Follow the Leader?

By human standards, the majority of mammal species are comparatively antisocial. Some actively avoid others of their kind outside of the mating season, but even those that are more tolerant are often found together in one place purely because that is where the food happens to be. But, of course, there are a great many exceptions to this; animals that habitually live in groups that socialise and travel together.

Animals that live like this have to have some form of decision-making process that all members of the herd, pack, or other grouping choose to abide by. The most obvious example of this would be deciding when and where to move, but it could also include, for example, determining the best way to escape predators. Lacking the sophisticated communication methods of humans, concepts of debate aren't likely to be applicable, but the decision has to be taken somehow, and, over the years, there have been many studies to determine just how egalitarian the process is and exactly which animals within the group are making the decisions if it isn't.

According to a recent review of such studies, there has been far more focus on some types of mammal than others. In part, this is due to the uneven distribution of group living across the mammalian family tree - questions about leadership and group decision-making aren't going to be relevant to animals that live their lives alone. But there are other biases as well, due to what sorts of things researchers are likely to be interested in, and how easy the animal might be to study. 

The review showed that nearly half of all studies on animal leadership are conducted on primates - mostly monkeys, but also lemurs and apes. One can easily see why those would be a favoured topic of research, given the likely richness of primate interactions and their obvious relevance to our own species. Just over a third of studies are conducted on cloven-footed herd animals, including those on farms (sheep, goats, cattle) as well as their wild relatives such as bison and deer - although there have been relatively few on antelopes, probably because there are so few in Europe and North America. 

The remaining fraction are more or less evenly split between horses, pack-hunting carnivores, and aquatic mammals. The last of these, of course, are going to fall into the "difficult to study" category, as would bats, which seem to have been almost totally ignored - presumably for this very reason.

When we're looking at a broad sweep of different species like this, there will inevitably be subtle differences in how each of them behaves so it can be useful to try and fit these into general categories. When we do this, we find that the most common pattern is for each group to have a single, identifiable leader that takes all the decisions and expects everyone else to follow them. 

We see this in herd animals, such as zebras, the herd stallion not only leads the group towards sources of water, but also tries to keep the herd together and goes so far as to threaten unruly subordinates who might sow discord within the group. In that case, the leadership position is long-lasting, but it doesn't have to be; it could switch between different individuals as the situation demands. In muskoxen, for example, dominant females are in charge of deciding when and where the herd moves in search of forage - but it doesn't have to be the same individual every time. There are, in effect, a small number of potential leaders who take turns. Among primates, female gibbons also take turns leading their groups, with pregnant or nursing individuals being prepared to take a back seat to focus on what's presumably more important to them.

Among wolves, we have the myth of the "alpha male", a term that used to be common in the scientific literature but that turns out to be rather misleading. In reality, the dominant female is just as likely to lead the pack to new hunting grounds as the dominant male and a better term for the resulting "alpha pair" turns out to be "parents". 

One would expect that the individual to physically leads the group, by heading to a new location, would also be dominant in other ways. This is certainly the case with wolves and it's also seen in many primate societies, as well as in cloven-footed herd animals such as cattle and sheep. However, this doesn't have to be so, and not all group-living mammals necessarily have a defined dominance hierarchy anyway. In which case, how do they decide who among them gets to choose where they should go next?

In some primate species, such as red-fronted lemurs, it has been proposed that the decision to move to new foraging grounds is made by the individual with the greatest investment in the answer - females that are pregnant or nursing young. Among Tibetan macaques, social connectivity appears to be the answer; individuals may not be more "dominant", but if they are more generally popular and have numerous close associates, enough other animals will follow when they move that the entire band is obliged to do so as well. It's almost rule by celebrity, rather than by coercion and physical dominance.

Another possibility, which might seem rather radical by human standards, is to follow those who know what they're doing. This has been proposed for sifakas (close relatives of lemurs) where it may be that females lead because they will have grown up in an area, and know where the best food or shelter is likely to be, while the males leave home when they reach adulthood and are less tied down to a particular place. A similar situation is seen in vervet monkeys, where there seems to be a distributed leadership where a group of experienced females (rather than a single individual) collectively decides where to go based on their knowledge of the environment.

This sort of oligarchic setup seems to be comparatively rare, but there are some mammal species that apparently do without leaders at all. Meerkats use what has been described as a "quorum" system, whereby if enough individuals signal their intention to do something by calling to one another, the entire group decides it's a good plan. Of course, they are unlikely to be literally counting the number of votes, it's more a rough idea of "that's enough to be worth listening to", but the idea is the same - the greater the number of individuals wanting something to happen, the more likely it is that it will. In a similar vein, one of the few studies of leadership among bats suggests that at least one species decides where to roost simply based on whatever is most popular with the bats that got there first.

African wild dogs also take group decisions on whether to leave an area,  sneezing to vote "yes". Although it's clear that the "vote" of a dominant individual is worth more than that of a subordinate one, they don't always get their own way if enough others disagree.

This is also what we would probably expect of group-living aquatic mammals, be they dolphins, whales, or seals. Out in the open waters, the lack of any boundaries and the three-dimensional nature of the habitat make controlling a territory difficult at best, and it's hard to monopolise control over resources that are equally mobile. As a result, aquatic mammals are not territorial and they usually have little, if anything, in the way of dominance hierarchies. 

Decisions such as when to dive into an extreme habitat where food may be patchily distributed are clearly going to be important to such animals. We know that they do, like land-dwelling herd and pack animals, coordinate their movements where they live in pods or other accumulations, but having a single decision-maker for such issues seems unlikely for animals that are generally egalitarian. 

In the case of seals, which certainly do have dominance hierarchies when it comes to mating, there do not appear to be any studies about leadership decisions out in the ocean. But, despite the difficulty of the studies, we do have evidence of this behaviour among dolphins - and it turns out that central leadership is much more common among such animals than we might think. In bottlenose dolphins, for instance, decisions about when to travel are apparently made by a single male individual. Which individual that is seems to be determined by their experience rather than being "dominant" in other respects over his fellows. 

Knowledge and experience, it seems, trump physical prowess or general bossiness among dolphins.

Similarly, in killer whales, it's the older, menopausal, females that lead the pod. Their ability to lend the benefit of their accumulated wisdom to their fellows is probably the main reason why killer whales even have a menopause; in almost every other mammal species, individuals don't survive past reproductive age, since they can no longer pass their genes on to their descendants, making them evolutionarily redundant. 

Normally, by consuming food that their children could have instead, a menopausal mother would actually harm her offsprings' chance of survival, but if they can help them survive by leading them to better food sources and knowing what to avoid, their accumulated wisdom can be a plus. Indeed, we see something similar among elephants, one of the few other mammals to experience menopause, although here, there's a clearer dominance hierarchy.

Having said which, it might not always be so simple. There is evidence that bottlenose dolphins, for instance, do have personalities, with some being shy and others more outgoing, so it's entirely plausible that the latter may be more inclined to "take charge" even if they aren't quite so experienced. Sperm whales also manage to coordinate their movements despite the fact that their groups consist of individuals that are literally miles apart from one another. Presumably, they do this by signalling audibly to one another, but so far as we can tell, they aren't following a single leader when they do so.

Sperm whales, at least, seem to have something approaching democracy, even if the slow and slightly confused manner in which their groups change direction at sea implies that it's about as messy for them as politics can sometimes be for us. 

[Photo by "Touhid biplob", from Wikimedia Commons.]