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.
Among these animals, it's notable that sexual segregation seems especially common where males are much larger than females. There are many possible explanations as to why this might be, and it would be foolish to assume that there is one single answer that applies to all species, or even that multiple answers all apply to the same extent. Nonetheless, we can look at particular species to learn what we can about them, especially if they are of interest to conservationists.
Perhaps the most iconic herd animal of North America is the bison (Bison bison). They are not considered an endangered species these days, having recovered from the effects of past overhunting over the course of the 20th century. They are, however, reliant on conservation efforts to stay that way and the free-ranging population may be smaller than you would think - it was estimated as no higher than 13,000 in 2017 with just five populations being considered truly viable. This is only just short of what would be required to list them as a "threatened species", and, while their population is not currently declining that we know of, it is "critically depleted", in part because bison currently occupy just over 1% of the land area that they did in Native American times.
Returning that figure to its natural level is obviously not an option, short of abandoning every city in the US. However, it does underscore the fact that bison require ongoing conservation just to stay as they are. Understanding how they use the landscape in which they live is a part of that, and if males and females aren't using the same parts of that landscape, it's useful to know why.
At least three possible reasons present themselves. It could be down to differing food requirements, with females seeking out areas with high-quality food to help with pregnancy and lactation, while males, being larger and not having infants to tend for, just look for wherever food is most plentiful, even if it isn't top quality. Alternatively, predators could be the driving force. Under this explanation, females use places that are safer for themselves and their calves, whether because there are just fewer predators about or because it's easier to see them coming. Meanwhile, the strong, muscular males brave more dangerous regions so long as they have good food to eat.
Thirdly, the different sizes of the sexes may mean that they have different patterns of activity through the day. One possibility that comes under this heading, for example, is that males often spend a lot of time aggressively sparring with one another, and females might prefer to avoid the hassle and attendant physical risk. But it could just be that they get up and travel at different times, which would obviously make herd cohesion difficult regardless of what else is happening.
A recent study looked at the herd of free-ranging bison on Antelope Island in Utah. This lies in the Great Salt Lake, just off the coast from Salt Lake City. With an area of just over 100 km² (40 square miles), this is uninhabited but does have campsites and other tourist facilities, primarily at the north end. It's open and often rugged country, dominated by sagebrush and grass, with just a few juniper and maple trees on the western side. It has long been home to mule deer, bighorn sheep, and pronghorn "antelope", with no predators larger than coyotes, which aren't much of a threat to adult bison.
Bison were first introduced to the island in 1893, having been imported from Wyoming. The herd currently stands at around 700 individuals, around a third of them immature. Surveys of the animals' behaviour were conducted over two years, focused on the mating and calving seasons. As the researchers point out, bison are good for this sort of thing because it isn't hard to spot a really big animal walking about on the open plains, compared with, say, small deer in a forest.
The study showed that while the bison gathered into mixed-sex herds during the rut, they tended to stay apart while the females were calving. The first part of that is pretty much a given; the sexes have to mingle during the mating season. The question is more about why they segregate at other times of the year.
The idea that they might be doing so because the females find the males aggressive and dangerous was supported by the fact that the all-male groups were extremely small, with an average of three individuals each. That's probably because even male bison find other male bison unnecessarily aggressive and would rather not get into fights with them when mating opportunities aren't at stake. All-female groups, where this isn't an issue, varied in size throughout the year, reaching an average of 27 adults each during the calving season... quite a bit higher than "three".
However, if segregation were down purely to wanting to stay out of each other's way, we would expect that the male and female groups would be evenly distributed across the island. They wouldn't be in the same place at the same time, but, over the course of a few days or weeks, they would be in the same place at different times. They would go where the food was, and that wouldn't change from day to day. They'd just pick somewhere else if their first choice was already taken.
But, while there was some overlap around the middle of the island, in general, male bison stayed in the north, and females in the south. This suggests that there is something different about those ends of the island that favour one sex over the other, if they're going to be apart anyway.
One possibility is elevation, with the highest hills on the island being at the southern end. It's not unusual for herd animals to give birth on higher, more rugged, ground, since it provides some degree of protection from predators amid steep terrain and shielding slopes. Bison are no exception, although, in this study, many of the females did use the lower, shoreward terrain in the south of the island, too, so that might be only a minor factor.
On the other hand, if they are trying to avoid predators, another reason to stick to the south is that most of the campsites, along with things like the visitors' centre and the most-used hiking routes, are all up at the northern end, where a bridge and causeway connect to the mainland. From a bison's perspective, humans might as well be predators, and it's well-known that many hooved animals will run away if we approach too close. The fact that all-female herds are larger when calving than during the rut would also support the idea that they are focusing on being protective.
We see much the same behaviour in yak.
Food quality seemed to play less of a role, at least among these particular bison. Having said which, the northern part of the island does experience some die-back of high-quality food at certain times of the year, which would benefit the males with their larger, more efficient digestive systems. Furthermore, those all-female groups that used the lowlands tended to congregate around patches of alfalfa and reeds along the southern shores, which could give them a benefit, even if their warier sisters in the hills were losing out.
This, the researchers suggest, gives us some clues as to how we can better help conserve bison. We could, for instance, limit visitor access to the southern parts of the island during the calving season, making it easier for female bison to avoid us at that time of year. We could also plant seeds of grasses and other nutritious plants along the relatively barren southwestern shoreline, giving females and their young more to feed on. Significantly, we wouldn't have to do the same in the north, where human activity is already greatest because the males are more resilient, both in terms of diet and adaptation to humans.
Male and female bison may not be different species, but sometimes it may help our conservation efforts if we treat them as if they were.
[Photo by Dallas Penner, from Wikimedia Commons].
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