Scientists have long been baffled why some species evolved to undergo menopause but findings of a new killer whale study offer new insights why orcas go through menopause.
Survival Of Family And Giving Birth
Earlier research suggested that whales undergo menopause so they can focus their attention on the survival of their families instead of giving birth to more offspring. Now, the same group of researchers reveal another reason for menopause in killer whales.
For the new study published in the journal Current Biology on Thursday, Jan. 12, behavioral ecologist Darren Croft, from the University of Exeter, and colleagues looked at the data from a study of killer whales in the Pacific Northwest.
Over the course of the 43 years of data collection, 525 calves were born. The researchers found that when older females and their daughters that live with them simultaneously give birth to calves, the calves of the older females have nearly twice the risk of dying in the first 15 years of their life. Interestingly, the calves did just fine when there was no other reproducing daughter.
"When mothers and daughters co-breed, the mortality hazard of calves from older-generation females is 1.7 times that of calves from younger-generation females," Croft and colleagues wrote.
Conflict For Resources
Croft explained that this does not mean that the older whales are not capable of raising their calves as well as the younger mothers. Conflict between the older and younger mothers for resources appears to have something to do with the survival of the calves.
When competing for resources, older mother are likely to lose out to their reproducing daughters so their calves are more likely to die.
The researchers said that the competition may primarily involve food because the older females may feel more pressured to share their food with the others around them. As a result, the younger females would have more food that they can give to their own calves.
Family Structure Of Killer Whales
The behavior of older female killer whales can be explained by the family structure of the species.
Killer whales mate with males from other groups and then go back to their families so the father is not around when a new calf is born. Female killer whales start their lives in a group where their kinship with others is low but when the female grows older and begins to have more offspring that stay with her, she became more related to the pod.
"When females are born, they have a relatively low relatedness to the males in their group, because their father isn't around," Croft said. "But as a female starts to reproduce, her relatedness to males increases, because her sons stay with her."
Older females are also known to play a crucial role in the survival of the family members of the group they live with. When old females die, their son's likelihood of dying a year after her death is eight times higher than when the mother was still alive.
Killer whales, short-finned pilot whales, and humans are the only species known to go through menopause.
