So far only three species are known to go through menopause: humans, killer whales and short-finned pilot whales.
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The survival of killer whale pods and individuals is thought to be the reason behind this phenomenon. The first instance of a killer whale going through menopause was seen by scientists two years ago.
According to those findings, young female killer whales spend a lot of time giving birth and participating in reproductive competition. When an older female whale notices that her female offspring is going through a gestation period, they themselves enter menopause so as to reduce competition and the resultant conflict over resources.
According to researchers, humans can relate to the above findings.
"What an interesting paper," Phyllis Lee, a behavioural ecologist at the University of Stirling in the United Kingdom, who was not involved in the study, told sciencemag.org.
"It brings two perspectives on menopause neatly together, and provides an elegant model for its rarity," Lee stated.
Behavioural ecologist, Darren Croft, from the University of Exeter, UK, along with his colleagues re-analysed the 2015 findings regarding killer whale menopause.
"That showed how killer whales helped and why they lived so long after menopause, but it didn't explain why they stop reproducing," Croft told sciencemag.org.
To solve this mystery, researchers examined 43-year-old data on two orca populations, living in the Pacific Northwest. The younger male and female killer whales were found to live with their mothers, while they copulated with members of other families of their species.
It was observed that male orcas had a life span of up to 30 years, while female orcas entered menopause in their 30s and 40s and lived for decades longer. The oldest female orca, Granny, lived till she was 105; and remained pod leader despite not giving birth past for the last 40 years of her life, sciencemag.org reported.
The 43-year-old data pointed to the fact that at some point, the mother whale would give birth to calves, coinciding with her daughter(s) in the same pod also doing so. Statistics showed 525 calves being born over these 43 years.
It was also observed that 161 of the births that took place in this period were co-generation births. It also found that 31% of the calves died and the older mothers were more prone to lose their young ones before the age of 15. The older mothers had an offspring mortality rate of 1.7 times higher than young mothers, said a Current Biology report.
"That's a high cost," stated Croft, "and it's led to the evolution of menopause."
The research also found that a mother whale required around 42% extra nourishment in order to look after and nurse her calf; and orcas are known to share the food they hunt. The older mother probably finds and catches most of the food, Croft and his colleagues think, but her daughters and grand-calves likely end up with most of it, possibly through fighting and hoarding, something the researchers plan to investigate further.
All these pressures combined means that calves born late in a mother's life may be neglected and likely die of starvation, scientists say.
"It's a cool study because we usually think about these kinship issues in terms of helping behaviour, not competition," stated Richard Connor, a behavioural ecologist at the University of Massachusetts in Dartmouth who was not involved with the research, Current Biology reported.
The researchers concluded that it's the family structure of killer whales that triggers rivalry amongst generations of whales, the prevention of which resulted in menopause.
"Although human families are not structured like those of killer whales, ours [like theirs] are built around the provisioning and sharing of food," Croft notes.
"The same dynamics of cooperation and inter-generational reproductive conflict might well underlie human menopause, too," the scientists further concluded.
