Groundhog Mortality
David P. Barash, in his book Marmots: Social Behavior and Ecology, has some interesting observations on groundhog survival behavior, and mortality. Groundhogs look up from their feeding on an average of once every 12 seconds, and spend up to 20% of their time sitting erect on their haunches, scanning their surroundings for approaching predators. They are munched on by a variety of species from foxes, coyotes, cougar and great horned owls, to domestic dogs. Mortality by humans is significant; automobiles take a high toll of groundhogs living near roads, and in rural environments, hunters kill numbers of them. It would be interesting to know how many are also removed by nuisance control operators. Mortality from predation is highest among the young of the year, in particular as they disperse into new and strange territory. Barash says that mark and recapture data suggests that about 30% of the adults die annually, whereas about 77% of the young don't survive the year. The oldest reported wild groundhog lived to be six and a half years old, but that's unusual. The average life span is only one year. However, if they survive the first dangerous year, adults do have a reasonable chance at living several years or more. Although predation is a major cause of groundhog death, there is another situation just as dangerous. Over-winter mortality, that is, death during hibernation, is a common fate. Simply, many groundhogs go into hibernation and never come out. The young in particular seem to be affected the most. A larger proportion of adults emerge from hibernation than do young of the previous year. It isn't certain exactly why groundhogs die in hibernation. If the young animals were simply starving in their dens, it would stand to reason that the ones that do survive would show some sign of malnourishment. However, yearlings that emerge in the spring rarely seem emaciated, so it isn't starvation alone that is killing them. Disease could be a factor. Free-ranging groundhogs often harbor bacteria that has been associated with lethal pneumonias. It is significant that a high incidence of bronchopneumonia is common in groundhogs in late winter and early spring, after hibernation; it could be that the moist, unventilated conditions in the underground dens help the disease flourish. Groundhogs also suffer from other diseases, among them the hepatitus B virus. In all probability, underweight or undernourished groundhogs are more susceptible to these diseases. Whatever the reason, the nutritional status of the animals seems to be a factor. Studies show that the later the groundhogs emerge from their dens, the less likely they are to survive the next hibernation. Simply, they have less time to put on weight. Yearlings, or young of the previous year, always emerge later than adults. One study showed that 58% of young marmots that appeared above ground before July 15, were recaptured after the next hibernation. Only 19% of the young that appeared after July 15 were recaptured. The ones emerging before July 15 were significantly heavier at hibernation time. Most data indicates that lighter body weight lessens the chances of surviving hibernation. An added
factor is that yearlings, who emerge later than
adults, have correspondingly later litters. Young
from these late litters have even less time to put on
weight--and even less of them survive hibernation.
Barash says that hard data on overwinter mortality of young groundhogs is difficult to obtain, because they all disperse, and dispersing animals are hard to follow and monitor. By observing a small group of young groundhogs that did not disperse very far, however, Barash was able to get some suggestive figures. Of 32 young-of-the-year groundhogs, he was able to locate and observe only six the next year. Of these 32 groundhogs, 19 were weaned before June 8th, and 13 were weaned after that. Of the six surviving young groundhogs, all had been weaned before June 8th. Of the above 32 young, nine were produced by yearlings; all of these were weaned after June 8, and apparently none of them survived to become yearlings the next year. Weight gain seems to be seasonal; it is most rapid from April to June, and markedly lower from June to October, in all ages. This is why early emergence is so important. Under natural conditions, says Barash, it appears that hibernation is induced by food deprivation. This effect is seasonal too; whereas it takes three or four weeks to starve a groundhog into hibernation in the summer, it can take only three or four hours in winter! Thus, weather can be a factor, too. If winter comes too early, animals are forced into hibernation before they are physically ready for it. Conversely, if winter stays too late, some animals may emerge successfully from the den after hibernation, but be unable to get enough nourishment to survive. It seems to be a straightforward equation; the more body weight a groundhog can obtain, the more chance it will survive hibernation. Summer is a time of constant eating and weight gain; hibernation is a time of dramatic weight loss. According to the data, there doesn't seem to be much margin of error here. Every day of uninterrupted, safe foraging is probably important. The groundhog with the longest uninterrupted period to eat, has the best chance to survive. And this brings up, once again, the issue of relocation of nuisance animals. It would
be interesting to know the percentage of relocated
nuisance groundhogs that survive hibernation. It
makes sense that if a groundhog is live trapped and
removed from its established territory, it would have
to expend considerable energy, and time, finding and
establishing another territory. In all probability,
weight gain would be minimal at best during The question is, which is more humane, euthanization, or relocation with its attendant stress and reduced chance for survival? The argument could be made that at least relocated animals have a chance, and death during hibernation is probably stress-free. More information on the effects of relocation on over-winter survival would be helpful in making a decision that is the best for the animal as well as the people involved. n
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