Staying Away from the Boys

Many large, hooved mammals live in herds. This has obvious advantages of safety in numbers, spreading the work of looking out for predators, as well as benefiting from the knowledge of more experienced herd members as to the best places to find food or shelter. The structure and composition of these herds vary from species to species, but one common trait is that males and females often form single sex herds that travel apart for much of the year.

This is referred to as "sexual segregation", and was first formally described by Charles Darwin in The Descent of Man. It isn't unique to mammalian herd animals, being seen in everything from fish shoals to bird flocks, as well as in non-hooved mammals (dolphins, bats, primates, etc.) Most zoological research, however, has tended to focus on large cloven-hooved mammals, such as deer and antelopes.

The Sounds of Mother and Calf

While for humans, speech is our primary means of communication, for many mammals scent-marking is more important. This is not to say, however, that audible signals are irrelevant. This can be especially true for social mammals (such as ourselves), which often have a need for something more immediate and complex than the sort of long-lasting "keep away" or "I'm ready to mate" signals that scent marks can provide. Communication through sound can help keep a herd together, signal aggression, provide instant warnings, and so on. It's key to many primates, for instance, because most of them live up in the trees where it may be difficult to keep sight of all troop members when they are out foraging.

It's also important for many hoofed herd animals and, at least in the wild, few North American mammals are more sociable than the bison (Bison bison). While herds are no longer as vast as they were 200 years ago, recovery plans for the species are underway, and, in many cases, may rely on some degree of fencing or other containment at least for the time being. Understanding bison behaviour, including communication, could help with that, making it easier to assess how comfortable the animals are feeling - and, perhaps, the likelihood of a 750 kg (1,600 lb) bull deciding it's had quite enough of that fence and heading off somewhere it doesn't realise is less safe.

Antilopine Antelopes: Tommy's Gazelle and Relatives

Thomson's gazelle

You probably don't need to live in Africa to be aware that there are a great many different kinds of antelope. (A couple of years ago I came across an online picture quiz of "can you name these African animals?" Over half of them were antelopes.) It's hard to say which of these are the most familiar to the general public, because quite a few of them probably are, at least in general terms. But one subtype of antelope that people will at least recognise are the gazelles.

Gazelles are smallish, fleet-footed animals; the word comes from the Arabic ḡhazāl, which literally means something like "slim/agile creature". Gazelles are widespread, perhaps surprisingly so, and there are many different species. Of these, the one that may be the most familiar to people outside of Africa is Thomson's gazelle (Eudorcas thomsonii) for the simple reason that it's the one that lives in the Serengeti and therefore gets into a lot of wildlife documentaries. Mostly getting eaten by big cats, to be sure, but it's a start.

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.

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.

Coming Down the Mountain

One of the features of behaviour that I've mentioned a few times while covering deer species this year is that, typically, the males and females spend much of their lives apart, only meeting up during the rutting season. While, clearly, many group-living mammals don't do this, deer are hardly unique, with many other species (and not just mammalian ones) having a similar lifestyle. Females gather together in herds, or whatever the group name is for the animal in question, where they can gather together to protect and nurture their young. Males of such species often live in bachelor herds, which are typically smaller.

Sometimes this is due to a simple imbalance in numbers. In species where males monopolise multiple females, a typical herd, while predominantly composed of females, will also have a single dominant male, forcing any subordinate males to live elsewhere. But this is not the case in deer, because, while stags do indeed mate with as many does as they can get away with, they don't live with them outside the rut. Nor are they alone, since other herd animals, such as goats and antelope, often do the same.

Let's Split the Party!

Last week, I described how, over the last few decades, populations of gray seals have recovered from a near-total local extinction in US waters. Clearly, other species facing similar threats, across the world, have not been so lucky. The rapid pace of human expansion simply overwhelms the ability of some species to cope, whether it be through direct effects (expansion of agriculture, cities, transport networks, and so on) or indirect ones (climate change, most obviously).

In the millions of years before humans appeared on the scene the world was changing at a slow enough pace that it was possible for mammal species to evolve their way out of a problem. Many didn't, of course, and went extinct, but others gave rise to newer species that outlived them. Today, it's not just the scale of the changes humans are making that has caused problems for some species, but also their speed.

When it comes to dealing with very rapid changes in the environment - whether human-caused or otherwise - mammals have both advantages and disadvantages compared to some other species. On the negative front, mammals, especially the larger ones, have long generation times, meaning that any effect of evolution, even on a relatively minor scale, is inevitably going to be slow.

Not the Pig Family: Peccaries of the World

Collared peccary

I've already described the anatomical and biological differences which mean that, despite appearances to the contrary, peccaries are not pigs. I've also described all the various living species of the pig family, which means that it's now time to do the same for the living species of the peccary family.

The peccary family is, however, a lot smaller than the pig family: it contains just three living species.

The one that's likely most familiar to readers of this blog is also the most widespread: the collared peccary (Pecari tajacu). It's so-named because of the thin 'collar' of whitish hair that runs from its shoulders across the throat, although, in practice, this isn't always as visible as it might be. The animal is found through pretty much the whole of central and northern South America east of the Andes, through most of Central America, coastal (but not central) regions of Mexico, and into neighbouring parts of the USA, where it can be seen in Arizona, New Mexico, and Texas. If a distinction is needed, this is the true "javelina", since it's the one that's common where that alternative word for "peccary" is most commonly used.

No Boys Allowed

Even in mammals that are otherwise social, it is quite common for males and females to live apart for most of the year. The most common pattern involves females living in groups with their children, until the latter approach adulthood, while males either live alone, or in much smaller bands, outside of the mating season. In many hoofed herd animals, however, the males live in herds that aren't much smaller than those of the females - in other words, both sexes live in herds, but the two don't mix until it's time to do the necessary.

Clearly, these are creatures that benefit from the group protection that living in herds provides, and, while there's obviously a limit on how large a herd can be before there isn't enough food to support them all, it's not always so clear why the herds should be single-sex. Why, in short, do so many hoofed herd animals practice sexual segregation?

There are a number of theories, and, as is often the case, they aren't mutually exclusive. Nor is it likely that the reason - or the balance of reasons, if there's more than one - will be the same for every species. It's something we have to examine on a case-by-case basis.

The Social Lives of Elephants

Indian elephant

Elephants are the largest land-dwelling animals alive today. They are also among the most intelligent non-primate species, with complex social lives, and, as I noted in passing a few weeks ago, seem to be able to pass the Mirror Test for self-awareness - the only mammal, other than apes or dolphins, for which there is reasonable evidence of this ability.

There are many reasons that we'd like to know more about the lives and habits of elephants; even leaving pure curiosity aside, on a practical level it might help us to find ways to manage elephant populations so that they can peacefully co-exist with our expanding agricultural and residential footprint.

But there is a problem to really getting to grips with elephant "societies" and how they function. That's because, in addition to being large, elephants are also remarkably long-lived. If you go out into the wild and study a herd of elephants for five years, you will get a lot of information, but it's really just a snapshot of what's going on in the course of their lives. Elephants can live for at least seventy years, and a generation lasts for about 25 years, so getting a really good picture of an elephant's life would take... well, a human lifetime.

The Kindness of Wallabies

The most dramatic encounters between wild animals often involved physical conflict of some kind. Animals fight for all manner of reasons, by no means limited to the eternal struggle between predators and prey. Even within the same species, there is often conflict, typically over access to things such as food or mates.

Such interactions are called "agnostic". (In an oddity of the English language similar to the flammable/inflammable divide, "agonistic" and "antagonistic" sound like they should be opposites, but actually have exactly the same meaning). In the case of solitary animals, they are often the main form of interaction within the species other than mating. But, for animals that live in groups, it necessarily has to be different.

Even if we ignore any breeding season, when competition, especially among males, is likely to be intense, animals within a group are still likely to come into conflict from time to time. In order for the group to remain cohesive, there has to be some mechanism to defuse this tension, to ensure that the pressures of competition don't outweigh the benefits of staying together. Such "affinitive" behaviour helps strengthen social bonds, and reduce stress.

How do Dolphins Sleep?

All mammals need to sleep. Quite why isn't entirely clear, although it may well have to do with saving energy, allowing the body to repair itself, consolidating the memories of experiences learned during the day, or any of a number of other reasons. What is clear is that it's very important, and that there's no way to avoid it in the long term. Indeed, it's not just mammals that do it, but also all species of bird, and probably just about every other creature with a brain, whether vertebrate or invertebrate. (Creatures without a brain, such as jellyfish, don't seem to sleep, although, to be honest, it's hard to see how you could tell if they did).

But sleep presents a problem, in that the animal is vulnerable while it's snoozing. This is why most mammals make dens of some kind to which they can retreat overnight - or during the day, if that's their preference. Dens, of course, can be remarkably varied, whether they are carefully constructed burrows, crevices in tree trunks, natural caves, or whatever, but most animals that live in the right sort of environment have them.

Hanging Out with Other Species

Spinner dolphin

Animals interact with members of other species in a range of different ways. Most obvious, perhaps, are predator-prey relationships, but not all interactions necessarily have to have the potential for violence. Often, we find members of different species living side by side because they simply happen to like the same habitat, or one species may steal the burrows of another rather than making the effort to dig their own. But there are also some more organised relationships, where two or more relatively large mammalian species actively congregate together for some sort of mutual benefit.

We commonly see this in herd animal, especially where one species is relatively rare within a given region. So long as they don't irritate the other species too much, it may be to the benefit of the rarer one to join the herd of the more common species, gaining the advantage from large herd sizes that it cannot achieve on its own. Aside from grazing herd animals, other social animals that often congregate with other, related, species, include examples among both primates and cetaceans. (Examples from other groups of mammal are rarer, but have been reported).

Broadly speaking, there are three different reasons why animals might want to actively hang out with members of another species. There is no particular why two or more of these reasons cannot be true at once, and disentangling them can take a fair amount of observation. Let's take a look at one recent study as an example.

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?

How many friends does a dolphin have?

Studying the behaviour of animals in the wild can be difficult, especially if they live in inaccessible or hostile environments, such as precipitous mountains or dense jungle. Animals that live out at at sea are particularly problematic. Seals and sea lions come ashore once a year to mate and give birth, and spend the rest of their lives some distance from land, where we have far less idea what they're doing. Fortunately, zoologists tend to be obsessed with sex, so at least they're on dry land when they're doing what we're most interested in.

Cetaceans, of course, don't even come ashore to do that (although they aren't quite alone in this respect, even among mammals). These animals are superbly adapted to an aquatic life, even more so than seals, and much, much more so than we decidedly non-aquatic humans. This means that studying cetaceans in their native habitat is a difficult, and rather specialised field - and, depending on what you want to know, studying them in the laboratory may not even be an option.