|Black-and-gold howler monkeys. The black one is male, the gold, female.|
One of the main reasons for living in a herd is that it makes it easier to watch out for things that are going to try and eat you - you don't all have to be watching all the time, which means that each individual can spend more time eating, or doing whatever else it wants. So a herd has to be large enough for that to be worthwhile, and there may also be a minimum size on viable herds based on such things as the way the animal breeds. On the other hand, a really large herd is going to require a lot of food to sustain it, so the amount of food available is likely to put an upper limit on the herd size.
Somewhere in the middle there's going to be a pay-off where it's "just right", and that will depend, not only on the animal concerned, but also on the local environment. Even so, some herds are always going to be bigger than others, for a variety of reasons. When it comes to deciding how to spend their day, we might expect that larger groups of any given species will travel more than smaller ones, because they're constantly on the look-out for enough food to sustain themselves. Mostly that means travelling from one place to another - they could just spread out over a wider area, but spread yourselves out too thin, and it rather defeats the point of being in a herd in the first place.
However, in most parts of the world, and for most diets, how much food is available doesn't just depend on where you are: it also depends on the season. Your herd may be the perfect size to balance protection and food availability in the summer, but what about the winter, when there isn't so much food around? Even in the tropics, there are wet seasons and dry seasons, and groups of animals will have to adapt to changing availability of the kinds of food they want to eat.
There's a number of different ways of dealing with this, and no reason why any given animal species can't use more than one at the same time. They could - and many do - switch to eating something different at different times of the year. But, if one food is less nutritious than the other, that only changes the nature of the problem. That leaves three strategies. Firstly, you could change the herd size, breaking it up into smaller groups in time of hardship, so they don't get in each others' way. Or you could travel more, searching further for new patches of food.
The third option is just to do less. If you don't burn as many calories, you don't need to eat so much, and it doesn't matter that there's less food around. An extreme example of this is hibernation, where you aren't merely spending all your time asleep, but actually stop burning the calories you need to stay warm-blooded. But, depending on the animal, you don't necessarily have to go that far for the tactic to work.
So far, I've been talking about 'herds', as if this only applies to hoofed mammals. Many studies examining these sorts of effects have, indeed, been performed on such animals, but the same concepts should apply to any animal that lives in reasonably stable groups. The effect on carnivores is likely to be more indirect, but, even if we restrict ourselves to herbivores, hoofed animals are hardly the only mammals to live in herd-like groups.
Most primates are essentially herbivorous, and a great many of them are highly social animals. While many hoofed mammals are grazers, however, primates are normally browsers, picking out more highly nutritious food from among the branches. The very best such food to eat is generally fruit, which is packed with nutrients, and usually juicy and tender into the bargain. Even where wild primates live, however, fruit isn't always available year-round, and so many primates feed instead (or as well) on leaves.
You're not going to run out of leaves at any time of the year, if you live in a tropical jungle. But even there, soft new leaves - and flowers for that matter - are better to eat than tough, chewy ones. The latter have less nutrients, and take more effort to eat, but, like fruit, fresh leaves and flowers aren't around in equal amounts at all times of the year.
Primates being cool and popular mammals, there have been a number of studies on how they respond to such factors. However, there have been relatively few examining how all these factors affect monkeys (as opposed to lemurs). A new study, by Ilaria Agostini and colleagues, not only did that, but added a further factor into the mix.
The researchers chose to look at two species of howler monkey, diurnal, and mostly leaf-eating monkeys native to South America. Brown howler monkeys (Alouatta guariba) live in the coastal forests of southern Brazil, while black-and-gold howler monkeys (Alouatta caraya) live in the more inland regions of southern Brazil, and also in eastern Paraguay, Bolivia, and Argentina. Since they are both howler monkeys, they are fairly similar (although not identical) in terms of how they live, and what they choose to eat.
Mostly that doesn't really matter, because they never meet each other, but this study took place in the El Piñalito provincial park next to the Brazilian border at the eastern end of Misiones Province in Argentina. In this area, and a narrow strip of nearby forest in southern Brazil, both species of monkey live together, virtually in amongst each other. When you have two different animals living in the same area, eating the same food, it's often the case that one, or both, of the animals has to adjust their lifestyle if they are to survive.
Howler monkeys Spider monkeys Woolly monkeys
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This shows the most recent information I can find for the relationships between the relevant kinds of South American monkey. However, while there is general agreement about how howler, spider, and woolly monkeys relate to one another - and that they form a single family of animals - quite how the other families of South American monkey relate to them is less clear. Other studies, for example, have shown that the saki monkeys might be closer than the marmosets. For our purposes today, though, that doesn't really matter.
In twelve months of continuous study, it emerged that by far the most popular daytime activity for howler monkeys of both species was... doing nothing. For over half of their waking lives, howler monkeys sit in trees, or lie down, or (being monkeys) hang from branches, occasionally scratching themselves, but otherwise just observing the world around them.
When they can be bothered to get up and do anything, when there's plenty of tasty fruit and young leaves around, the black-and-gold howlers are noticeably more active, whether they're actually off looking for more, just wandering around aimlessly, or socialising (which, for these purposes, includes fighting each other, as well as more friendly activities such as grooming and mating). They were also more active in general than the brown howlers, presumably because they also lived in larger groups, and, as predicted, needed to spend more time searching for food if everyone was to get enough of it.
The effect was far less noticeable with the brown howlers, although not entirely absent. It's not really clear why, but perhaps there's some subtle difference in how they feed that we've overlooked. In general though, this suggests that howler monkeys respond to a limited diet by choosing not to burn calories, rather than expending effort on looking for dinner that might not be there.
More surprisingly, perhaps, neither type of monkey seemed to do anything differently because the others were around. Generally, when this sort of thing happens, you'd expect the two species to either concentrate on different kinds of food, or to be active at different times of the day, so that they can stay out of each others' way. But, no, not only did they eat the same food, but they both fed at about the same time.
In fact, their feeding times were actually quite rigid, and that may be part of the explanation. Like humans in Western society, they typically ate three meals a day: breakfast at around 8 a.m., lunch at around noon, and an evening meal about three or four in the afternoon. Well, not quite that rigid, perhaps, but the pattern was certainly noticeable.
They probably do this for a similar reason to us: three or four hours is about how long it takes them to get hungry again. Leaves take a fairly long time to digest, and, unlike cows, they don't have the advantage of multi-chambered stomachs. That's not to say that they don't have adaptations to a diet rich in relatively low-nutrition leaves; they have an enlarged hind-gut, as horses and rabbits do, but even so, it isn't as efficient as in those animals. As a result, it probably takes howler monkeys three or four hours to fully digest a meal, and, rather than stuffing their faces constantly, they've evolved to eat at particular times of the day, spaced about that far apart.
However, if that's the case, they don't have a lot of choice about when they eat. They can't easily skip meals, or eat at night (when they're asleep), so they pretty much have to eat when they do. That prevents them from adapting to the presence of the other species by changing their habits when they're around. Sometimes an animal's biology just prevents it from doing that.
The usual consequence of this sort of thing is that one species wins out over the other. When American mink escaped from fur farms in Europe, they drove the native mink to near-extinction because of this. I see another example frequently in the grounds around my workplace. We have squirrels living in the trees, and I often see them darting about. But they aren't native squirrels; they're American grey squirrels. Once they arrived in Britain, grey squirrels rapidly displaced the native, red, species, and there's now just a few left in the wilder corners of Scotland. Red squirrels as a species are doing fine, but only because there's so many of them on the continent, where grey squirrels have gained little foothold: in Britain, though, they're almost gone.
This may explain why the two species of monkey normally live in different areas, and only overlap in a very narrow zone where those two areas meet. Normally, they do stay apart, and prefer subtly different habitats, and so neither wipes out the other. Where they do live together, they are similar enough that they sometimes live together, even mating to produce hybrid offspring that may, or may not, be fertile.
It's evidently confusion that, in general, they prefer to avoid.
Synapsida will be taking a break next week, but will return on the 29th.
[Picture by X from Wikimedia Commons, Cladogram adapted from Wildman, et al, 2009.]