As part of their management of their animals, zoos get together to create breeding recommendations for each species. But sometimes the animals have their own plans. Despite being on contraceptives, a Los Angeles Zoo hippo delivered a surprise baby on Halloween.

When the zoos that comprise the Association of Zoos and Aquariums get together to evaluate the breeding potential of each species - either through a "Species Survival Plan" or a "Taxon Advisory Group" - they do so with the intention of maintaining maximum genetic diversity from within a small population, usually the animals within the North American zoos that comprise the AZA. That's why animals are transferred so often between zoos. A baby is born, grows to breeding age, and is moved to a zoo where he or she might breed with their intended. It's like an arranged marriage, but for animals.

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When animals aren't given breeding recommendations - perhaps their genes are already sufficiently represented within the population - they can be housed in single-sex groups. If being housed in a single sex group isn't appropriate, perhaps because of the nuances of the animals' social ecology, then animals in mixed-sex enclosures might be given contraceptive drugs. That was the case for 10-year-old female hippo Mara, who was housed at the Los Angeles Zoo with a 3-year-old male, Adhama.

Unlike the human versions of the drugs, which work by preventing ovulation in the first place, wildlife contraceptives are often immunocontraceptive vaccines (we were unable to verify this with the LA Zoo as of this writing, but it's likely that this was the sort of birth control the hippo was on). That means they prevent pregnancy by tricking the female's own immune system into binding antibodies onto her own eggs. That prevents any sperm from doing the same, though she still ovulates as usual.

For more on wildlife contraception, see this piece I wrote at BBC Future:

But wildlife contraception is a tricky business, and it doesn't always work the way it should. (Just as for humans, even with contraception, there is always the possibility of unintentional pregnancy.) The zoo's hippo keepers suspected that Mara might be pregnant despite her contraceptive vaccination program, because thanks to their careful monitoring, they noticed an unusual weight gain.

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And so it was that on Friday afternoon, October 31, Mara went into labor while on exhibit - an incredible treat for zoo-goers. Two and a half hours later, she delivered a healthy young calf. Given the eight months in which hippos gestate, the keepers and vets estimated - if their hunch was correct - a mid-November birth. They were only off by a couple weeks. Trick or treat, indeed.