Sunday, 19 October 2014
How Elephants Bring the Rains
Which isn't so bad, if you happen to live in a densely vegetated environment. The rather smaller African forest elephant (Loxodonta cyclotis) does just this, inhabiting jungles north of the Congo, but was only recognised as being a separate species in 2010. The bush elephant, while it, too, often lives in dense forest, also seems happy to inhabit less favourable habitats. For example, a significant number live in the savannah of the Serengeti in East Africa, and in similar environments. At the extreme, some live on the edges of deserts in places like Mali and Namibia.
These are not, however, places that are entirely devoid of rain. In Namibia, for example, there are distinct wet and dry seasons. The transition between the two is extremely abrupt, and we might reasonably expect elephants to change their behaviour with the seasons, to take as much advantage as possible of the rains, when they do come. They might, for example, travel from one place to another, especially since they do, indeed, have good memories for things like geography and the location of water holes.
It's not directly relevant to the African species, but, in India, Hindu myths say that Asian elephants are associated with water and fertility, and help to bring the rain. In one version, the god Indra rides a white elephant, symbolising cumulus clouds, and hurls his thunderbolt at a herd of dark elephants thundering across the sky, so ushering in the monsoon. In Kenya, local folklore apparently has it that sighting an elephant towards the end of the dry season means that the rains are coming at last.
Of course, for that to be true, the elephants would have to start moving before the rains actually begin, although not necessarily by very much. Indeed, studies have shown that, not only do elephants move around more in the rainy season, but that they may well start this behaviour before they experience rainfall. To be sure, though, and to figure out how it is that they know, we need to look at detailed studies that specifically address this question.
Kunene is an arid region in the northwest of Namibia, including part of the border with Angola to the north. The local elephant population was wiped out in the late twentieth century, as much by Namibia's protracted war of independence against South Africa as by poaching, although both played a significant part. Since about 2002, however, elephants have returned to the area, and the local population now numbers in the hundreds.
An eight-year study of elephants in Kunene included GPS monitoring of a number of animals wearing radio-tagged collars. Because the region is so marginal, even though individual elephant herds stick to particular home ranges within it, those areas are huge. While the ranges do overlap, each may be anything up to 14,000 km2 (5,500 square miles) in area. Those areas stay pretty much fixed throughout the year; these elephants don't migrate between different pastures, possibly because the terrain here is too rugged and mountainous. In fact, the only part of the region where the land is flat and low-lying is where it approaches the sea - a region aptly known as the Skeleton Coast because of it's near-total lack of anything worth eating. Unsurprisingly, the elephants don't go down there if they can help it.
The collars used in this study transmitted only once each day, in order to conserve battery power over what was a very long time period. So they're basically giving us a snapshot of where the elephants are on any given date, rather than, say, how fast they're moving. But that's sufficient to tell us whether they really do change their behaviour on or around the day that the rains first come.
It also shows us that females move about less than males, tending to stick more closely to specific places within their home range. The males in this study were all sub-adult, so it's not likely that they were searching for lonely females. It's more likely that they moved more simply because they could. That is, the females, burdened by having to look after their calves, tended to stay within a few miles of water sources, rather than ranging more widely. Males only have to drink once every three to five days, and while the females could probably do the same if they had to, their calves can't, so once they find water they stick close to it. Since we're talking about a region where one of the two major rivers, the Hoanib, only had running water in it for five days in the whole of 2003, the reluctance to abandon any you've found does rather make sense.
But, for all the remoteness and desolation of the Kunene, modern satellite technology means that we know exactly when the rains start, and when they finish. Tying these dates up, the researchers could tell that the elephants did suddenly change their behaviour at the start of the rainy season, before returning to their usual habits when it finished. How it changed seemed to vary between individuals, possibly because of factors in the local geography, so that part of the picture isn't clear, but the key lesson is that the changes were significant, and closely tied to the coming of the rains.
Sometimes, as one might expect, this change occurred after the elephants had got a drenching, and the vegetation around them had suddenly burst into life. But that was the exception, rather than the rule; in most cases, the elephants started acting differently a few days before the rains came.
Which brings us to the obvious question: how do they know? The study can't conclusively answer that one, but it does give us some significant clues. For instance, it can't be something along the lines of changing day length, because the rains don't come on the same date every year. Furthermore, when brief bursts of rain came unexpectedly during the dry season, the elephants often responded in the same way - and, again, before they actually got wet. That also makes it unlikely to be random chance, especially since many different elephants across the region all responded at about the same time.
But here's the thing: the elephants may not have got wet before they started moving, but that doesn't mean that it hadn't started raining somewhere else. In other words, the elephants began to move about, not necessarily when it rained where they were, but when it started raining up to 200 km (125 miles) away. To a local observer, it looks as if the elephants get agitated, and then it rains, but, in reality, they can tell what's coming because, somewhere else, it's already started.
The most likely explanation - although not necessarily the only possible one - is that the elephants can hear the coming rain. That may sound remarkable, given the distances involved, but elephants have hearing more sensitive than we do, and are particularly able to hear infrasound - very low rumbles that, by their nature, can travel great distances without fading. A previous analysis showed that, at least in theory, elephants should be able to hear thunderstorms at these sorts of distances - and it's possible that here, we have some evidence to back up that theory.
So it seems that elephants can hear the distant rumble of thunder from a hundred miles away or more, or perhaps the constant drumming of rain on the ground closer to home. Armed with this information, they can get ready for what they know is coming - and provide a handy forecasting tool for humans into the bargain.
[Photo by Hans Hillewaert, from Wikimedia Commons]