Detecting Pregnancy by Proxy

Subtle changes in how long northern elephant seals dive show when they’re pregnant.

Forget pee sticks, researchers have developed a way to remotely detect whether a northern elephant seal is pregnant using nothing more than basic tracking data. In a recent study, Luis Hückstädt, an ecologist at the University of California, Santa Cruz, and his colleagues showed that pregnant elephant seals dive for shorter durations than those that aren’t carrying a baby. It’s kind of like using data from a Fitbit to show that a woman is pregnant just because she’s running a bit slower than usual. This pregnancy test by proxy opens a window into a major life stage of the elephant seal that has been largely invisible until now.

The effect of pregnancy on wild animals is understudied, and northern elephant seals make it even trickier thanks to their habitat. Other than a brief stint on land during mating season, the animals spend most of the year out at sea, Hückstädt says. “It’s pretty much impossible for us to even detect pregnancy.” But there’s another good reason why studying pregnancy in northern elephant seals is particularly important—it’s their typical state; most of the time, most adult females are pregnant.

This odd fact results from the animal’s hectic sex life and long gestation period. Every winter, elephant seals haul out in huge rookeries where they give birth to the pups conceived during the previous mating season. A female elephant seal will nurse its pup, wean it, and then, to cap off a frenzied month, mate again. Meanwhile, male elephant seals on the rookery fight each other—sometimes to the death—for the opportunity to reproduce. It’s a brutal melee that leaves one adult male alive for every two females. After mating, they all head back out to sea. Now, scientists may be able to learn a bit more about what happens next.

Hückstädt was inspired to investigate the effects of pregnancy on northern elephant seals when he was looking at tracking data from closely related southern elephant seals, which live off the coasts of Antarctica and South America. He noticed that the dive durations of females changed throughout the year, and that the numbers were different than he expected.

Hückstädt had assumed the southern elephant seals’ underwater forays would get progressively longer throughout the year since bigger seals—fat from feeding on summer’s bounty—can store more oxygen in their blood and tissues to support lengthier dives. Instead, he found that while females’ dive times did initially increase, most of them stalled about halfway through the year and then began to decline, while the rest continued to increase. By the end of the summer, the difference between the two groups was about six minutes. This was a small change for dives that can last up to two hours. Hückstädt wondered if this tiny deviation could signify a physical change such as pregnancy.

To find out, he looked for the signs of a similar change in the dive data of California’s northern elephant seal, a population that has also been studied with tracking tags and through extensive rookery observations. He found that like their southern cousins, 90 percent of female northern elephant seals also make shorter dives in midsummer. Confirmation that pregnancy was behind this phenomenon came when these shorter-diving seals gave birth the following winter and the longer-diving seals did not.

Hückstädt can’t say exactly why pregnant seals take shorter dives, though he suspects developing fetuses may draw oxygen from the females, forcing them to surface to breathe sooner.

Whatever the reason, Andrew Trites, a marine biologist with the University of British Columbia, suspects the finding may spur similar, and much-needed, research. Females are the most ecologically important members of a polygamous species, Trites says, so framing more studies around them and their biology will be illuminating.

Hückstädt wants to tag other marine mammals to see if he can use the same strategy to learn about their pregnancies. He’s also studying the pup-less elephant seal females to see if they miscarried or just didn’t conceive. Miscarriage can be an early warning of problems such as food stress or pollution.

Pregnancy may seem like an exceptional state, but for elephant seals it’s often the default. It’s smart to study them with that in mind.