Waist-Deep in Silverbacks

By Sherry Symington
Mar 1, 1993 6:00 AMNov 12, 2019 6:08 AM


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More male gorillas are being born in American zoos than females. This is not good for the long-term health of the population.

Since 1980, for reasons that are unclear, the number of male gorillas born in American zoos has exceeded the number of females by a third--105 males to 70 females. At first the skewed ratio was merely baffling, but after a decade it has become alarming.

The roots of the problem may go back to a well-intentioned reform. In the 1970s zoos began keeping their gorillas in groups rather than in isolated pairs, and in naturalistic compounds rather than cages. That’s when the animals first started to breed in captivity. Initially they produced more female offspring than male, so that now, after the tables were turned in the 1980s, there are roughly equal numbers of both sexes. But because gorillas are polygynous--a single male breeds with a harem of females--there is already an effective male surplus.

Zookeepers are thus sitting on a powder keg. Although gorillas are basically peaceful, when cornered they can do damage to one another. As the young males in captivity beef up into silverbacks, dangerous conflict with their peers gets more likely, says Myron Sulak, curator of mammals at the San Francisco Zoo.

What’s a zookeeper to do? No one is seriously considering returning male gorillas to the wild. That notion has romantic appeal, but it would probably fail; a zoo-born gorilla is not likely to have mastered the behavior it would need to gain acceptance in a wild troop.

The only solution seems to be to do an even better job of mimicking the gorillas’ natural habitat. The sex ratio in the wild is also roughly one to one, but there confrontations between males usually don’t end in violence; the more senior silverback simply drives off his junior rival. In zoos there is no way for the loser to beat a dignified retreat-- there’s nowhere for him to go but the opposite side of the compound.

The most promising approach has been to establish harmonious groups of bachelor gorillas, like those occasionally found in the wild; without females to fight over, males are more likely to tolerate one another. In 1987 the St. Louis Zoo began experimenting with this concept. It took the researchers until 1991 to find a winning combination: two mature silverbacks (ages 14 and 11) and three 8-year-old blackbacks. Getting the right mix of personalities was critical, according to Ingrid Porton, the curator in charge of the St. Louis experiment, and so was adhering to gorilla protocol. The strangers were first placed in adjacent cages, where they could safely size one another up. They were also given plenty of projectiles--cardboard boxes and so on--suitable for harmless gorilla-style displays of dominance.

The St. Louis bachelors appear to be content. But bachelorhood is no solution to the long-term problem. Researchers still don’t know why zoos are being inundated with male gorillas. Their best guess at the moment, says Joanne Earnhardt of Chicago’s Lincoln Park Zoo, is that the problem is the good life in zoos.

According to what biologists call sex allocation theory, male births are favored in polygynous species when the mother is in good condition. A healthy, well-fed mother can raise a healthy, well-fed male-- one that has a good chance of becoming a harem master and passing on the family genes to the maximum number of offspring. When the mother isn’t in great shape, however, a daughter is a safer choice because she will rarely fail to breed, albeit on a more modest scale.

In zoos females all tend to be in pretty good shape. Sex allocation theory thus predicts the current excess of sons over daughters-- but then why was the situation the other way around in the 1970s? And what can be done to correct the excess of males now?

The theoretical answer to the second question (for the moment there’s no answer to the first) would be to expose gorillas to more hardships, to simulate what they would experience in nature--which is why gorilla researchers are hoping they’ll find another explanation. Perhaps we will find a dietary key, says Earnhardt. We certainly don’t want to add stress to the gorillas’ lives.

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