Tracking the monogamous mouse

December 09, 1990|By J. E. Ferrell

BERKELEY, CALIF. — Berkeley, Calif.--Two scientists have identified a species of monogamous mouse, the female of which produces a chemical in its urine that induces the male to take care of their babies.

University of California ecologist David Ribble and David Gubernick, assistant professor of psychology at the University of Wisconsin, also discovered that, after being weaned from their mothers, virgin female California mice (Peromyscus californicus) roam into new territory to find a mate, while virgin males stay near the place where they were born to wait for a female to find them.

After a female selects a mate, it waits, sometimes for several months, before going into estrus to mate, perhaps to ascertain the suitability of the male and its territory for raising offspring.

Scientists have long suspected that the California mouse, which ranges from Northern California to Baja California, was monogamous since it was first studied 40 years ago. "It's extremely rare in mammals as a whole," said Mr. Ribble. Although monogamy is common in bird species -- perhaps as high as 60 to 70 percent -- the best estimates are that only about 3 to 10 percent of mammals mate exclusively with one of their own species.

But this year Mr. Ribble, a doctoral student who does his research at Cal's Museum of Vertebrate Zoology, finished experiments using a technique new to field biology -- genetic fingerprinting -- to demonstrate the exclusive relationships between male and female California mice.

The data backed up field observations he made in 1987. At that time he dusted female California mice living in the canyons of the Hastings Natural History Reservation in Monterey County with different fluorescent pigments to see how many other male mice would pick up the same pigment. He found that each adult male picked up only one pigment -- the color of the female with whom it was living.

To obtain the DNA necessary for the genetic testing, he anesthetized trapped mice and snipped off a tiny tip of their tails. He chose the procedure after noticing that many wild mice were missing their tail tips, perhaps from encounters with predators. He couldn't take blood samples, because the red blood cells don't have nuclei, where DNA is located. In testing 98 offspring from 27 families, he found that their DNA matched the DNA of their parents.

"It turns out they're as monogamous as they can be," said Mr. Ribble.

From his observations during the field studies, he also found that males spend as much time caring for the baby mice -- known as "pups" -- as females do.

By putting tiny plastic collars with radio transmitters around the necks of several male and female mice, he tracked their position for several days before removing the collars. He found that they spent equal amounts of time in the nest. Because pups cannot regulate their body temperature, the adults must attend them to keep them warm for survival.

This behavior was also observed in a Wisconsin laboratory, where Dr. Gubernick began his studies of 600 California mice several years ago with 1,000 hours of time-lapse video recordings. He found that both parents care for the pups: They build nests, carry the young, huddle over the pups to keep them warm and lick them to keep them clean and stimulate blood flow.

Only two behaviors differ between males and females. Although males lick the pups longer than do females, the mothers lick the pups' anogenital area. They do so to stimulate urination by the pups, which can't urinate on their own.

In addition, the mothers drink pups' urine to recover the water provided to the young in the milk. This behavior has also been noticed in rats and is a method for the female to maintain its body's water balance. "It reduces the time the mother has to leave the young and go foraging," said Dr. Gubernick.

When looking at what kept the parents parental, Dr. Gubernick stumbled across the physiological basis for male parenting. Although he found that females needed their pups to act parental, pups did not induce a male to be parental. "It was the presence of the mother that was important to keep the males parental," he said. After exposing males to other female mouse urine and distilled water, Dr. Gubernick determined that it was only a volatile chemical signal from the urine of the male's mate that induced it to take care of the pups.

"We're trying to identify the chemical now," he said, also noting that some changes may be occurring in the brain during parenting. The medial preoptic area, which affects maternal behavior in rats, is larger in virgin males than in virgin females, but the same in mice that are parents.

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