We humans have a strong urge to reproduce, but if the environment steers us into putting off having children, we may be rewarded with both longer life and a bigger genetic footprint in future generations.
So concludes a new University of Minnesota study that reveals what may be a major force in shaping the evolution of most living things, including humans. Harnessing this natural effect could open the door to new means of delaying reproduction while promoting longer, healthier lives.
The work, led by ecology, evolution and behavior graduate student Will Ratcliff, was published online June 25 in the Public Library of Science.
The basic idea is simple. When environmental cues like food shortages signal that the population is about to shrink, individuals who can afford to wait until this has happened should do so; then their offspring, when they come, will represent a bigger fraction of the gene pool.
"When the population is declining, future kids make a greater splash in the gene pool than current kids," Ratcliff explains. "If there are tradeoffs between reproducing now versus later, delaying can be a good idea even if it reduces the number of kids you have during your lifetime."
Conversely, if hard times turn to good times and the population is about to boom, it's better to get those kids out there sooner, while the population is still small.
Rules of the waiting game
Over evolutionary history, early reproduction has reduced life expectancy due to the risk of complications in pregnancy, death in childbirth, damaging fights for mates or social status, and the demands of caring for and protecting offspring, says Ratcliff. Though lessened for modern humans, these risks shaped the evolution of our responses to stress.
For example, in some parts of Africa that suffer chronic food shortages--an environmental signal that the population will decline--girls experience their first menstrual period at later ages.
"Delaying reproduction to age 16 instead of 12 can really increase your chances, and your offspring's chances, of survival because having children very young is fraught with risk," says Ratcliff.
But in Western countries where girls have been getting richer food in recent years, the age of menarche has been receding. Rich food is an environmental signal that the population is poised to rise, and so the age of fertility has dropped.
Besides food availability, the environment may signal an imminent population decline chemically. Many food plants produce toxins that tend to depress reproduction and extend the lifespan. Humans may have eaten more of such plants when meat and other rich foods were relatively scarce, a sign that a population is facing a decline.
"A lot of these toxins extend life in ways that mimic dietary restrictions and have been shown to extend life in mice, fruit flies, roundworms, and yeast," says Ratcliff. "The whole point is that if a population is headed downhill, an individual who trades early reproduction for longevity can come out ahead."
One mechanism may involve testosterone, which suppresses the immune system, says R. Ford Denison, Ratcliff's faculty adviser and adjunct professor in the University's College of Biological Sciences. Thus, a toxin or other cue that reduces testosterone levels would tend to extend life as well as dampen reproductive behavior. Someday, the researchers say, harbingers of population decline may result in new drugs or lifestyle changes that lead to delayed reproduction and, potentially, longer and healthier lives.
What counts is the message organisms get from the environment, not necessarily the actual situation, the researchers say. For example, while the stress of regular fasting can delay reproduction and extend life, animal experiments have shown that the mere odor of food can reverse this effect.
Other authors of the paper were graduate student Peter Hawthorne and professor Michael Travisano of the Department of Ecology, Evolution and Behavior.
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