Reproductive Physiology of Richardson's Ground Squirrels
(also known as gophers)
Richardson's ground squirrel litters first emerge above ground about 2 months after adults emerge from hibernation and mate
Male Richardson's ground squirrels time their emergence from hibernation to maximize their mating opportunities with females. In order to be reproductively successful, male Richardson's ground squirrels must be above ground and producing viable sperm by the time the first female emerges from hibernation and is ready to mate. For males, emerging too early has no reproductive benefits and may be costly in terms of survival, but emerging too late results in lost mating opportunities, as many females will have already mated. Typically, males emerge from hibernation about 2 weeks before females start emerging.
At the end of the previous year's mating season, testes regress and return to an abdominal position. Hence, male Richardson's ground squirrels must re-grow their testes before each subsequent mating season. This re-growth starts as soon as males arouse from their last torpor bout. Males that have seeds cached in the hibernaculum can remain underground for a week or more before they need to come above ground, by which time their testes are large and descended into a darkened scrotum. In contrast, males without food caches come above ground within 1-3 days of their final torpor bout; on emergence, their testes are still small and the scrotal skin is still pink. In either case, by the time females start to emerge from hibernation, all males have large descended spermatogenic testes. Testes are at their maximum size prior to and during the peak mating period, then begin to regress.
Within 4 days of emergence from hibernation, female Richardson's ground squirrels exhibit a swollen bright-pink vulva, indicative of estrus. Behavioral estrus lasts several hours on one afternoon. Thus, a female's mating season is limited to a period of a few hours, whereas the mating season for males lasts for 2-4 weeks, from whenever the first female in the population enters estrus until the last female mates. Occasionally a female fails to conceive during her estrous period and she may enter a second estrus about a week later. Adult female Richardson's ground squirrels can produce only one litter per year. Even if the litter is lost during pregnancy or lactation, the female cannot mate again until the subsequent year, after she has been through another hibernation season.
After insemination, some female ground squirrels have a copulatory plug firmly adhered to the vaginal wall.
Fluid ejaculated by the male coagulates quickly to form a creamy-white copulatory plug which is rubbery to the touch.
Copulatory plugs dislodge within 15-17 hours of being formed. The function of copulatory plugs is uncertain. They do not prevent subsequent inseminations, and some females mate with several males during their brief estrous period. The coagulated material is virtually sperm free, so plugs do not serve as a sperm reserve. A possible function of plugs is to push the semen through the cervix and aid movement of sperm toward the ova.
Gestation lasts 23 days in Richardson's ground squirrels. Nipples of yearling females evert for the first time during the third trimester of pregnancy. Nipples of all females enlarge as parturition nears. During lactation, nursing by pups results in darkening and elongation of the nipples and fur wear around each nipple.
Nipples are conspicuous on lactating Richardson's ground squirrel
Lactation lasts about 5 weeks. Maximum milk demand occurs when pups are 3-4 weeks old and still totally dependent on mother's milk. At this age, the combined mass of the pups in a litter is greater than the mother's own mass and mothers consume large amounts of fresh green vegetation to support milk production. The emergence of female Richardson's ground squirrels from hibernation (in mid-March in southern Alberta) is timed so high-quality food is plentiful when the milk demand of the growing litter is highest (late April to early May in southern Alberta).
Juveniles are weaned shortly after they first come above ground at 29-30 days of age. After the litter is weaned, the female's elongated nipples flatten and dry, then the dry tissue is shed leaving an inconspicuous pink nipple. Weaned juveniles spend several months foraging in order to grow and fatten before their first hibernation.
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. 1989. Reproductive effort during gestation and lactation by Richardson's ground squirrels. Oecologia, 78:77-86.
- Michener, G.R. 1985. Chronology of reproductive events for female Richardson's ground squirrels. Journal of Mammalogy, 66:280-288.
- Michener, G.R. 1984. Copulatory plugs in Richardson's ground squirrels. Canadian Journal of Zoology, 62:267-270.
- Michener, G.R. 1983. Spring emergence schedules and vernal behavior of Richardson's ground squirrels: why do males emerge from hibernation before females? Behavioral Ecology and Sociobiology, 14:29-38.
- Michener, G.R. 1980. Estrous and gestation periods in Richardson's ground squirrels. Journal of Mammalogy, 61:531-534.
- Michener, G.R. 1973. Climatic conditions and breeding in Richardson's ground squirrel. Journal of Mammalogy, 54:499-503.