Defying birth control, siamang Eloise has baby at San Diego Zoo

Much to the surprise of the San Diego Zoo, Eloise, a 37-year-old siamang, delivered a baby this month.

Eloise and partner Unkie, 35, had already contributed six strapping siamangs, a species of gibbon, to the gene pool. So, in keeping with a species survival plan the zoo cooperates with, Eloise has been on chemical contraception for years.

Nature found a way.

Eloise didn’t show any signs of pregnancy, Jill Andrews, animal care manager at the zoo, said in a statement. This is the zoo’s first siamang birth in more than 12 years.

“We’re not certain why birth control didn’t work in this case, but as with humans, it is not uncommon for contraceptive failure to happen from time to time,” Andrews said. “Still, we are overjoyed — because any birth of an endangered species is a reason to celebrate.”

All gibbon species are listed as endangered on the International Union for Conservation of Nature’s Red List of Threatened Species. Their main threats are loss of habitat from logging and agriculture.

In addition, siamangs are sometimes killed to capture the young, who are illegally sold as pets. Since siamangs are fierce in defense of their young, it’s extremely difficult to capture the babies without killing the mothers.

Eloise is quite old to be a mother. The life span of a wild siamang is about 30 years, although in captivity they can live past 40.

With certain animals that are solitary by nature, breeding can be prevented by keeping them apart. But gibbons are social creatures and require companionship to thrive, zoo spokesman Andrew James said.

Siamangs pair-bond, which is unusual for apes. Mother and father usually stay together for life, sharing responsibility for raising their offspring.

“They’ve been together for a long time,” James said of Eloise and Unkie. “The better option was to put her on birth control.”

Siamangs are noteworthy for their inflatable throat sacs, which amplify their resonant calls. These calls are used for communication, such as identifying themselves to members of their group or to rival groups.

For their social health, Eloise and Unkie are housed with the zoo’s orangutans. In the wild, the two ape species occupy overlapping territories in tropical forests in Malaysia and Indonesia.

Since Eloise hadn’t had a baby since 2006, contraception had been mostly successful, although not foolproof, James said.

As for the big reveal — boy or girl — James said people will have to be patient. With Eloise and her baby in constant contact, there hasn’t been an opportunity to determine gender. An exam will be performed in the next few months.

“We’re just going to allow her to be a mama for right now,” James said.

After the exam, the zoo can think about giving a suitable name.

Suggestion: What are the Malay and Indonesian words for “little surprise”?

bradley.fikes@sduniontribune.com

Fikes writes for the San Diego Union-Tribune.