Sunday 20 August 2023

Love on the Mountain Tops

Caprines - members of the goat subfamily - are amongst the mammals most adapted to harsh environments, with the majority of species adapted to living in the cold, barren, and precipitous slopes of mountains. There are some exceptions; sheep (which are taxonomically a subtype of "goat") originally evolved to live in barren rocky hills rather than on true mountains, while some of the East Asian species inhabit forested slopes. 

There are, as with many animal groups, more species of caprine than one might at first think, and I covered them all individually about ten years ago. Looking through that series, it should be possible to appreciate that the group is also varied, not only inhabiting a range of environments but also living varied lifestyles, from those that are near-solitary to those that prefer large herds. This is also reflected in their mating habits which, are as one might expect, related to the size of the community in which they live. One would also expect that the habitat would have some effect on how the animals choose to live, and, in turn, on that mating behaviour.

Studies comparing how habitat shapes sociality and mating behaviour across different species are not unusual but, inevitably, some groups of animal have been studied more than others. Birds and primates, for example, have garnered a fair bit of attention in this regard. Caprines are members of the cattle family and here, perhaps the primary such comparative study is one that dates back to 1974. That looked at African antelopes, a group that's even more varied than the caprines, but none of which live in such a harsh environment. There are dry seasons to contend with in many parts of Africa, but the extreme seasonality of temperate zone mountain tops and the patchier distribution of food put the caprines in a different situation.

Nonetheless, the 1974 study provides a framework for thinking about mating arrangements in horned mammals and how those are shaped by the local ecology. While it's not true to say that caprines have been ignored in this regard in the last fifty years, it's only recently that a review looking at them specifically has been published, to see if the general principles that hold for antelopes in the savannah and jungle also hold for goats on cold, barren, mountainsides.

Perhaps the most basic way of classifying mating systems is one I've mentioned here many times before: dividing them between monogamy, polygyny, polyandry (which is rare in mammals), and promiscuity. Because we tend to think of goats as herd animals, it's natural to assume that males are going to be in contact with a lot of females, leaving little motivation for them to stay monogamous. That is true of most species, but not of the serows (Capricornis spp.), a kind of forest-dwelling goat native to eastern Asia.

This is because serows live, at best, in very small herds, and most are solitary. They can do this because their habitat is, by the standards of goats, a rich one, where it makes sense to hide from predators in vegetation and defend small patches of high-quality food. This means that the sexes have territories of approximately the same size, and the uneven distribution of that high-quality food means that, on average, each male's territory only has one female living in it. Thus, serows end up being monogamous and may form long-lasting pair bonds.

It's interesting to note that it's generally thought that serows are the goats that most resemble the ancestral caprines that first evolved during the Miocene, having changed less than other species since that time. As most other caprines headed out to more open habitats, however, we see a change in their behaviour. Hiding from predators becomes much more difficult with the lack of bushes or trees, and it's harder to defend a wide stretch of grassland than it is a small clump of bushes. 

Now males are exposed to many more females at a time, and potentially could mate with any of them.  Thus, for example, American mountain goats are strongly polygynous, with the males competing to monopolise access to as many of the females as they can get away with. On the other hand, chamois live in what might best be described as "semi-open" habitats close to the treeline, feeding on bushes that can be defended but in places where there isn't quite enough vegetation to hide them from predators. This is probably an intermediary stage in caprine evolution between the solitary serows and the more typical goats, and the result is smaller herds and a mating system that's only weakly polygynous, with it being difficult for males to monopolise too many females, but at least giving it a go.

In fact, it has recently been shown that where serows venture out into alpine meadows from their preferred woodland habitats, not only do they group together more, but the males start to act polygynously. This provides a picture of how the shift from one mating pattern to the other might have happened, as the relative dispersion of the females affects how the males respond to them.

However, while most caprines are polygynous, this does not necessarily mean that the males of every species behave in the same way. This is because a male animal wishing to mate with several different females has multiple strategies available to him, and different species may employ different ones. One way of classifying polygyny identifies four such possible approaches, not all of which are mutually exclusive.

Firstly, there is "group-guarding", where the male stands over a herd of females and drives away rivals. Secondly, there is "range-guarding", where the male patrols a territory that is so much larger than that of the typical female that he has several available partners so long as he can keep[ intruders out. Third comes "female-guarding" which basically means serial monogamy and can include "sneak mating", where weaker males dash into a guarded territory for a quickie and try to get out again before being spotted. Finally, there is "site-guarding" where the male either defends some essential food or water resource that females will have to approach eventually or simply stands about acting sexy and hoping that works ("lekking").

It turns out that, among caprine species, female-guarding is by far the most common tactic employed. This, too, is probably due to the habitat in which they live. Food sources are often temporary in the mountains, and the females wander about so much in search of new forage that it's probably difficult to guard more than one of them at a time - the male's best bet seems to be to find a receptive partner and stay with her for long enough to mate before moving on. 

There are, however, exceptions. Chamois and serows, living in more vegetated terrain, practice resource-based site-guarding because, with the food being relatively permanent, that works, and probably isn't much effort. Gorals, Asian animals that live in a similar habitat to chamois, apparently go for range-guarding at least some of the time, although this doesn't seem universal

Group-guarding might seem another obvious tactic - collect a harem from the herd and stand watch over it. However, in mountainous terrain, it's probably easy for at least some of the females to wander off behind rocks or cliffs where the male can't see them, rendering it all a bit pointless. The only caprine that seems to do this, therefore, is the muskox - not coincidentally the only species to live on flat, lowland terrain. As for lekking, which is common enough in deer, caprines just don't do this, although their closest living relative, the chiru, does, perhaps because of their comparatively high population densities.

The mating system of mammals can, in evolutionary terms, affect more than just their behaviour. In polygynous species, since the number of males and females is usually about equal at birth, there is intense competition to become one of the minority of males that get to mate at any given time. This, in turn, leads to males becoming larger than females, the better to fight off their rivals, and there is a general rule that, the more polygynous the species, the greater the disparity between the size and appearance of the sexes.

Although the data is incomplete, this rule seems to hold for the caprines. Serows are noted for having very little difference in size between males and females, with the two sexes being hard to visually distinguish. Chamois, with their weakly polygynous mating system are not much different, although the males do put on more weight around the rut; in their case, this lack of difference may be enhanced by the fact that male-male competition seems to favour agility and speed more than it does raw physical power.

In most other caprines, the males are consistently larger than females year-round and, moreover, tend to have much larger and impressive head ornamentation. To measure which species are the most polygynous, we would need to know, for each of them, to what degree the most successful males are able to monopolise mating opportunities - something that can be determined through paternity testing (to rule out any "hidden" successes by those using the sneak-mating strategy). Unfortunately, we only have these numbers for bighorn sheep and Alpine ibex, where the bias is certainly strong; it would be interesting to compare these with, say, American mountain goats.

It's probably worth pointing out that all of the above takes a male-centric view of mating strategies. That may be a bias on the part of (mostly male) researchers, or just the fact that it's easier to observe, especially where the species is polygynous and the males are visibly competing with one another. It's likely, however, that the females get at least some say, too. In pronghorns, for example, which are also polygynous, there is some evidence that females "egg on" the males to fight, encouraging them to prove their prowess so that they can mate with the winner. Might goats do the same, or affect things in more subtle ways? 

That much, for now, remains a mystery.

[Photo by Manfred Werner, from Wikimedia Commons.]

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