Annual Weight Cycle of Richardson's Ground Squirrels
(also known as gophers)
Richardson's ground squirrels, also commonly known as gophers, follow a regular pattern of weight fluctuation during their annual cycle. Basically, they gain weight during the active season, then lose it during hibernation. However, the cycle is strongly influenced by the age, sex, and reproductive status of the squirrel.
At emergence from hibernation in southern Alberta, adult male Richardson's ground squirrels weigh about 350-400 g. Males maintain or slightly increase in mass during the 2- to 3-week interval before females emerge from hibernation and start mating. During the mating season males expend energy as they seek females for mating and attempt to drive away other males, but these activities reduce the time available for foraging. Consequently, energy expenditure increases but food intake decreases, so males use up fat reserves and lose weight. By the end of the mating season, males reach their lowest body mass. Thereafter, males slowly gain weight; then, a few weeks before the start of hibernation, males rapidly store fat and enter hibernation weighing around 500-600 g.
When adult female Richardson's ground squirrels emerge from hibernation they are at their minimum body weight, weighing aout 200-225 grams if yearling and 250-275 grams if older. Adult females begin increasing in mass immediately. Except for a brief interruption on the day of mating, weight gain continues during gestation. Although yearling females weigh less than older females on emergence from hibernation, they rapidly increase in mass during pregnancy. Yearlings finish growing to adult size during pregnancy, and by early lactation yearling and older females are similar in weight.
At the time of parturition, females lose about 60-80 g in association with giving birth to 5-8 offspring. During lactation, weight gain slows due to the energy demands of producing milk for the litter. Weight increases again after the pups are weaned, and continues to increase as females begin fattening in preparation for hibernation. At immergence in July, all adult females weigh around 400-450 g.
Newborn pups weigh about 6-7 g, and the entire litter (usually 5-8 infants) weighs about 50 g at birth. During lactation, litter mass increases 10 fold, and the combined weight of the infants exceeds that of the mother herself. Litters of yearling and older Richardson's ground squirrels are of similar size and weight.
Richardson's ground squirrel pups first emerge from their natal burrows when 29-30 days of age when they weigh about 65-85 g each. Juvenile females grow to about 80% of their adult size before entering hibernation in August, then finish their growth during pregnancy the next spring. In contrast, juvenile males remain active into October, allowing them to grow to their final adult size before entering hibernation.
Sources: PDFs of many of these articles can be downloaded from the Michener Publications page
- Broussard, D. R., G. R. Michener, and F. S. Dobson. 2006. Age-specific resource investment strategies: evidence from female Richardson's ground squirrels. Journal of Zoology (London), 268: 389-394.
- Broussard, D. R., G. R. Michener, T. S. Risch, and F. S. Dobson. 2005. Somatic senescence: evidence from female Richardson's ground squirrels. Oikos, 108: 591-601.
- Michener, G. R. 1998. Sexual differences in reproductive effort of Richardson's ground squirrels. Journal of Mammalogy, 79: 1-19.
- Michener, G. R. and L. Locklear. 1990. Overwinter weight loss by Richardson's ground squirrels in relation to sexual differences in mating effort. Journal of Mammalogy, 71: 489-499.
- Michener, G. R. 1984. Sexual differences in body weight patterns of Richardson's ground squirrels during the breeding season. Journal of Mammalogy, 65: 59-66.
- Michener, G. R., and J. W. Koeppl. 1985. Spermophilus richardsonii. Mammalian Species, 243: 1-8.
- Michener, G.R. 1985. Chronology of reproductive events for female Richardson's ground squirrels. Journal of Mammalogy, 66:280-288.