Being an elite athlete, at least as a mouse, does not shorten lifespan or limit how many children you can have. This is the finding of a study 30 years in the making, and it runs counter to accepted evolutionary theory.
According to one of the central ideas in the field of evolutionary biology, animals have to make trade-offs. Energy devoted to becoming stronger, faster, or more athletic is energy that cannot be spent on repairing the body, living longer, or producing and raising offspring.
To test this idea, the UC Riverside research team documented the lifespans and number of offspring born to regular mice and compared them to “high runner” mice bred to run extraordinary distances. These mice run on wheels up to three times the distance of unaltered mice on a daily basis.
“We did expect to find a reproductive cost for this athletic behavior,” said Theodore Garland, a distinguished professor of evolution, ecology, and organismal biology who led the research team, along with doctoral student Natalie Whitehead.
However, the team found that the average number of litters did not differ between the high runners and the control lines of mice. “We also thought the runners wouldn’t live as long, but they did,” Garland said. These findings are detailed in a new Behavior Genetics paper.
The project represents the culmination of more than three decades of research, starting in 1993, making it one of the world's longest-running animal selection experiments. Garland said lifetime breeding studies like this are rare because they require years of continuous monitoring and a major commitment of laboratory space and resources. The work was made possible through National Science Foundation funding.
Throughout the experiment, pairs of mice lived under standard laboratory conditions with unlimited food and water. The high runner mice did not have access to running wheels during the breeding study, but previous research has shown they remain substantially more active than control mice even without wheels. The researchers monitored each litter from birth through weaning and recorded the total number of offspring that survived over each pair’s lifetime, along with the age at which the mothers and fathers died.
Garland cautioned that the findings should not be interpreted as direct evidence about human athletes. Although some studies suggest certain groups of elite athletes live longer than average, people’s exercise habits are shaped by genetics, culture, family influences, education, and countless environmental factors that are difficult to separate. By contrast, laboratory mice can be studied under carefully controlled conditions that eliminate many outside influences.
“These are mice. They aren't humans,” Garland said. “In people, it would take decades to conduct a comparable study, and there are all kinds of environmental and cultural factors that can influence the results.”
The researchers did not find any relationship between how long the males and females within each pair lived.
“Going into the study, we thought that if one animal died, the other might die soon after due to some sort of grief, loneliness or heartbreak, a phenomenon for which there is some evidence in human beings,” Garland said.
“An alternative hypothesis was that, if a pair did not get along, then the death of one might lead to a ‘free at last!’ response in the survivor. Of course, both things might occur, depending on the pair, leading the two effects to cancel out.”
Garland suggested that future studies with laboratory rodents could potentially tease apart such phenomena.
The researchers believe the absence of energetic trade-offs in the mice may reflect the fact that the mice always had enough food to meet the demands of both activity and reproduction. Rather than forcing the animals to divert resources away from one biological function to support another, the plentiful food supply may have allowed them to eat more while maintaining both athletic performance and reproductive success.
It is also possible the high runner mice have evolved metabolic efficiencies or other “evolutionary countermeasures” that help them cope with elevated activity levels, though that question was beyond the scope of the current study.
The findings also suggest evolutionary trade-offs may not be as inevitable as biologists have often assumed.
“When people see a negative relationship between two traits, they often assume there’s an unavoidable biological trade-off,” Garland said. “But another possibility is simply that natural selection never favored having both traits at high levels. In this system, we bred mice for extraordinary activity, and we didn't find evidence that they had to pay for it with shorter lives or fewer offspring.”
(Athlete cover image: nicoletaionescu/iStock/Getty)
