Sunday, 4 May 2014

Too Many Monkeys

You can never have too many herbivores.

Well, actually, yes you can, but it doesn't happen very often in the wild. Populations of animals are naturally limited by a number of factors. For large predators, for example, the primary limiting factor is how many other animals there are around that they can eat. If the population of predators grows too large some of them will starve, and the population drops.

For small to medium herbivores, however, the limiting factor is rarely food supply, at least in the long term. (In the shorter term, something like a summer drought may be a different matter). For them, the main thing that prevents their population growing too large is that any excess population will get eaten by the predators.

This is, of course, an oversimplification. Large herbivores, such as elephants and rhinos, have relatively little to fear from predators, especially once they reach adulthood. Small predators, such as weasels, have just as much to fear from large ones as herbivores do. And there can be all kinds of other factors coming into play, especially where mankind has mucked up a long-standing ecosystem. But it's not entirely false either, and the upshot is that we rarely get to see what would happen if herbivores multiplied as fast as they'd obviously like to.

To get an idea of what happens then, and of just how many herbivores the environment can stand, we would need two things. We'd need an environment lacking in predators, and one in which there is a clearly limited supply of food. Wait a million years, and you might get something like insular dwarfism or insular gigantism evolving. But if you haven't got quite that long, you may well want to study what happens on an island that's really, really small.

Construction of the Guri Dam in Venezuela took place in a couple of phases between 1963 and 1978. It's a typical hydroelectric dam, albeit quite a large one, and it stands astride the Caroni River, the largest tributary of the Orinoco. The lake created by the dam is immense: at 32 cubic miles it is the sixth largest artificial lake in the world by volume, and the tenth by surface area. (In comparison, the largest reservoir in the US, the one created by the Hoover Dam, is less than nine cubic miles).

The creation of this vast lake also created a number of islands from former hilltops and ridges around the old course of the river. The largest of these is around three square miles, but many are much smaller. And these islands are not without wildlife.

The Guyanan red howler monkey (Alouatta macconnelli) lives in swamps and jungles between the Orinoco and Amazon Rivers, and they mainly eat leaves, topped with in-season fruit and flowers. They are members of the spider monkey family, equipped with a prehensile tail to help them swing through the tree tops where they make their homes.

If you've been following my series on marmosets, you'll know that a number of monkey species have been recognised in recent years, many of them consisting of known animals previously thought to be subspecies of something else. Guyanan red howlers are another example of this, only having been widely recognised as distinct from other red howlers for a little over a decade. Today, we recognise something like fifteen species of howler, two of which are considered "endangered species", and a couple more on the borderline of that status. Fortunately for the Guyanan species, they are not among them, and they're about as well off as any species of monkeys can be.

Nonetheless, while, taken as a whole, their forests aren't under much threat at present, a large chunk of them were flooded by the construction of the Guri Dam. Filling a reservoir is a slow process, and many would have had a chance to leave, but some found themselves trapped on the islands, and remain there to this day. A study published a couple of weeks ago by Gabriela Orihuela and co-workers  examined how those monkeys have coped with the changes to their circumstances over the last few decades.

Red howlers live in troops of up to eight individuals, led by a single male, accompanied by one or two females, and with the balance made up by their children. Each group normally wanders over an area of about 45 hectares (110 acres), howling loudly at any other groups they should come across, just to make it clear that the patch is "theirs". Their familiarity with their territory is clearly important; a report from 1988 describes how one such group, forced out of its former home by the creation of the lake, was picked off one by one by a single jaguar until only the youngest survived.

In this study, however, the researchers looked at troops trapped on two islands within the lake. One island measured 190 hectares (470 acres, or about three-quarters of a square mile), and was only a short distance from the lake shore. It was large enough to support a small population of predators, including jaguars, pumas, ocelots, and snakes, or perhaps was close enough to the shore for some of them to have swum across. It was also visited by eagles, which are large enough to prey on monkeys.

So, while it was an island, it was large enough and "normal" enough that the monkey troop living there should have been reasonably comfortable. The second island was somewhat different.

This island was much further out in the lake, and had no predators that could possibly attack the monkeys. Apparently, even the eagles didn't bother flying out there. It was also smaller - much smaller. At just 0.6 hectares (1.5 acres), it's a tiny fraction of the area over which the monkeys would have ranged given the chance. Yet there were just as many there as there were on the larger island - a single troop of six, in both cases - and they had to have been there since rising floodwaters created the island in 1985. The study was conducted between 2000 and 2002, but since howler monkeys typically live for fifteen to twenty years, there must have been some successful population turnover in that time.

In fact, when you think about it, it's remarkable that such a small island could support a troop of six monkeys at all. It goes to show just how fertile the jungle is, that cramming the monkeys into an area barely a tenth of what they'd be used to didn't actually result in them all dying of starvation. Nonetheless, this is an island on which the local population is limited, not by predators, but simply by what the land can support.

And, while not being under threat of being eaten is surely a good thing, there was very obviously a downside to that.

The monkeys on the other island, were, so far as the researchers could tell, just the same as those observed on the mainland. They were well fed, and behaved in all the normal ways of their kind. Not so those on the smaller islet.

For those were clearly monkeys living on the edge. With nothing else to restrict their numbers, they were living at the absolute limits of the island's capacity. For one thing, they were smaller, and scrawnier, than their preyed-upon kin. Although the females could raise youngsters to adulthood, it was a rare event. Indeed, they rarely gave birth at all, although whether because they were too stressed to mate, or because their poor nutrition led to miscarriages (or both) is unknown.

Their behaviour was odd, too. The monkeys on the other island howled at troops across on the shore, chased one another through the branches in friendly competition, and generally behaved like a happy family. But the other group, while they could sometimes hear the howls of their kin wafting across the waters of the lake, never once replied. Howler monkeys that don't howl; that's not normal.

The adult male dominated the best trees, driving the females and juveniles away from his prized source of food. Indeed, of all the monkeys on the island, he was the one that was closest in size and fitness to those elsewhere. In his absence, the adult females fed before the juveniles. (That might seem cruel, but, from an evolutionary perspective, a child needs a mother, but a mother can always have more children).

The troop was oddly broken, with individual monkeys wary and often staying on their own, not cooperating as they usually would. They chased each other, yes, but the chases often ended in physical violence, and they almost never groomed one another. Adding to the air of gloom, the babies apparently never played - although there were so few that it's hard to see who they would have played with anyway.

This was a troop, then, in which all the norms of social cohesion had either weakened, or broken down altogether. A troop, indeed, dominated by antisocial behaviour. It may well be that part of the reason monkeys live in troops is that it helps to discourage predators (this is certainly thought to be the case for herd-dwelling herbivores). But when that's pointless, and food is in short supply, motivations may change.

The paper concludes as follows:

"Thus, there is little to be envied about a [world free of predators]. Under normal circumstances, howlers experience a constant threat of predation but enjoy the benefits of a relative abundance of food resources, which, in turn, allows amity and social coherence. Might there be a lesson for us humans here?"

[Photo by Sean McCann, from Wikimedia Commons]

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