The benefits of cohabitation

mouse in palm of gloved hand

For years, researchers have known that with age comes a decrease in fertility.

What they haven’t been able to figure out is how to stop or slow down this decline.

A new study by Ralph Brinster, professor of physiology at Penn’s School of Veterinary Medicine, may have the answer, and the work could have far-reaching implications for breeding programs for livestock, zoo animals and even endangered species.
The findings, published in the journal Biology of Reproduction, reveal that aging male mice housed with fertile female mice have a longer reproductive life than those males living alone. In fact, the difference in fertility is significant: Males living with females have a reproductive lifespan that is as much as 20 percent longer than those mice living alone.

“I think the magnitude was surprising to me,” says Brinster. “Twenty percent was a big effect.”

Brinster and his research team hypothesize that the presence of female mice is one of many environmental factors that delay aging of the “niche,” or environment where sperm stem cells live. This delay in aging leads to an increase in male fertility.

“We’ve already shown that when males stop breeding, it’s got to be a niche effect,” says Brinster. “Males fail to breed when the niche fails.”

This research is the result of years of work by Brinster and his team on the relationship between the niche and spermatogonial stem cells, known as SSCs. During his work with aging, Brinster says he surmised that the female would have an effect on the male, so he set up an experiment with mice, dividing males into two groups—one between the ages of 16 and 32 months that lived with females for two month intervals, and one group that lived by themselves.

The group of females, aged two to four months, were then monitored to see if they gave birth three weeks later.

The team analyzed blood samples and testes from the aged males and found that males housed alone showed reduced fertility by 26 months of age, while males that lived with females did not show a reduced fertility until 32 months of age—approximately a 20 percent increase of the normal fertile period. “The ones without females began to fail, but those kept with the females did better,” says Brinster.

One of the most surprising findings occurred with the rate of reproductive decline. Brinster discovered that, indeed, reproductive function lasted longer in males living with females, but once it did begin to decline, when the males were about 32 months old, the rate of decline was the same as that in the males who lived alone.

This suggests that a similar mechanism is responsible for the decrease in fertility in both groups of male mice, and the presence of the female mouse simply delays this mechanism.

While there’s no definitive evidence that this extension of reproductive function would occur in other species of aging males, Brinster says it’s not out of the realm of possibility.

“We don’t know that it affects other species, but I think intuitively you would expect one thing that affects one mammal to have an effect on the others,” he says. “In humans, married men live longer than single men.”
If this effect is mimicked in other species, then it could have a significant impact on how the farming industry cares for livestock, zoos house animals in their care and experts protect and increase the numbers of endangered species.

Explains Brinster: “This really shows what the final point is, and what nature’s real intent is—reproduction.”

Originally published on February 5, 2009