Sunday 5 April 2020

Musth and the Older Elephant

Male Indian elephant, probably practising his social distancing
rule of "stay at least 1.2 km apart."
As with many other group-living mammals, male and female elephants not only spend much of their lives apart, but also live in quite different sorts of society. Females live in small family groups, dominated by the eldest among them and consisting of her children and other close relatives. These groups, in turn, form larger aggregations known as 'clans', the membership of which can change over time, with individual families joining or leaving as circumstances dictate. The precise details vary between the three different species of living elephant, but these basic rules seem to be universal.

Males, on the other hand, leave the family unit of their birth as soon as they reach puberty. From then on, they spend much of their lives alone, without the benefit of the matriarch guiding their sisters. . From time to time, they may meet up with other males, or even join female groupings, but these are always short-term arrangements. This creates a situation where solitary males regularly travel about, hoping to encounter different female groupings as they do so; similar behaviour is seen among mammals as diverse as giraffes, polar bears, and killer whales.

Which is all very well, but does put some pressure on the males when it comes time to mate. Female elephants spend very little time sexually receptive, not least because pregnancy lasts about two years, and it takes so long to raise the resulting young that it's then at least another four before the mother is ready to mate again. Since she then spends only a few days in heat, there is considerable competition among the males for mating opportunities.

Elephants have a polygynous mating system, where one male mates with several females, and tries to prevent other males from getting a look-in. But, perhaps partly because of the unpredictable timing of female fertility, they don't rut or form harems in the way that, say, deer or seals do. Instead, male elephants periodically go into an unusual state referred to as musth.

While in musth, male elephants experience a surge in testosterone levels. This leads to the elephant becoming highly aggressive. At least in captivity, they will attack almost anything in their vicinity, and to escape from whatever enclosure they may have been placed in. Which, given the strength of an enraged bull elephant, is certainly a cause for concern and can make it difficult to breed elephants in zoos.

In addition to this, elephants in musth also become incontinent, regularly dribbling urine onto the ground (presumably as an intentional signal) and producing a sticky, smelly, secretion from glands on the sides of their heads in sufficient quantities to produce a noticeable wet patch. These behavioural and physical changes are naturally costly to maintain, in terms of nutrition, and that's a large part of the point. By going through musth, and the process being so apparent, the elephant is advertising his physical fitness and ability to withstand hardship.

The details may be different, but this sounds similar to the behaviour of, say, stags in rut. But there's a key difference: male elephants are perfectly capable of breeding when they aren't in musth. Nor does it necessarily always occur at the same time each year, or last for the same amount of time. So what factors influence when and for how long an elephant goes into musth?

There are good reasons to suppose that the answers might not be the same for each of the three living elephant species. In particular, there are differences in how musth manifests between Asian and African elephants, with the latter able to experience temporary boosts to their testosterone levels and accompanying libido without going all the way into musth. That's something that Asian elephants apparently can't do.

Indeed, since African and Asian elephants last shared a common ancestor over 7 million years ago - which is probably before the human-chimpanzee split - such differences really shouldn't be all that surprising. (As an aside, mammoths split from the Asian elephant only around 6 million years ago, so it's plausible that something similar would apply to them, too).

In the case of Asian elephants (Elephas maximus) much of the research on musth has been conducted on animals in captivity - whether in zoos or being kept as beasts of burden. Which makes sense as far as accessibility goes, but does limit the particular forms of behaviour you can study. For instance, they may have little choice as to which other elephants they come into close proximity to. So, to try and learn more about that, a recently published study watched elephants in the national parks of Nagarahole and Bandipur over an eight-year period.

Perhaps the most significant finding, since it's exactly the sort of thing you couldn't tell in captivity, was that male elephants visibly in musth were more likely to travel into or out of the study areas than those that were not. This suggests that, among its other effects, musth induces a sort of wanderlust in the animals, making them travel further than they otherwise would. Which makes sense if they're trying to find relatively rare receptive females to mate with.

But there was also the question of how musth varies with age. One possibility, for example, is that the heightened aggression induced by musth helps younger males compensate for their smaller size when competing for mates with older, larger, ones. That's been proposed for the two species of African elephant, but it turns out that it's probably not the case for the Asian one.

There's a couple of reasons why the study made this seem unlikely. Firstly, younger elephants (i.e. under the age of 30) were simply less likely to be seen in musth than older ones. Asian elephants first enter musth at around the age of ten, and do so roughly once a year from then on. But it seems the older they get, the more likely they are to experience it in any given year, and probably for longer periods of time, too. That makes no sense if it's a strategy to help younger males, and instead suggests that they'd rather spend their hard-earned calories bulking up their bodies to give themselves a better chance in future.

The other reason is that, at least according to this study, young elephants that did enter musth spent less time in the company of females than they did at other times of the year. Whether they're actively avoiding them or the larger males are successfully driving them off is unclear, although the latter seems more likely. We do know that females find males in musth more sexually attractive so, presumably, if the young male does happen to get close to a female there's a fair chance he'll get lucky... which explains why it happens to youngsters at all.

On the other hand, since older males don't seem to be bothered by the presence of young males out of musth, and since the latter are still perfectly capable of mating if a female proves willing enough, it's not as if they won't get a chance at all. Indeed, the researchers spotted a young male mating while a much larger male, currently in musth, had done no more than briefly turn his back and walk a couple of hundred metres away. When the older male returned, he continued to ignore his younger counterpart apparently on the grounds that the latter wasn't in musth at the time, and was presumed not to be a rival.

The older male mated with her twice so the advantage of his sexually attractive state had evidently paid off. But, at least this time around, the younger one had benefited almost as much from the exact opposite...

[Photo by "Srinidhi BS", from Wikimedia Commons.]

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