|Spot the male|
Sea lions and "true" (or "earless") seals belong to different, if closely related, families, but their reproductive strategy is much the same. In both cases, they spend most of their lives at seal, but they can't give birth there without their pups drowning. The majority of species breed once every year, with the females coming ashore, first to give birth, and then, almost immediately after, to get pregnant again before returning to the sea.
The precise details vary from species to species, but the South American sea lion (Otaria flavescens *) is a typical example. In December each year (that is to say, in early summer, this being the Southern Hemisphere), males haul themselves out onto beaches and fight one another for the best spots. Naturally, the largest and most aggressive males win, with the smaller and younger individuals forced out, to either wait another year, or to make daring raids when the larger ones aren't looking.
By the time the females get there, the males have already sorted themselves out, and the females aggregate around them as the males build up as large a harem as they can, defending it from any potential rivals. It's then that the females give birth, tending their pups for the few months necessary for them to enter the water. By the end of February, it's all over, and everyone returns to the sea, with the two sexes spending the other nine months of the year apart.
However, while the males may avoid female company for most of the year, they don't avoid one another. While they spend a lot of their time out at sea, when they do haul themselves out onto the land, they often do so in groups, sharing the same beaches year after year. But these are not the same beaches on which they breed, being apparently reserved for males only - perhaps to avoid the two sexes competing too much with one another for food.
South American sea lions are found across much of the southern and western coasts of the continent, from Peru (only just south of the equator), round the Cape, and all the way back up to Uruguay. Over this wide area, they naturally have quite a number of favoured beaches, both for breeding and for out-of-season socialising.
Two such male-only beaches are located near Mar del Plata in northern Argentina. Male sea lions have coming there for at least 40 years that we know of, despite the fact that the beaches are about 700 km (430 miles) from the nearest breeding rookeries. What's perhaps more significant, though, is that the rookeries in question lie in both directions. That is, there is one about on an island 700 km to the north, in Uruguay, and another at about the same distance to the south, in Patagonia. We know, from some genetic studies, that the females who visit these two rookeries are different populations, each apparently returning to the same place each year.
But what about the males? If they do spend much of their year about half-way between two equally promising breeding colonies, how does that affect the social dynamics of the sea lions in the region?
Over a period of six years, researchers studied what the males at these two sites were up to, and where they went in the summer. With over 500 males to study, it was still necessary to mark them individually, in order to see which of them are doing what. This required the researchers to sneak up on the animals and poke them with a six-foot pole (presumably one wouldn't want to touch them with a ten-foot one...) ending with a numbered stamp covered in bleach. This, apparently, doesn't both the sea lions all that much, and, since it's only on their fur, the number comes off anyway the next time they moult.
The sea lions, of course, didn't stay at the beaches for the whole of the nine months - they do have to hunt for food. But feeding trips rarely last much more than a week, and typically don't head out more than about 400 km (250 miles) along the coast from the animal's base. These were the only kinds of trips that the youngest sea lions - those that had yet to grow their mane - made, otherwise staying at the male beach even during the breeding season. Presumably they must follow older males to get there the first time round, since they stay with their mothers for the first three years of their life, only turning up at the male beaches later.
During the summer, of course, the older animals were much more likely to abandon the all-male beaches, heading off to the two breeding sites. For the most part, they seem to head to the same ones, although there is some genetic crossover between the Uruguayan and Patagonian sites, and it certainly doesn't seem to be the females who are travelling that far, it must be (at least some of) the males who keep the gene pools mixed, either by travelling to different sites in different years, or just by heading to whichever one they weren't born in.
The males return to their beaches a few weeks after the breeding sites have been abandoned, often with signs of injury suggesting some violent encounters in the interim. Those that leave first take the longest to come back, indicating that there really is an advantage to staking claim to the best bits of the rookery before your fellows arrive. These also tended to be the largest males, which are presumably the best able to travel the long distance, and then to guard their harem from all comers for weeks at a time without starving to death.
But the big males weren't the only ones to head out early. Some of the younger ones made long trips between July and November, before the breeding season proper had started. They ended up, not in the main breeding rookeries, but much smaller ones on the periphery of the main sites, perhaps hoping to catch a female that had given birth early and was still on her own. This apparently worked for at least one young male, his mane not fully developed, who nonetheless managed to mate at least twice during November, before any of the big males had turned up.
While at the all-male beaches, the males seem not to fight, something they do rather a lot of during the breeding season. A sudden rise in testosterone levels during the summer probably has a lot to do with both that, and with when they choose to make the trip to the rookeries. But the fact that they can tolerate one another's presence doesn't necessarily mean that they have to, and this new study doesn't really go much of a way to explaining why they're travelling so far in the first place.
Presumably, these are good feeding grounds, and the fact that there is a nearby fishing port, with some inevitable overspill of recently dead fish, may well be helpful. But another possibility, at least in this case, is raised by the fact that this general area was apparently once the site of a breeding colony itself. That ended in the late nineteenth century, with the all-male beaches being established from about the 1960s onwards. That's obviously too long for individuals to be returning to the sites of their birth, or whatever, but perhaps the sites were overall good ones for sea lions, and males aggregated there again, even while the females preferred to stay closer to their birthing beaches.
* Or Otaria byronia - take your pick. O. flavescens is the older name, and would therefore normally have priority, but there is some controversy as to whether the specimen first described and given that name really is what it was supposed to be, so that the newer name might be the first for this animal.
[Photo by Nestor Galina, from Wikimedia Commons]