Saturday, 1 October 2011

Following the Herd

A number of mammal species live in large groups, including dolphins, monkeys, and wolves, among others. Among the most familiar, though, are the various hoofed herd animals. In order to maintain a herd - or any similarly sized group - it is important that the animals all move together, and that there has to be some kind of communal decision-making process that everyone agrees on. It's no good one animal wandering off on its own, if nobody will follow it, but, equally, if all animals have an equal right to decide where the herd should go, its just going to mill about, not going anywhere.

Undoubtedly, different species will have different methods for making such decisions, depending on their biology, the nature of the environment, and so on. If the group is really big (as might be the case with, say, bison or wildebeest), options are fairly limited, and there is unlikely to be one single leader - if only because not all the members of the herd will be easily able to see him. Among animals that live in smaller groups, leadership by single individuals, or by a small group of individuals, becomes more of a realistic possibility, although alternatives do exist. But which individuals do the leading?

Perhaps surprisingly, there have been relatively few studies of this sort of thing in wild herd animals. Certainly, there have been studies on domestic cattle, and the like, and a number on wild primates, such as baboons. Mathematical modelling using computers, has also been fairly common - a process that has been quite effective at predicting the behaviour of human crowds. But wild herd animals, in wide open countryside, can be fairly difficult to observe. One recent exception is a study by Claudia Ihl, of the University of Alaska Fairbanks, and co-workers, examining one the large ungulates native to that state.

You might think, looking at musk oxen (Ovibos moschatus), that they are some kind of cattle species, perhaps related to yak or buffalo. Indeed, they are members of the cattle family, but then so is almost everything with horns and cloven hoofs, including, for example, gazelles. However, within that family, musk oxen are not true bovines - that is, they are not closely related to bison, domestic cattle, and so on. Instead, the musk ox is the world's biggest species of goat.

Well, perhaps calling it a 'goat' is stretching a point. But it is a caprine, belonging to the same group as the goats and sheep (and a sheep is, biologically speaking, just a goat that can't climb mountains). Essentially, if you make a goat large enough and heavy enough, it's going to look somewhat like a cow, and that's what we have with the musk ox.

Goats,    Chamois,     True      American
 etc.       etc.       Sheep   Mountain Goat
  ^          ^           ^           |
  |          |           |           |        Serows       Musk
  |          |           |           |        & Goral       Ox
  ------------           -------------           ^           |
        |                      |                 |           |
        |                      |                 |           |
        ------------------------                 -------------
                   |                                   |
                   |                                   |

Exactly how the serows and goral (both from eastern Asia) relate to each other is a little unclear, but there seems little doubt that they are the closest living relatives of musk oxen, despite their much more obviously goat-like appearance. On the other hand, when it comes to the larger groups, the above scheme is not universally accepted, and it may even be that musk oxen are actually closer to goats than 'true' sheep are. I say 'true', incidentally, because there are a number of animals, such as the barbary sheep, that have 'sheep' in their name, but that do not belong to the same evolutionary branch as the domestic sheep and its wild kin.

Musk oxen live in herds of up to forty members, although most seem to average about half this size. They inhabit cold barren environments, with relatively little in the way of vegetation, which makes their decision as to where to seek food particularly important. The herds in this study all live in the Cape Krusenstern National Monument, a region of lagoons, low hills, and long ridges along the western coast of Alaska, dominated by open heathland vegetation. The study took place only during the summer and early autumn, so we don't know whether the animals behave similarly during the winter - when, aside from the cold, there may be as little as an hour and a half of daylight.

Be that as it may, for much of the summer, the herds consist primarily of females and their calves, with just a small number of adult males. The remaining males, presumably, are off on their own, or at best, in small bachelor herds. That changes as the rut approaches, normally around the middle of August. Now those isolated males begin to gather females, luring them away from their existing herds. The end result is that herds are only about half size - ten individuals or so - during the rut, but that each herd contains a higher proportion of adult males.

That the herds are predominantly female, even during the rut, means that, all things being equal, we would expect to see females more likely to be the ones leading a herd off in a new direction. This does indeed appear to be the case, but to an even greater extent than their numerical superiority would suggest. For short movements, in the direction of a tasty-looking patch of tundra grass, this isn't quite so noticeable. True, the females are more likely to be the ones in front when the herd starts moving, but, once you take into account their greater numbers, not by very much.

For longer distance movements, such as heading off in search of an entirely new pasture, the pattern is much more pronounced. For most of the summer, such movements are almost always led by the females, with the males just content to follow. There does not, however, seem to be a single leader for the herd; pretty well any female over two years of age can decide to head somewhere that looks interesting, and have a reasonable expectation that the others will follow.

That it's only adults doing the leading is hardly surprising; they're more likely to know where they're going, and if a calf wanders off, the most sensible thing to do is to fetch it back to the herd before it gets eaten by a wolf or grizzly bear. Yearlings too, don't bother to lead, again most likely due to a lack of experience. But why should the females be more likely to lead than males of the same age?

There may be a couple of reasons here. Firstly, they might be more hungry than the males, having a greater need to keep themselves fit in order to raise young - the so-called 'gastrocentric model'. However, it seems at least as likely to me that baby musk oxen learn to follow their mothers, as well they might, and never quite shake the habit. Musk oxen herds, like those of most other mammals, are based around a core of related females, drawn together by sisterly and motherly bonds, while the males wander off in search of someone they're not related to.

But all of this changes during the rut. Suddenly, the males are more likely to take charge, although they are far from getting their own way all the time, especially when it comes to the longest of movements. But not only are the males more likely to decide where to go, they don't make their decision known in the same way. The females seem to move on a basis of consensus and trust in experience - somebody decides its a good time to move, and the others follow. The males, however, prefer aggression.

Their main motivation here is evidently to stop females leaving them, or worse still, finding another male. If a female tries to leave, they will block her path or chase her about, and continue doing this until she finally gives up, and goes where he wants to instead. Initiating mating may be another tactic to distract the females, but overall, its clear that the males have to go to a lot more effort to get anyone to pay attention to them. For most of the year, they just can't be bothered, but during the rut they have plenty of motivation to keep the females where they can see them.

[Picture from Wikimedia Commons. Cladogram adapted from Laluleza-Fox, et al, 2005 -  for an alternative view, see Ropiquet & Hassanin, 2005]

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