News Release

Male groundhogs hibernate less to visit the ladies

Peer-Reviewed Publication

Penn State

Credit: Penn State, Stam. Zervanos

Female groundhog in front of her burrow greets early rising male visitor.

Full size image available through contact

The days of gentlemen callers leaving their visiting cards are over, but if you are a male groundhog, visiting a few female burrows before the ladies come out in the spring may prove to smooth the way for later mating activities, according to a Penn State biologist. "Field observations indicate that males immerge later and emerge earlier than females," says Dr. Stam. M. Zervanos, associate professor of biology, Penn State Berks-Lehigh Valley College.

Zervanos defines the date of immergence as the last day they monitored the groundhog above ground in the autumn and he defines the date of emergence as the first date they monitored the groundhog above ground in the spring. Groundhogs do not simply crawl into their dens and hibernate, but rather they experience a series of torpor and arousal events throughout winter. During arousal events they stay in their burrows, but in the spring, they emerge and move around above ground. They then return to the den for some more deep sleeping episodes before the final arousal for the season.

"Upon emergence, males tended to move within a given territory, often visiting female burrows," says Zervanos. "Females tended to stay close to their burrows."

The Penn State researcher observed one male at the entrance of a female's burrow about 300 yards from his home burrow. The female emerged and the male stayed with her for two days before moving on to another female's burrow. Afterwards, all three groundhogs stayed alone in their burrows experiencing episodes of deep torpor before final arousal.

"For males, these early excursions are an opportunity to survey their territories and to establish bonds with females," says Zervanos. "For females, it is an opportunity to bond with males and assess food availability."

Typically, groundhogs do not exit hibernation for good until early March, which is when they mate. These episodes of early visitation occur in February. It does not appear that mating occurs during these early encounters.

"The length of the hibernation season at a given location appears to be consistent for groundhogs – also called woodchucks – and is characterized by a predictable timing of immergence and emergence," Zervanos reported in theProceedings of the Fourth International Conference on Genus Marmota held in Montreux, Switzerland. "This is important, because if mating occurs too early, young would be weaned at a time in the spring when food is still limited. If mating occurs too late, young would not have sufficient time to gain their critical hibernation weight."

The Penn State scientist studied 32-free ranging groundhogs over four hibernation seasons. Radio telemetry was used to monitor hourly body temperature from the groundhogs. A portion of the groundhogs at any one time were implanted with temperature transmitters. During the first two seasons, straw at the burrow entrances indicated if an animal had exited or entered a burrow, but for the final two years of the study, infrared motion triggered cameras were placed at the burrow entrances. These cameras recorded date and time of emergence as well as supplying a photograph.

On average, Zervanos found the groundhogs experienced first torpor on Nov. 7 and final arousal on Feb. 28. The average length of hibernation was about 114 days, but males exhibit significantly shorter hibernation seasons of about 106 days compared with females at about 117 days. The timing of hibernation activities did not vary significantly from year to year. Although males "tended" to immerge later and emerge earlier than females, no significant difference was observed.

"It would appear that the early bonding activity and establishment of territories in preparation for mating insure optimum conditions and timing for reproduction and offspring survival," says Zervanos.


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