Sunday 19 November 2023

Learning to Hunt at Sea

One of the distinguishing features of mammals is that they invest a lot of time and effort in caring for their young. Of course, birds do this too, as do some other vertebrates, but the provision of milk is one of the key defining traits that's true of all living mammal species. (Probably most extinct ones, too, although it's hard to tell with the very early species). This means that newborn mammals are entirely dependent on their mothers in a way that the young of, say, most reptiles are not. 

But there comes a point where any mammal has to be weaned and make its way in the wider world. Even then, the maternal investment doesn't necessarily end with the mother continuing to raise and train her offspring for what can be an extended period. Brown bears, for example, are weaned at around six months, but they commonly stay with their mothers for at least another year, and often for two or more. We can see similar patterns in other mammals including, perhaps most obviously, primates.

In others, however, once the young are weaned, they're basically shoved out to fend for themselves. This is most common with small, fast-breeding mammals, such as mice. Perhaps the most extreme example comes from certain small marsupials, such as the antechinus, where raising children requires so much effort that the mother literally drops dead from exhaustion as soon as her young leave home. However,  a similar "not my problem any more" approach is seen in some larger species, too.

Among these are seals and sea lions. The problem here is that baby seals can't swim, and even if they could, their fluffy white coats wouldn't insulate them in the water, so they'd soon die of hypothermia. But they have those fluffy coats because they need something to keep them warm on land while they build up their fat reserves to develop the blubber their parents have. Until then, their mother has to stay with them on land to protect them, which means that, until they're weaned, she isn't getting any food at all. 

Understandably, as soon as her young are old enough not to need her milk, mum is going straight back to sea to gorge herself as much as she can before she dies of starvation. That leaves her with no time to make any further investment in her children, and they're just going to have to manage on their own. (Outside of the mammal world, we see something rather similar with penguins).

But what about those children? They have to teach themselves how, what, and where to hunt, all without any kind of guidance. And they'd better learn quickly, because they need to build up their weight and physical fitness before sharks or killer whales try to eat them. It's a very risky time of life for them, and there's obviously a strong evolutionary pressure on them to know what they're doing as soon as possible.

I've discussed grey seals (Halichoerus grypus) before as part of my 2017 series on seals. They're one of the larger species, found off both the northern coasts of Europe and off the northeast coast of North America. Although some use ice floes, they're typically born on rocky shores, and, like other seals, they bulk up very rapidly after birth. They're weaned at just 17 days old, at which point their mother makes the mad dash back to the sea to finally get something to eat. 

By this point, the young seal may have nearly half of their body weight composed of fat, which you'd think would be enough blubber to insulate them. But now they've got another problem, because 17 days is just not long enough for them to develop the ability to store oxygen in their muscles when they dive. Add that to the fact that, being smaller, they have a higher metabolic rate than adults, and they just can't dive for long enough to feed. So they end up hanging around on land for something like a further three weeks before putting their flippers in the water.

But what happens once they do? Without any parental guidance, such as many other carnivores would receive, how do they learn what's good to eat and where to find it? Some of this is clearly going to be instinctive - for instance, they obviously know that food is out there in the sea somewhere, and not inland - but they can't start out immediately hunting as adults would, because they still aren't physically capable of holding their breath for that long. How that changes physiologically is something we know a bit about for various seal species, but how it affects their behaviour is (if not a total mystery) not so well studied.

What is probably the world's largest breeding population of grey seals is that on Sable Island, a 43 km (27 mile) long sandspit about 175 km (110 miles) off the south coast of Nova Scotia. It's estimated that 87,500 grey seals were born here in 2016 - around 80% of all the grey seals born in the northwest Atlantic (there are harbour seals, too, so at the right time of year the island is pretty busy for something that's basically just sand and marram grass). In that same year, 25 of the newly weaned grey seals were fitted with satellite tracking transmitters and then released to see what they did and where they went over the two to six months it took for the batteries to run flat.

Two of the transmitters failed before the seal pups even entered the sea, but those that were still working showed that almost all of the seals initially swam to the waters south of the island. The authors speculate that this might be due to the more gradual slope of the seafloor in that direction, making it easier to orient themselves, assisted by the sudden appearance of the deepwater current over the continental slope beyond it, marking the start of the deep ocean depths. But, in all honesty, that's just a guess. It's worth noting however, that some other seal species also show a marked preference for a particular first direction of travel, influenced by the local geography. 

After that, the young seals moved on to explore the surrounding shallow waters across the region south of Nova Scotia. Nonetheless, all of them went on to spend at least some time in much deeper waters beyond the continental shelf, with some going on to explore the deepwater Laurentian Channel between Nova Scotia and Newfoundland. One went the opposite direction, spending some time in the Gulf of Maine and in the shallow waters around Cape Cod and Nantucket.

In fact, over the months of the study, the young seals travelled much further than adults would over the same period. Adults are more inclined to find somewhere where food is plentiful and spend a lot of time there, diving repeatedly before eventually heading back to land for a rest. But these seals did not, suggesting that what was most important to them was exploring their neighbourhood and learning what was out there. In fact, the older they became, the less of this exploring they did, moving towards the adult behaviour of more thoroughly investigating a particular area where they already felt comfortable rather than seeking out pastures new.

Moreover, those seals whose wanderings led them to the shallowest sea banks that are still remote from land (such as, say, the Banquereau Bank about 100 km [62 miles] northeast of Sable Island) tended to stick around and explore less. This is the sort of place that many adults feed, so they'd probably hit it lucky and found a good place on an early trip. Similar refinement of search patterns is seen in other sea-feeding animals, including birds such as shearwaters.

Although most of them explored deep water seas, it was clear that they didn't like them, and they turned round again not long after reaching them. For instance, those that reached the Laurentian Channel never crossed it, heading back south when they realised how wide it was. In waters over 200 metres (660 feet) deep even adults wouldn't be able to reach the bottom, where there is not only more easily available prey, but possible subsurface features that allow them to navigate. For grey seals, deep water is not a good place to be, and while the young pups clearly didn't know where it was until they got there, it wasn't somewhere they wanted to stay.

For all that they are presumably less physically resilient than older animals and have difficulty diving to similar depths, the young seals spent longer at sea than adults would - on average about nine or ten days at a time - and then spent longer recovering once they had returned to dry land. This probably reflects their inexperience. The whole point of swimming out to sea is to find food, but if you're not very good at catching it, or knowing the best places to dive to find it,  you may have to spend longer travelling before you no longer feel hungry. That they spent some of their time travelling to places that, in retrospect, weren't good places to feed supports this; they just hadn't figured out what they were doing yet. 

For many, this doesn't work out. Only around a third of grey seals survive long enough to breed, and it's likely that mortality is highest amongst the youngest individuals. But they have to learn somehow, because their parents won't help them; they can't even follow them to see where they go, because they've leave the short much earlier and, in any event, probably dive in places too deep for the youngsters to manage. But, even over the six months of this study, it was apparent that, week after week, the seal pups got steadily better, making shorter, more effective, trips and gradually figuring out how to get the food that their parents won't teach them about.

[Photo by Francesco Veronesi, from Wikimedia Commons.]

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