Miocene (Pt 31): Terror Mice and the First Howler Monkeys

Phoberomys

As the heat of the Middle Miocene gave way to the more moderate temperatures of the Late Miocene, grasslands grew across much of the Northern Hemisphere. In South America, however, the changes were, perhaps, less significant due, in part to its more equatorial position. Nonetheless, even aside from the general worldwide cooling trend, the South American climate was changing as the Andes continued to rise, affecting weather patterns across the continent. It was also continuing to edge closer to North America, so that, towards the end of the epoch, it became possible for a few animals to make the crossing using the islands of what is now Central America as stepping stones.

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.

Sharing Resources

Black-and-gold howler monkeys. The black one is male, the gold, female.

I've mentioned previously how animals have to juggle their time between different activities, such as feeding, watching for predators, and reproducing, and how that can be affected by things such as the size of the herd they live in (if any). Unsurprisingly, this works both ways, with the size of a herd being affected by the animal's lifestyle.

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.