Although reliable data on the use of contraception is not kept by zoo associations, officials say that it is much more prevalent in North America but that it is starting to expand in Europe.

American zoos began developing contraception for highly fertile animals like lions in the 1970s, after breakthroughs in human birth control. Contraception use then expanded as it became quite difficult for zoos to sell or give away animals they could no longer accommodate.

This kind of family planning meant males and females no longer had to be kept apart to avoid unwanted pregnancies, which was ideal for the transition to more natural zoo environments. There were benefits, too, for zookeepers: hormones in contraceptives given males can take the edge off aggressive behaviors surrounding competition for a mate, which can result in mayhem and unsettle visitors.

There was a time when no one could have imagined that contraception would be needed for the Mexican wolf, a species hunted nearly to extinction in the 1970s. Zoos began with only seven survivors and bred a captive colony of nearly 300 wolves, saving the species. Ninety-two were reintroduced into the wild by the federal government starting in 1998, but then four years ago, the government used up the limited space that had been allotted for the program in New Mexico and Arizona.

So, for the last four years, birth control has helped the zoos breed selectively to maximize genetic diversity but not run out of holding space — or Mexican wolves — as a compromise is worked out with the states.

Genetic diversity is key to long-term species survival because it prevents inbreeding and preserves a broad array of traits that animals might one day need to survive in the wild.