Survival and Longevity of Richardson's Ground Squirrels
(also known as gophers)
Survival of Richardson's ground squirrels differs between males and females, with males experiencing much higher mortality than females throughout their lives. Only 5-15% of juvenile male Richardson's ground squirrels survive their first year and reach adulthood. As adults, males have about a 25% chance of surviving each succeeding year. Thus, male Richardson's ground squirrels rarely attain the age of 3 years. In contrast, 35-45% of juvenile females survive to adulthood, and adult females have a 50% chance of surviving each subsequent year. Thus, female Richardson's ground squirrels often live 3 or 4 years, with a few surviving as long as 5 or 6 years. Of many thousands of free-living Richardson's ground squirrels followed for their entire lifetimes in a multi-decade study in southwestern Alberta, Canada, one female survived to 7 years and one male to 4 years.
The difference in life-span between male and female Richardson's ground squirrels is attributable to the different reproductive strategies adopted by the two sexes. Juvenile males pursue the high-risk strategy of dispersal, emigrating from their natal area to an unfamiliar location where they encounter squirrels that are not kin. Dispersal has the reproductive advantage of avoiding inbreeding but incurs the disadvantages of traversing unfamiliar territory, exposure to predators, contact with machinery or vehicles while crossing roads, and being attacked by resident ground squirrels as they attempt to settle in a new location. Adult male Richardson's ground squirrels experience extreme pressures during the mating season due to intra-sexual competition for access to estrous females. Males engage in vigorous fights, and the resulting wounds and stress can cause fatalities. Additionally, males are less vigilant about watching for predators during the mating season.
Female Richardson's ground squirrels are more sedentary and conservative in their reproductive strategies than males. Females tend to stay in a familiar location in or near their natal home range throughout their lifetime, and females do not engage in fights during the mating season.
Even though the sex ratio at birth and at weaning is 1:1, with similar numbers of males and females being added to the population each year, the difference in mortality between the sexes results in a skewed sex ratio among adults. At the start of the mating season, females outnumber males by 3- to 5-fold, but by the end of the mating season females outnumber males 8- to 20-fold.
Sources: PDFs of many of these articles can be downloaded from the Michener Publications page
- Michener, G. R. 1998. Sexual differences in reproductive effort of Richardson's ground squirrels. Journal of Mammalogy, 79: 1-19.
- Michener, G. R. and I. G. McLean. 1996. Reproductive behaviour and operational sex ratio in Richardson's ground squirrels. Animal Behaviour, 52: 743-758.
- Michener, G. R. and L. Locklear. 1990. Differential costs of reproduction for male and female Richardson's ground squirrels. Ecology, 71: 855-868.
- Michener, G. R. 1990. Use of body mass and sex ratio to interpret the behavioral ecology of Richardson's ground squirrels. pp. 304-338 in Interpretation and explanation in the study of behavior. (eds. M. Bekoff and D. Jamieson). Westview Press, Boulder.
- Michener, G. R. 1989. Sexual differences in interyear survival and life-span of Richardson's ground squirrels. Canadian Journal of Zoology, 67: 1827-1831.