This Bachelor Life

The social structure of a mountain zebra isn't that different from that of most ungulates you'll see in zoos.  A single male leads a herd that consists of several females, all of which he will mate with until such a time comes as he is overthrown by a new male.  Male offspring are tolerated in the herd until reaching sexual maturity, at which point they are run off.

There is no magic difference between the sex-ratio at birth of zebras or gazelles as there is with people.  About half the babies born will be males.  If one male possesses many females in a herd, it stands to reason that there are a lot of males at any given point who don't have any females.  What's their story?

In the wild, these surplus boys have three options.  Firstly, they can... die.  It's hard, but true, that young males, pushed out and forced to find their own way, suffer high mortality.  Secondly, they can go it alone, living a solitary life until they eventually find their own social group.  The third option is that the boys can band together.  These social groupings are called bachelor groups.

A typical antelope herd exhibit in a zoo, Zoo Miami in this case.  A single male greater kudu poses with his herd of females and their calves.

Bachelor herds have become something of a trend lately in zoos.  The problem of surplus males exists there as in the wild, but without the likelihood of predators or natural disasters thinning their numbers (at least we would hope so).  Some zoos have tried to even up the disparity between males and females by putting their animals in a somewhat more monogamous relationship - one boy, one girl - just to make sure everyone has a partner.  This may actually discourage breeding in some species, as it's not a natural grouping.  A bachelor herd is.

Bachelor herds provide a reservoir of young males which can be switched out for breeding programs.  At the same time, they'll be kept in a socially appropriate group where they can demonstrate appropriate behaviors and skills.  The concept of bachelor herds is spreading beyond ungulates, now being used with a variety of other mammals, such as elephants and gorillas, in which the "one male, many females" social grouping applies.