When Sloths Roamed the Grasslands

The Altiplano - where ground sloths may once have grazed

I've mentioned before that the living tree sloths are the last remnant of a much larger group of ground-living animals. These animals originated in South America during the long time that that continent was an island, connected only to Antarctica. Eventually, the land mass collided with North America, bringing with at an array of more modern mammals, such as foxes, deer, and cats, as well as a few that have died out elsewhere since, like tapirs and llama.

The ground sloths actually did reasonably well after the other mammals arrived, and a very wide array of fossil species are known, the last of them having died out as recently as 9000 BC. Indeed, they crossed Central America heading in the other direction, leaving fossils in the northern continent, and at least one species, Megalonyx, even crossed the Arctic Circle to live in the cold wastelands of northern Canada. Many inhabited forests, but others preferred grasslands, or even semi-desert, while Thalassocnus may have been semi-aquatic. There was also a huge range of different sizes, from some that were barely larger than modern tree sloths, up to the multi-ton Megatherium.

The Not-Naked Mole Rats

Silvery mole rat

I recently discussed moles, and their adaptations to their unusual, underground, lifestyle. But, of course, they are not the only group of mammals to adapt to such a life. A great many rodents are burrowing animals, spending a significant proportion of their time below ground, often in a bid to escape predators and to have a safe place to store food for later use. However, the majority travel above ground to forage, and are not truly subterranean in the way that moles are. Not so the mole rats.

The most famous mole-rat is surely the naked mole-rat, a bizarre, hairless species known for reproducing in the manner of bees or ants - with a single reproductive queen supported by numerous infertile workers. But this is not the only mole-rat, and the other, furrier, forms are also highly adapted to underground life. Indeed, the naked species is regarded as exceptionally odd, even by the standards of a group that's fairly strange to begin with, and seems to have branched away from its kin at an early stage in their evolution.

Common Mole     Damaraland      Dune & Cape   
 Rat, etc.     Mole Rat, etc.    Mole Rats 
     ^               ^               ^     
     |               |               |       Silvery
     |               |               |       Mole Rat
     -----------------               |           |
             |                       |           |
             |                       |           |       Naked
             -------------------------           |      Mole Rat
                          |                      |         |
                          |                      |         |
                          ------------------------         |
                                      |                    |
                                      |                    |
                                      ----------------------
                                                 |
                                    (original African mole rats)
                                                 |

Until recently, there were believed to be around sixteen species in the African mole rat family, but recent genetic studies have shown that there are considerably more than this, with the "common mole rat" actually turning out to be several different species, some of which aren't even particularly closely related. Such are the problems of studying animals that are almost always down below the ground where you can't see them.

To complicate matters further, a second group of subterranean rodents are also referred to as "mole rats", and have at least as many species. To keep things simple, I'll just discuss the species related to the naked mole rat (technically called the "bathyergids") for the moment. I'm going to call them "African mole rats", because that seems to be the most common name, even though its a bit misleading, since some of the others also live in that continent. Animals just want to make it difficult for us sometimes, don't they?

Anyway, African mole rats are only ever found south of the Sahara, although they may have ranged further in prehistoric times. Most are found in southern Africa, with just a few species further north, in places like Nigeria and Ghana. Again, the naked mole rat is unusual, living across in Somalia, far to the north and east of any of its relatives, and it may be this isolation that allowed it to evolve into such an odd animal.

African mole rats have many adaptations reminiscent of those of the moles, including a relatively cylindrical body that fits well into narrow burrows, velvety fur that won't clog with dirt, and almost invisible ears. But there are some significant differences. Where moles burrow with their spade-like paws, mole rats instead use their teeth. As with all rodents, mole rats have a single pair of large incisors in each jaw, which grow continuously throughout their life, allowing them to constantly gnaw on tough materials without fear their teeth will ever wear down.

The incisor teeth of mole rats are particularly large, and appear even more so because the animal's lips are actually positioned behind them. Because the lips are behind the teeth, instead of where you'd expect them, mole rats can keep their mouth closed even as they use their teeth to gnaw away at the soil in front of them, digging out a tunnel as they go. They then scoop up the loose earth with their front paws, push it underneath their body, and kick it away with their hind feet. The need to keep the tunnels clear means that, eventually, they have to push the excavated earth up to the surface, where they leave mounds very similar to mole hills, that may be the only visible sign of their presence.

For the most part, they prefer relatively solid ground to burrow in, which stops the tunnels collapsing, and probably also makes them more difficult for predators to dig out. The two species of dune mole rat are slightly different, however, and live in soft, sandy soils. These are much easier to dig through, and, unlike the other species, they mainly use their front paws to do so, instead of their teeth, which are smaller than those of their relatives.

The eyes of mole rats are very small, and it had been assumed for a long time that they were completely blind. However, over the last decade or so, it has become clear the structure of their eyes isn't as odd as you might expect if this were true. They have an iris and pupil, a lens that is small, but nonetheless fairly normal, and crucially, the retina seems to have all the light-sensitive cells you'd expect to find in an animal that can see. There are far less nerve cells connected to the retina than in other mammals, but they do exist, which suggested that mole rats probably are able to see, albeit very poorly. When scientists looked at the visual centres of the brain, they found a similar story; the brain structures that would be expected to respond to the presence of light are all there, even though the parts involved with actually seeing objects are highly degenerate.

Of course, examining the anatomy is all very well - can they actually see, and if so, why? The first part of the question was answered in 2005 for the Zambian mole rat (Fukomys anselli). The animals were kept in an artificial burrow, with a choice of sleeping chambers, some of which had light, and some of which were dark. They chose the dark chambers, showing both that they could tell the difference, and that they considered light unpleasant - as well they might, living underground all the time.

More recently, studies have been conducted on the giant mole rat (Fukomys mechowii) and the silvery mole rat (Heliophobius argenteocinereus). Again, both species avoided light, although they didn't seem to mind red light, and presumably couldn't see it. However, while they clearly don't like light, they don't always run away from it; by poking holes in the artificial burrow and shining light through it, the researchers were able to persuade the mole rats to rush towards the breach and plug it with earth. In the wild, there would presumably be many other clues that a tunnel had a hole - the sound of a predator digging its way in, or just a change in air currents or the like - but it does seem that light by itself is enough to be disturbing to the animals.

The dune, cape, and silvery mole rats are all solitary animals, leaving their home burrow when they reach adulthood. It seems that they simply burrow away and block up the tunnel behind them, although they do venture above ground to find mates, a potentially very dangerous time for them. All the other species of mole rat are at least partially social, living in communal burrows. It is well known that the naked mole rat takes this to a remarkable extreme, but it is not alone since the Damaraland mole rat (Fukomys damarensis) of Botswana and Namibia has a similar lifestyle, albeit much less studied. It is generally thought that the other species of mole rat are less extreme, living socially, but not having a single reproductive queen, although even they tend to be dominated by a small number of long-lived individuals that limit breeding by their relatives to some extent. And, of course, its always possible that the lack of detailed research means that there are a few extreme social breeders that we've missed.

Given that they aren't very closely related within the African mole rat family, its perhaps unsurprising that this social system works slightly differently between the naked and Damaraland mole rats. The former are the more extreme, with a distinct 'worker' caste of infertile individuals incapable of breeding. Damaraland mole rats, however, live in smaller colonies consisting of a queen, a small number of reproductive males, and a number of offspring. Unlike their naked kin, Damaraland mole rats will not mate with their own parents or siblings, so it may not be that their workers are truly sterile, just that they don't have anyone to mate with. Nonetheless, there is at least some evidence that the queen is able to control access to visiting males from outside the colony as well, so it may not be quite so simple.

The question of quite why they evolved to live like this is an interesting one, especially since some of their relatives didn't. There are a number of possible explanations, but it may be worth noting that the two most social species live in the driest parts of Africa south of the Sahara, while other mole rats live in generally more hospitable savannah or veldt habitats. Unlike moles, mole rats are vegetarian, eating roots, tubers, and even above-ground plants that they can physically pull down into their burrows by their roots. In fact, while they do, of course, use their burrows for shelter and breeding, most of the tunnels are only dug in the ever-present search for food.

In a dry environment, food is most often available for only a short time after the rains, and the mole rats have to dig quickly in order to find suitable food, and store it where possible. That the soil is also likely to be hardest to dig in these environments makes it especially difficult, and it may be that a large colony working cooperatively makes this all much easier to do.

For the most part, the mole rats are not endangered by human activity, and they have few natural predators. A few snakes, most notably the mole snake of southern Africa, are able to get into their burrows, but in general, their cosy, relatively cool, and difficult-to-find underground burrows keep them safe. Although no species are officially listed as endangered, one, the Kafue mole rat (Fukomys kafuensis) of Zambia, is considered vulnerable, due to its very restricted range, and because locals consider it something of an agricultural pest.

[Picture from Wikimedia Commons. Cladogram adapted from Van Daele et al. 2007]

Surviving on the Edge

The spread of humanity across the globe has been bad news for a number of animal species. Species have always gone extinct, as the climate or other circumstances change, but the pace of extinction, and the number of species that are currently teetering on the brink has greatly increased over the last few thousand years. In many cases, humans are the direct cause of the problem: we kill animals for food, for sport, for make-believe medicines, to protect our livestock or crops, and so on.

But just as often the effects are indirect, as natural habitats are broken up into smaller isolated pockets or destroyed altogether. A few species have adapted well to human environments, and are rarely, if ever, found far from our settlements - the house mouse and the pigeon are both examples, before we even begin to look at pets or livestock. But most have not fared so well, and the question of how our destruction of the habitat affects animals is obviously an important one in conservation.

Going Underground

European (or "common") mole

Mammals have adapted to a wide range of habitats and lifestyles, from Arctic wastelands to the deep sea. In addition to taking to the water and to the air, there are a number of mammals that spend almost their entire lives underground. I'm not talking here about living in caves, or digging a burrow to sleep in, but animals that spend their whole lives burrowing through the ground.

There are a number of disadvantages to this lifestyle. You have to constantly dig for one, and your fur and other body parts will get constantly clogged with earth. There is no light, and its difficult to find food or mates. On the other hand, there are also some clear advantages. For one, burrowing animals are safe from most (though not all) predators. Anything small enough to get into their burrows is probably small enough not to want to eat them, and, being underground, they're pretty difficult for anything above ground to find. In addition to that, unless the ground really gets saturated, they don't have to worry much about the rain, and they are protected from extremes of heat and cold.

Although there are a lot of mammals that dig burrows, there are four main groups that have taken the subterranean lifestyle to an extreme. Perhaps the most familiar example are the moles. The mole family consists of forty-one species, although a number of them aren't truly underground creatures, and two of them are semi-aquatic. The group seems to have diverged from its closest relatives, the shrews, not long after the dinosaurs went extinct, and possibly even earlier, making them quite an ancient group. The true underground lifestyle probably evolved at least twice, because the "common mole" of Europe (Talpa europaea) and the "common mole" of North America (Scalopus aquaticus) are not particularly close relatives within the family.

  European      "American"    Star-nosed
& Asian moles     moles       mole, etc.
      ^             ^             ^
      |             |             |
      |             |             |       Desmans
      |             ---------------          ^
      |                    |                 |
      |                    |                 |       "True"
      ----------------------                 |     shrew-moles
                |                            |          ^
                |                            |          |
                ------------------------------          |
                              |                         |
                              |                         |
                              ---------------------------
                                           |
                                     Earliest moles
                                           |

The chart above is somewhat simplified; the "American" moles in fact include one species from China, while the close relatives of the star-nosed mole include some species commonly called "shrew-moles". Brown represents fully subterranean species, blue the semi-aquatic species.

Moles are reasonably widespread, being familiar animals across Europe, much of Asia, and North America. They avoid the colder environments where the ground freezes solid for much of the winter, and so truly subterranean moles are not found in places like Norway or most parts of Canada. They also avoid the deserts, which explains their absence in the Middle East, North Africa, and the central US, despite the presence of two species in the eastern US and three along the west coast. They've never colonised Africa or (less surprisingly) Australia, although there are other mammals with a similar lifestyle on both continents, which I plan on discussing in later posts. Perhaps more surprising is their absence over much of India and in South America; it's not that they dislike tropical environments because there are moles in South East Asia, from Vietnam and Myanmar down to southern Malaysia. For its size, there are also rather a lot of mole species in Japan, along with several species from China, Korea, and the warmer parts of southern Siberia.

True moles are well adapted to their burrowing life. They have powerful front legs with spade-like claws for digging through the earth, and their velvety fur clogs with soil less than that of most other animals would. Unlike bats, which can usually see pretty well when they need to, the eyesight of moles really is quite poor. The European mole (that is, the one we call the "common mole", and which is the only one found through most of Europe north of the Mediterranean) has eyes that superficially appear quite normal, but the cornea is degenerate, and has been compared to that found in humans with the rare visual disorder keratoconus.

Nonetheless, while it prefers to feel its way about with the help of sensitive whiskers and specialised touch receptors on the snout (the ears are also very small, again, to stop them clogging with dirt) the European mole can see, if not very well. The Iberian mole (Talpa occidentalis) of Spain and Portugal, and the eastern mole (that is, the American "common mole", which is found throughout much of the eastern US) are a different case. Their eyelids never open, permanently protecting their eyes from the dry gritty soil of their homelands at the cost of their vision. Still, while the fact that their eyes are permanently shut presumably prevents them from making out any shapes, they aren't completely blind.

The structure of the eyes in Iberian moles has been studied in some detail. The cornea is even more degenerate than in European moles, the iris has no muscle fibres to allow it to widen or narrow in response, and in any event is stuck to the lens, the lens itself has a disorganised structure and is less transparent than in most other animals, and the retina is unusually small. Nonetheless, apart from its size, the retina does look relatively normal, which suggests that it must be at least partially functional. Indeed, Iberian moles do respond to light - by running away from it, which makes sense if you live underground, where there really shouldn't be any.

Nor does the strangeness of moles end with their poor eyesight. In a further bid to stop holes filling with dirt, the female's vagina is sealed shut for most of the year, and only opens when they are sexually receptive. Rather odder, though, is the fact that female moles are, in a certain sense, actually hermaphrodites. This is perhaps rather stretching the term, but it is the case that the ovaries contain a large amount of what certainly looks like testicular tissue - indeed, there's rather more of it than there is ovary. It doesn't produce any sperm, but it does produce male hormones. Of course, all female mammals produce some male hormones - they're made by the adrenal glands - but the female mole produces considerably more than usual.

Eastern (or American "common") mole

This only seems to have been studied in detail in four of the five species of mole found in Europe - the European and Iberian moles, and the Balkan (Talpa stankovici) and Roman moles (Talpa romani) of the Balkans and Italy, respectively. Those four species are quite closely related, having diverged only a few million years ago after their ancestors left Asia, and with their current distributions probably having been heavily shaped by the need to avoid the advancing permafrost during the Ice Ages. But it isn't unique to them, and has also been reported in the star-nosed mole (Condylura cristata), that remarkably distinctive beast of the eastern US and Canada, and, for that matter, the Japanese mole (Mogera wogura) - although, not, apparently, in the eastern mole.

Quite what the point of this is is a little unclear, although the glands do shut down during the breeding season and when the female is pregnant. Being awash with male hormones for the rest of the year though, does lead female moles to develop some masculine anatomy as they grow; they have an enlarged clitoris, and what appears to be a prostate gland. None of this hampers their ability to be good mothers, though, because moles have a higher reproductive effort (that is, the mass of offspring one mother can produce in her lifetime) than their closest relatives, the shrews. This seems to be due, at least in part, to the production of some very nutritious milk, and may offset the high death rate of young moles when they leave home and make the dangerous journey above ground to find somewhere to establish their own burrow.

They have to make their own burrow, because moles are pretty anti-social animals, and adults normally only meet up to mate. Having said that, they don't need particular large burrow systems, so there can be several moles in a relatively small area. Population densities of 5 to 25 moles per hectare have been reported for the European mole, and 5 to 12 for the eastern mole. Moles, of course, are mostly carnivorous, eating insects and worms, largely, it would appear, by hoping that they accidentally wander into the tunnels.

While the less subterranean members of the mole family are basically nocturnal, its a little more complicated for the true moles. Day and night aren't really an issue for them, and they sleep three times each day for most of the year - although males can be active for rather longer periods when they're looking for a mate.

Because of the protection that hiding below ground offers them, and the fact that they generally don't mind agricultural land, most mole species are not overly threatened by human activity - which isn't to say that farmers don't poison them, just that they don't do it enough to place entire species at risk of extinction. The clearest exception is the Echigo mole (Mogera etigo), which lives in only one small area on the west coast of Japan's main island, Honshu. It prefers soft soils, and changes in agriculture in the region make those less common than they used to be, and the animal is officially considered Endangered.


[Pictures from Wikimedia Commons. Cladogram adapted from Mikko's phylogeny archive and Motokawa 2004]

Endangered Cats of the High Andes

A few weeks ago in the UK, we had the 10-yearly treat of filling in our national census forms. Once all the information from these forms is collected, we will have an official figure for the number of humans living in our country. Unfortunately, finding out the same figure for wild animals isn't quite so easy. Yet studying the population densities and numbers of wild animals is often of more than academic interest. A great many species are endangered, and without accurate ideas of how many of them are, we can't tell how endangered they might be, what strategies we might have to use to protect them, and whether or not those strategies are working.

Of course, animals can't fill in census forms, but there's more to it than that. If the animal lives in large, highly visible herds, it may not be so difficult - although even there, you have to be sure you're not counting the same animal twice, and that you're picking a suitably representative area to study (since, realistically, you're not going to go out there and do a head count of every such animal in existence). But many animals aren't so obliging. They may be small, they may actively hide from humans, they may be nocturnal or crepuscular, or they may just live somewhere its hard to get to, or in a jungle so full of plants its hard to spot anything. The rarer they are, the harder it becomes to find them, and yet those are exactly the ones we're most interested in.

Horns of the Thunder Beasts

Megacerops, one of the largest brontotheres

Today, the group of odd-toed ungulates - those large mammalian herbivores that support most of their weight on their middle toes - is represented by just sixteen species in three families: the horses, rhinos, and tapirs. But they were once much more extensive, with whole families of animals that didn't make it through to the present day. One of these was the brontotheres, a family whose name literally means "thunder beasts".

These were big, heavily built animals, some of them standing over six feet tall at the shoulder. In many respects, they would have looked quite like rhinos, having a rather similar body-shape, and crucially, large blunt horns on their snout. However, the horns weren't really so rhino-like as they appear, since the horns of rhinos are actually made of compacted hair, while those of brontotheres were made entirely of bone. The surface of the horns are rugged, suggesting that they were probably covered in skin during life, which perhaps makes the closest living parallel the projections on the heads of modern giraffes.

In the grand scheme of things, the brontotheres didn't last very long, dying out around 34 million years ago, but they must have been very impressive animals when they were alive. Despite their appearance, they seem to have been more related to horses than to rhinos. This is more apparent when you look at the very earliest brontotheres, which were relatively small and hornless, and do look rather more like the early multi-toed ancestors of the horses.

Muscular Jocks and Randy Nerds - the Sex Lives of Wild Guinea Pigs

Brazilian guinea pig

The domestic guinea pig (Cavia porcellus) is a fairly familiar animal, but they've changed enough in the 5000 years of their existence that we don't actually know quite what they were domesticated from. There are at least five species of wild guinea pig alive today, and while one of them was presumably the origin of the domesticated form, we don't know which it is. In addition, the guinea pig family includes eight other species with a fairly similar appearance, and three that are manifestly different, including the largest living rodent, the mighty capybara.

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
     ^             ^              ^             ^          |
     |             |              |             |          |
     |             |              |             |          |
     ---------------              |             ------------
            |                     |                  |
            |                     |                  |
            -----------------------                  |
                       |                             |
                       |                             |
                       -------------------------------
                                      |
                                      | 

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"

What of the largest rodent of them all, the capybara (Hydrochoerus hydrochaeris)? As you might expect for something weighing 140 pounds, capybaras don't look that much like guinea pigs, and they were once thought to belong to a different family altogether. Nonetheless, genetic analysis has shown that they are, in fact, gigantic, semi-aquatic relatives of the rock and climbing cavies.

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]