Scientists have long speculated why, from an evolutionary perspective, a woman's reproductive history should influence her ability to have children in the future. Now, biologists at The University of Manchester have been given some clues after studying the mating behaviour of the dusky roach (Nauphoeta cinerea).
"This cockroach is unusual in that it gives birth to live young, rather than laying eggs, even nurturing them in their first few hours of life," said Dr Patricia Moore, in the University's Faculty of Life Sciences.
"The females also experience reproductive cycles and show age-related decline in fertility and so provide an excellent opportunity to examine the mechanisms by which females lose reproductive potential as they delay breeding."
Earlier research by Dr Moore showed that female dusky roaches that were prevented from mating did indeed lose fertility in later life. They were also far less choosy about who they mated with once reintroduced to males.
"The females would normally spend a good few minutes sniffing and climbing all over potential mates but these older females spent much less time choosing a partner," said Dr Moore.
"We also found that they lost fertility in later life – they had fewer clutches and fewer young in those clutches; I was curious to find out why."
Dr Moore's latest research, published in the science journal Evolution and Development, suggests a natural biochemical reaction is to blame.
"The female cockroaches reach sexual maturity at six days but we delay mating by two weeks. Adults live for about a year, so even at this age they are still quite young.
"As we expected, the eggs that would have been fertilised had the female been allowed to mate were discarded through a natural, controlled process of cell death, called apoptosis.
"But we also observed that apoptosis began to occur in the cells of eggs that wouldn't yet have been used, eggs-in-waiting if you like."
Dr Moore believes that there are complicated evolutionary arguments behind why perfectly healthy eggs should begin to die.
"We suggest that reduced fertility brought about by delayed reproduction occurs because evolution has not been able to adapt to what is an artificial situation; the cockroach is unable to refine getting rid of eggs that are no longer needed.
"In humans, of course, this artificial situation is created by cultural influences that see women not giving birth until long after reaching sexual maturity.
"Although it's hard to compare the experiences of the female cockroach to humans, the biological mechanisms are similar and so an inappropriate apoptosis response to the 'mistiming' of reproduction may explain the evolution of the loss of fertility with age."
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