Sunday, 17 January 2021

Sperm Whale Bromance

While there are a few exceptions, there is a general rule among mammals that males travel far from home when they approach adulthood, while females tend to stay in their local neighbourhood. The purpose of this separation is to ensure that, once they become sexually active, the females that a male is going to have the opportunity to mate with aren't all his own sisters or close cousins. Many mammals live essentially solitary lives, so this is merely an issue of the male travelling further than the female when he leaves home; other than that, their social lives aren't really all that different.

In social species, however, the end result is that herds (or other groupings) are united primarily by their female relationships. The females in a herd are likely sisters or other close relatives, while the males have travelled from elsewhere and are not only not closely related to the females, but may not even be closely related to each other, either. Often males spend some time living on their own before they find a suitable herd to join (perhaps because the existing dominant male is getting on a bit) with the result that there's a distinct female-bias in membership of the group. 

Sperm whales (Physeter macrocephalus) go a step further than this. Living as they do in the open ocean, sperm whales don't have specific territories, and tend to wander across a vast area during the course of the long lives. As a result, they can be found almost anywhere that the water is deep enough, save for those places where there is a risk of getting trapped under an ice sheet. 

Nonetheless, when they travel, females stay together, forming social groups of up to two dozen members. Although there is some degree of membership turnover as individuals come and go between different pods, most females seem to remain within the group of their birth throughout life, forming long-term social bonds.

When males approach adulthood, however, they leave the pod of their birth and travel much further distances than females, often into the high latitudes, where they live in much smaller, looser, groups than the females do. Indeed, these groups get smaller as the whales get older until, by the time they are about 40 years old, each male travels entirely on his own, occasionally heading into the warmer equatorial waters where the breeding grounds lie.

Most studies on sperm whale sociality have tended to focus on the larger, longer-lasting, female pods, where there is a fair degree of complex interaction going on. But what about those younger males, old enough to have left their original family, but not yet old enough to brave a solitary life? How do they form the small groups they live in, and, come to that, why bother? Male mammals often aggregate to jointly defend a territory from rivals, but sperm whales travel wherever they please and don't have a territory, so that can't be it.

Indeed, there's a bit of a mystery here, and it's one that relates to mass strandings. The exact causes of such strandings are unclear, and they are more common in highly social species such as pilot whales (Globicephala spp.), but they do also occur in sperm whales. As you'd reasonably expect, they are usually of females, since they are the ones travelling in the sort of large groups that could come to collective misfortune.

But sometimes, mass strandings of all-male groups of sperm whale have been reported. These groups can sometimes be surprisingly large, and genetic analysis of some strandings has shown that the individuals concerned weren't close relatives, originally hailing from widely disparate parts of the ocean. Of course, it would actually be quite difficult for a male sperm whale to travel with his brother, since they tend to be born many years apart, and they still want to leave home at roughly the same age. So that part isn't surprising, but that so many males can strand together at the same time suggests a degree of sociality that doesn't fit with what else we know.

Plus, being dead, it's hard to know how they were associating when they were still alive and healthy.

Researchers recently published a study of the sperm whales living in the Nemuro Strait, a body of water separating Hokkaido, Japan from Kunashir Island (which is jointly claimed by Japan and Russia, but currently governed by the latter). They used commercial whale-watching boats to do this, operating out of a small fishing port on the Japanese side of the strait, using photographs to identify specific individuals by the shape and markings of their tail flukes. 

Over the course of several years, they were able to identify over 200 different male sperm whales, which stayed in the area for an average of two years. (The boat trips only took place in summer, though, so this doesn't mean that the whales didn't go elsewhere at other times of the year). But what they found was that certain pairs of male sperm whales were more likely to be found close together than would be expected by chance - which is to say that the whales did have preferred "friends" with which they tended to associate.

These were clearly not the close associations that we'd expect to see with females, so the two sexes do, expected, live very different lives, but the males did show more evidence of a social life than had been seen in previous, smaller-scale studies in places such as Hawaii. The data in the study suggest that such pairings last for over 2.7 years on average but, since that's longer than the average time spent in the area, the real figure could well be much higher - it's just that the pair tends to wander off elsewhere at some point and so can no longer be observed.

Of course, these males aren't literally following each other about all the time. In fact, they're mostly a few kilometres apart, but that's well withing earshot of one another, given how far the sound of their clicks travels underwater. From time to time though, they did come close to another, often to rest on the surface, not showing any of the mutual aggression we might expect from competitors. This had previously been thought to be a rare occurrence, but apparently not, at least in this particular body of water.

So, why do they do it? The fact that they are within hearing distance of one another might mean that they are sharing useful information, such as where the best food is. Two whales can cover a wider area while searching for food, both benefiting from the extra pair of eyes.

Another possibility, one often seen in other animals, is that this may be a means of defending against predators. One might think that a sperm whale has relatively little to fear, but they have been shown to react to the sounds of killer whales, rising rapidly to the surface and sending out specific sounds that are presumably alarm calls intended to alert their neighbours. Again, that doesn't work if you don't have any neighbours close by.

This could explain why the male sperm whales in this particular area were apparently more sociable than those studied in other parts of the world. Perhaps the pressure from killer whales is different there, or it may even be something to do with the fact that the Japanese whaling fleet kills considerably more sperm whales than killer whales ever do. If the whales perceive themselves to be under pressure, they may be more cautious, and more likely to seek safety in numbers.

For many land-based animals, even when males live apart from females, it's usually not by very much. They do after all, want to mate with them at some point. The situation in sperm whales is different, with the sexes literally thousands of miles apart for much of the year. There is, perhaps, some benefit at such times from knowing that at least someone familiar has your back.

[Photo by Mathijs van den Berg, from Wikimedia Commons.]

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