|Brazilian guinea pig|
The guinea pig family therefore represents a fairly diverse group of South American rodents, adapted to a wide range of different habitats, and found across much of the continent. Considering that, by the standards of rodent families, there aren't very many species in the group, and that, with a few exceptions, they don't look all that different, its perhaps surprising just how variable their behaviour is. That's largely because they have adapted to so many different environments; there are forest dwelling species, a couple of semi-aquatic forms, and others living in mountains, deserts, and grasslands. The different ways that these species behave was the focus of a couple of recent reviews in the Journal of Mammalogy.
Guinea Pigs Mountain Yellow-toothed Rock Capybara
Cavies Cavies Cavies
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The Brazilian guinea pig (Cavia aperea) is one of the ones that might be the wild ancestor of the domesticated animal. It prefers savannah with plenty of long grass and undergrowth. They are moderately sociable animals, where males live in a stable home range with one to three females. (One suspects those with a single partner just haven't been able to find any others, rather than having a preference for monogamy). Obviously, this means that there are some males left over, and these, which are generally younger and smaller individuals, wander about quite widely, although never too far away from the mated groups - presumably waiting for the male to die, or at least weaken.
As you might expect, this leads to quite a bit of violence between the males, and if you keep them together in captivity, they will try and kill each other. The females, on the other hand, tend to ignore each other, although males have been seen trying to break up fights between their partners. All of this means that males are quite effective at ensuring a female's pups are their own. As a result, Brazilian guinea pigs have evolved so that males are noticeably larger than females - their primary means of passing on their genes is to beat up their rivals, and for that, you need to be large and muscular.
The greater guinea pig (Cavia magna) lives in regularly flooded wetlands, and isn't a strong contender for the ancestor of the domestic forms, not least because it has webbed feet and enjoys swimming. Unlike the Brazilian guinea pigs, they don't seem to stick to a home territory, possibly because they can't predict when the ground is going to be flooded. Indeed, they are relatively antisocial for guinea pigs, largely ignoring each other and wandering about on their own. In their fast-changing environment, males have to mate with whatever females they can find, but they also need to fight off rivals, and so the males are larger and more muscular.
The common yellow-toothed cavy (Galea musteloides) has a third tactic. Although there is apparently some debate on how they live in the wild, they generally seem to be quite sociable, with several males and females living together, and communally raising their young. There is a dominance hierarchy, with an identifiable alpha male, and we might therefore expect that, as with the Brazilian guinea pigs, the males would be bigger, to fight off their many rivals. But they're actually slightly smaller and weedier than the females, and that's probably because of the way the females behave.
When they are ready to breed, the females get very excited, and dash about all over the place, something that the males notice quite quickly. Eventually one of the males will catch her, and mate, but that isn't enough for the female, who will immediately start running around again until another male manages to catch her. She often doesn't stop until everyone has had a go (and usually more than once each), which makes male dominance a bit pointless. There isn't much purpose fighting off rival males if it's the female that calls the shots, especially if she's going to mate with everyone else no matter what you do.
So, if they can't fight off rivals, how do the males ensure that they sire the most offspring? Well, they might not be able to out-compete rivals in physical combat, but they can try to do it sexually. Relative to its body size, the common yellow-toothed cavy has amongst the largest testicles of any mammal. In fact, they are about three times the size of those of Brazilian guinea pigs. As with most mammals, they shrink dramatically outside the breeding season (you'll probably have noticed that humans don't have a breeding season...), but that's par for the course. The individual sperm cells are also smaller, and less likely to be damaged, which means that not only can they produce a large quantity of the stuff, but it contains more healthy sperm for a given volume. If you can't beat your rivals with teeth and claws, you can try and do it by producing more, and better, sperm.
|Capybara - a 140 pound "guinea pig"|
They live in herds, which can, at times, reach quite large numbers. The females in a herd tend to breed at around the same time, so that their young are born together, and can be raised in a nursery group, with all the females cooperating. Their testicles are not particularly large, and, when examined under the microscope, seem to have a high number of testosterone producing cells, rather than sperm-producing ones. That suggests that, despite the large number of potential rivals, breeding males can be confident nobody else will get at their females, and that they would rather divert their energy into being larger, muscular, and generally more aggressive.
Which is what we see; the males are larger than the females, and establish a clear dominance hierarchy, with an exceptionally large alpha male controlling access to the females. He isn't perfect at this, of course, and younger males do get a look in from time to time, but he does have a few advantages on his side. Females are only receptive to mating for a few hours at a time, which gives the male the chance to stand guard, and courtship and sex last around ten minutes - quite long enough for the alpha male to step in and put a stop to anything he doesn't like the look of.
Many other species of guinea pig show variants on these themes, with, for example, the southern mountain cavy (Microcavia australis) showing sperm competition, and having correspondingly large testicles, and the rock cavy (Kerodon rupestris) fighting off rivals. But there is a third way.
The Muenster yellow-toothed cavy (Galea monasterensis) was only identified in 2004, and is only known from one small location in Bolivia. In experiments, when females are given a choice between two or more males, they pick one and stick with him, largely ignoring the others. After they have been together for some time, if the female is removed, the male becomes stressed, and he visibly becomes much happier if she is returned. In short, the species appears to be monogamous.
The males are no larger than the females, since any potential rivals are likely to have mates of their own, so there's no need to fight them off. But equally, they don't need large testicles or increased sperm production, because the female is likely to be faithful. Significantly, and quite unlike the aggressive common yellow-toothed cavies, fathers of this species help to raise their young, something that makes sense in a monogamous partnership where you can be sure the child is your own. This may be explain why the behaviour evolved; Muenster yellow-toothed cavies inhabit the Andes mountains, where food is scarce, so having two adults to look after the young may well be advantageous.
Cooperation, for this species, beats sexual competition.
[Pictures from Wikimedia Commons. Cladogram adapted from Dunnam & Salazar-Bravo, 2010]