Richardson's Ground Squirrels

Reproduction and Development of Richardson's Ground Squirrels
(also known as gophers)

Richardson's ground squirrels are not sexually mature until they emerge from their first hibernation season, when they are about 11 months old. Richardson's ground squirrels are seasonal breeders, with mating restricted to a 2- to 3-week period in early spring. Soon after the brief mating season is over, the male's testes regress and return into an abdominal position, where they remain for the next 48 weeks. Each female mates on only one day during the mating season, after which she is pregnant and then lactating for the following 2 months.  Females are incapable of having a second litter in the same year.

After males arouse from the final torpor bout of hibernation, their testes undergo recrudescence. Males frequently spend several days in the underground hibernaculum before they emerge above ground, so their testes are already enlarged and descended into a darkened scrotum by the time they appear at the surface.

Adult male Richardson's ground squirrel
Adult male Richardson's ground squirrel recently emerged from hibernation with testes descended and scrotal skin infused with black pigment.

Once females begin to emerge from hibernation, males defend areas that change on a daily basis as each male attempts to maximize his proximity to females that are in estrus on that day. Males compete fiercely with each other for access to estrous females.

Female Richardson's ground squirrels enter estrus shortly after emergence from hibernation, and they usually mate on their third or fourth day out of hibernation. Each female's estrous period only lasts about 2-3 hours. During this time the female mates with 1 to 4 males. Copulation usually occurs underground.

Once they are impregnated, female Richardson's ground squirrels become intolerant of all males, including any males with whom they have mated. If a female Richardson's ground squirrel loses her litter during pregnancy or lactation, she is incapable of breeding again until the following year. Only one litter can be produced a year.

The testes begin to regress during the mating season, and by the time the last female to emerge from hibernation has mated all males have small testes. The testes cease producing sperm and then return into the abdominal cavity until the following spring.

testis data
Average length of the left testis of adult Richardson's ground squirrels in the spring mating season. Range in testis length is indicated by thin vertical lines. Ns is the number of males on that date with testes descended into the scrotum; Nns is the number of males with testes not in the scrotum. Solid line above the graph indicates the period over which all females mated, and the wide bar indicates the peak of the mating season when 50% of all females mated.
Source: Michener (1983)

Gestation lasts 23 days. The litter is born, usually overnight, in a chamber that the female uses for sleeping during late pregnancy. Litter size at birth is most commonly 5 to 8 young. Most mothers are able to rear all their offspring to weaning age. Consequently, litter size at first emergence above ground is also usually 5 to 8 young. The overall sex ratio of infants is 1:1, but individual litters sometimes have more of one sex than the other.

litter size data
Distribution of litter sizes for 999 newly emerged litters of Richardson's ground squirrels observed in an 18-year study in southern Alberta.
Source: Risch et al. (2007)

The majority (over 85%) of female Richardson's ground squirrels wean a litter, unless a nest predator is resident in the area. Badgers and long-tailed weasels, predators that can invade a mother's underground nest chamber, kill and eat entire litters, though they usually do not capture the mother squirrel. In one study year, a badger killed 40% of litters. Some mother ground squirrels manage to save young from predation, but usually they can rescue only a single infant before the predator reaches the nest. Infanticide by other Richardson's ground squirrels is extremely rare.

After it is born, the litter remains permanently underground for 29-30 days. The mother is the only Richardson's ground squirrel that comes into contact with the infants. She usually sleeps with the infants at night, and visits them several times during the day to suckle them. The mother raises her litter on her own, with no help from the father(s) or any female kin.

Litter development

Richardson's ground squirrels at birth

At birth, infant Richardson's ground squirrels weigh an average of 6.5 grams. They are hairless, with eyes and ears closed, digits fused and teeth un-erupted. Infants are helpless and totally dependent on their mother.

Richardson's ground squirrels at 15 days

At 15 days of age, infants look like miniature ground squirrels, but they are incapable of locomotion and their eyes are still closed.

Richardson's ground squirrels at 20 and 21 days

Even at 20 and 21 days old, the eyes have not yet opened but infants can hold the head off the ground.

Richardson's ground squirrels at 26 days

The eyes finally open at 22- to 23-days of age. By 26 days, the fur coat is well developed and infants begin exploring nearby tunnels, but they do not yet come to the surface. They start to nibble on food if available, but still depend on the mother for milk.

Richardson's ground squirrels emerge

At 29-30 days of age, the litter first emerges from the natal burrow. The young, now referred to as juveniles, weigh between 65 and 85 grams.

Newly emerged juveniles immediately begin eating solid food, and soon they are weaned and become nutritionally independent of the mother. Small young from large litters emerge at the same age as larger young from small litters, and make the transition from milk to solid food at the same age. At 50 days of age, juveniles moult their baby fur, and begin to grow in the summer fur coat.

Juvenile Richardson's ground squirrel profile

Juveniles grow throughout the summer, then begin to fatten in preparation for hibernation. Juvenile female Richardson's ground squirrels enter hibernation in August and juvenile males in October.

When they emerge from their first hibernation at about 11 months of age, Richardson's ground squirrels are sexually mature.

Richardson's ground squirrel emerges from hibernation

Related Pages:
Reproductive Behavior
Reproductive Physiology

Sources: PDFs of many of these articles can be downloaded from the Michener Publications page

  • T. S. Risch, G. R. Michener, and F. S. Dobson. 2007. Variation in litter size: a test of hypotheses in Richardson's ground squirrels. Ecology, 88: 306-314.
  • Michener, G. R. 2004. Hunting techniques and tool use by North American badgers preying on Richardson's ground squirrels. Journal of Mammalogy, 85: 1019-1027.
  • 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. 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. 1982. Infanticide in ground squirrels. Animal Behaviour, 30: 936-938.
  • Michener, G. R. 1980. Estrous and gestation periods in Richardson's ground squirrels. Journal of Mammalogy, 61: 531-534.
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