Defending the Troops

A few weeks ago, I talked about how group-living mammals decide when and where to move, and how their decision-making leadership is structured. But there are other aspects to how animals living in a herd or pack might travel, or, indeed, position themselves when they are not travelling. One of these is the perceived risk of predation. 

It's well-known that predators will tend to pick off weaker individuals if they can, largely to save themselves the effort of capturing something that's more able to escape or fight back. But it's also likely that some positions within a herd are going to be inherently safer than others, and merely being fit may not help much if you're an obvious target. The question then arises as to whether certain sorts of individual are likely to occupy safer or more dangerous positions and as to how the group as a whole arranges itself.

However, which position in a herd is the most exposed may depend on what sort of predator it is that you're worried about. In the case of large mammalian predators, there are basically two different strategies that they use for capturing prey, and these have different implications for how herding herbivores might choose to defend themselves. Firstly, there are pursuit (or "coursing") predators, such as wolves, cheetahs, and hyenas, which will pursue their targets and try to outrun them. Here, the main factor that affects a prey animal's risk of being attacked is simply how close to the edge of the group they happen to be. 

Ambush predators, on the other hand, tend to sneak up on a group, relying on taking down their target almost immediately, and using cover to get as close as possible before doing so. Here, the primary risk is not being close to other prey animals - if the group is spread out, the ambush predator could manage to get well inside it without being spotted and strike without warning at individuals nowhere near the edge. In the case of large land mammals, the primary concerns here are cat-like predators such as leopards or tigers; for smaller ones, eagles or owls might be a bigger risk, since being in the middle of the group isn't going to help at all.

Another factor to consider is that there may be other reasons for taking up a particular position within a herd or other grouping. The dominant individuals, for instance, may tend to be at the centre, where they can best take control of the group and see what most of their subordinates are up to. Or, then again, if the group is travelling, the dominant individuals may well be at the front, leading the way. In that position, the chances of coming across an ambush predator unexpectedly are presumably at their highest, but, equally, it's the animals at the back that have the most to fear from pursuit predators, which will likely figure that anything having difficulty keeping with their fellows is an ideal target.

Given all of these possible factors, it's unsurprising that studies of even how a single species gathers together have often been ambiguous or contradictory - a lot may depend on the exact circumstances. But this has not, of course, stopped researchers from trying to answer such questions.

A study published last week looked at the grouping behaviour of olive baboons (Papio anubis) in Kenya, using GPS collars to avoid the risk of the animals changing their behaviour when they thought that humans were watching them (given that humans are, themselves, something that baboons are going to be wary of). The olive baboon is the most widespread of the six species of baboon, being found across a wide swathe of Africa just north of the equator, from Guinea in the west to Ethiopia in the east, taking in 24 other countries on the way. They live in large mixed-sex groups, sometimes of over a hundred members each, with a complex social structure, and their primary predators are leopards, lions, and spotted hyenas, in that order.

The baboon troop in question lives in a wide area of scrubby bush near the Mpala Research Centre in Kenya and has been studied for various purposes since 2011. At the time of the study, it consisted of eight adult males, sixteen adult females, and 34 adolescents and infants. All three of the major predators of baboons also live in the area, although, at least in modern times, they are not very common and mostly focus on other local prey.

Tracking the movements of the group - or at least the 26 of its members that had been fitted with the GPS collars - showed that the adult males tended to position themselves at the outer edge of the troop, leaving themselves more exposed to any potential predators. This is not surprising, as we see a similar pattern in vervet monkeys, where males stay on the outer edge of a group even when it is stationary, and they aren't leading the way during travel. Indeed, we also see the same pattern of behaviour in other mammals, such as male gazelles, which are, indeed, more often killed by cheetahs as a result.

Why would this be? There are at least three possibilities, and they are not mutually exclusive. One is that it's a simple trade-off, a consequence of the fact that the males of most mammalian species are larger than the females. In olive baboons, for example, the average adult male weighs about 24 kg (53 lbs), while a typical female weighs only 15 kg (32 lbs). Fully-grown males, therefore, need more food, and it's better for them to forage at the edge of the group where there is less competition, even if it means that that puts them at greater risk.

But that may not be the only reason. Especially if the baboons are worried about pursuit predators, such as hyenas, it makes sense to have the biggest, strongest individuals at the outer edge of the group, so that they can protect their more vulnerable troop-mates further in. Many of the youngest animals are likely to be their own children, which they naturally have an interest in protecting, but even if they aren't, no male is likely to want his potential mates to be killed, either. Such behaviour also makes it easier to look out for rival troops, whose males may want to take over their own.

The third possibility is related to the second: the males may simply be showing off, proving their bravado and physical fitness by exposing themselves to danger in the hopes of impressing the females. Because some primates do that sort of thing.

But it's not just pursuit predators that male baboons put themselves at risk from. It would still be possible for males to be reasonably close to one another while staying at the edge of the group, assuming they didn't space themselves out evenly. But, in fact, in this group, they did space themselves out, while females and young were more likely to clump together, gaining protection in numbers from ambush predators that might sneak up on them. Being relatively isolated isn't going to help the males protect anyone else more effectively, although it would give them more access to food sources, and it fits with the "showing off" hypothesis, too. At least as likely, though, is that the adult males in a troop tend to be aggressive towards one another, since they are not close relatives (they will have been born outside the troop and moved in on their own, while the females stay living with their sisters).

It's all very well being exposed to predators, but the males would probably rather avoid being exposed to each other, too. There's no point getting into a fight if you don't have to.

The researchers expected that the dominant male in the troop would be the exception to these general rules. He shouldn't have to worry about other males pinching his food or otherwise trying to challenge him since, most of the time, they wouldn't dare. So staying in the middle of the group, where he can protect his children and have greater access to the adult females, makes a lot of sense. (Not even necessarily for the obvious reasons; male baboons will apparently leap to the defence of a female even if they don't expect anything in return). 

But, in this case, that isn't what happened; the alpha male stayed near the edge of the troop, just like his subordinates. The researchers speculate that, since the dominant male in this group hadn't been alpha for long, he didn't yet have enough children to make it worth his while... but it's equally possible that our intuitions in this area may be wrong, even if they do seem to hold for some other primate species.

Rank may have its privileges, but, in this one baboon troop at least, that doesn't include keeping yourself safe behind the front lines.

[Photo by Harvey Barrison, from Wikimedia Commons.]