently been published, following eighteen Weddell seal pups living on the shores of McMurdo Sound, the southernmost stretch of navigable water in the world.

The study used pressure sensors to monitor the pups' activity, allowing them to determine when, and how far down, the young seals had been diving. The researchers clipped them to the pups' hind flippers when they were around a week old, retrieving them again six weeks later, by which time they would have been weaned and already starting to lead an independent life.

Unsurprisingly, as they get older, the seal pups dive deeper and more frequently although, even at seven weeks, these dives are much shorter and more shallow than those made by adults. They seem to begin in earnest around the end of the second week of life, but it's notable that some made a dive on the same day that they were fitted with the sensors. This could literally have been their first ever dive but we can't know that, and it's possible that some had made the attempt the day before - at six days old.

In addition to the dives being shorter than those of adults, averaging two-and-a-half minutes each, the pups also spent less time recovering between them than adults do. This is likely because, even at that age a Weddell seal can hold its breath for around six minutes, so the dive simply isn't long enough for it to need much recovery, whereas adults are far more likely to push themselves to the limit and come up gasping for air.

Another difference is that adult Weddell seals do most of their deep diving in the afternoon, when their favoured food (Antarctic silverfish, in this part of the world) has itself retreated further down the water column. It turns out that the pups don't do this, at least until after being weaned. Instead, they do most of their diving between about midnight and 9 a.m. and virtually none during the afternoon when their mothers are feeding 300 metres (980 feet) down. 

This could be because the fish aren't swimming so deep at night, so even the relatively shallow 10-metre (30-foot) dives that the pups make have a chance of finding something to eat. But it might also be that they aren't foraging much at all, at least until they're weaned, instead focussing on getting experience at deep swimming. For instance, they may be learning how to navigate beneath the frozen surface of the sea, since getting trapped being trapped beneath the ice and unable to come up for air is a common cause of death in young seals. If you can't get that bit right, the feeding is redundant...

Unlike hooded seals, they can afford to take the time to do this, because they're less threatened by predators at an early age (there isn't much that eats them apart from killer whales, and they don't like swimming beneath solid ice). Because hooded seals are using comparatively thin and floating sea ice, instead of thick sheets anchored to the land at one edge, they are both at more risk of being eaten if they can't get off it and at a lower risk of being trapped beneath it when they do. Their food also doesn't swim so deep, so even a shallow practice dive can get them ready to learn how to hunt for it.

These advantages, however, only remain true so long as the ice where they live is thick and firmly attached to the land. So far, it is - the Antarctic isn't warming that much yet - but that could change in the future. At the opposite end of the world, where Arctic ice sheets are receding much faster, some native seals are already at risk of losing the habitat they need to survive.

[Photo by Armando C.L. Genta, from Wikimedia Commons.]