Menopause is pretty unusual in the animal kingdom — up until now only humans, orcas (or killer whales) and short-finned pilot whales were known to stop reproducing decades before the end of their lives.

But two new species have now joined the exclusive menopause club: beluga whales and narwhals, says a new study published in the journal Scientific Reports.

Researchers say the discovery could have implications for whale population management, while shedding light on how beluga whale and narwhal societies work.

Most species reproduce until the end of their lives, although they tend to have babies less often as they age. That holds true even for humans' closest living relatives, like chimpanzees and gorillas, and many kinds of whales.

That's why scientists think it's striking that killer whale females generally stop reproducing in their early 40s, even though they regularly live to be 60 or 80, with some known to reach 100.

Strange phenomenon

"They've got this really long post-reproductive life, which is a really strange biological phenomenon," said Sam Ellis, a postdoctoral researcher at the University of Exeter and the lead author of the new study, which also involved researchers at the University of York in the U.K. and the Whale Research Center in Washington state.

Narwhals live into their 60s, but females only reproduce until their late 30s or early 40s. (Paul Nicklen/WWF)

He wondered if other whales might also go through menopause, but that still remains unclear.

By looking for the right anatomical data in the scientific literature on whales that have been examined after they died, Ellis and his colleagues turned up the two new examples.

Based on what is known about humans and killer whales, he suggests they share some commonalities in social structure. Members of both species, for example, live a long time and use their experience to help their children and grandchildren to survive.

When resources are scarce, an older whale female has more incentive to help the group than the rest of the group does to help her. "So they stop reproducing to sort of favour their grandchildren," Ellis said, citing a possible evolutionary reason for menopause.

Studying species that also have menopause can help researchers understand how menopause evolved, and this information has possible impact for humans, Ellis said.

Belugas appear to travel in family groups. The new discovery suggests that females become more related to the rest of the group as they age. (Submitted by Lisa Barry)

It's also information that's needed so scientists can better estimate and predict how whale populations will change over time.

Data from dead whales

Whale reproduction is difficult to study, since they spend their whole lives at sea and travel long distances.

Ellis realized that their bodies might carry information about their reproduction that was accessible when dead whales are cut open and studied by other scientists.

That was the case, and he found that when female mammals ovulate or release eggs, they leave behind a follicle. It degrades over time in most animals, but not in whales. The number of follicles is a permanent record of whale ovulation, and can be compared to the whale's age.

A whale's age can in turn be measured by other physical characteristics. For example, beluga whales add an extra layer to their teeth every year, and the layers can be counted like tree rings.

Based on that information, the researchers found that beluga and narwhal females stop reproducing in their late 30s or early 40s even though they live into their 60s or longer.

Robert Michaud, who has been studying the social structure of St. Lawrence beluga whales for three decades, was not surprised when he read the results of the study.

"I said, 'Yes, I knew it,'" he recalled.

Michaud, the scientific director for the Group for Research and Education on Marine Mammals, a non-profit based in Tadoussac, Que., had observed that the whales spend their summers in sex-segregated groups, and that females born into a particular group stay in it their whole lives. He had also noticed that older females weren't seen with calves of their own, but said it was difficult to prove scientifically that they weren't reproducing.

He said the proportion of older females' lives spent after reproduction, according to the new study, is amazing and suggests they have a very important role in their communities.

"It's very challenging to demonstrate that," he said, but his own study is one of the few long-term studies that could do it, so it's something he plans to look into.

Lisa Loseto, a research scientist at Fisheries and Oceans Canada and an adjunct professor at the University of Manitoba, studies belugas in the Western Arctic. She said the findings are consistent with what she's heard from Inuit elders based on their traditional knowledge of beluga whales' social structure. But to find scientific evidence supporting the role of beluga grandmothers is "so cool."

Steve Ferguson, another researcher at Fisheries and Oceans Canada and adjunct professor at the University of Manitoba who studies the population ecology of marine mammals, including whales, said in an email that he believes the results for beluga whales.

But, in the case of narwhals, it's harder to tell how old the whale are, making the data on them "somewhat suspect," he said.