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Spermophilus richardsoniiRichardson's ground squirrel

By Jillian Reimer

Geographic Range

Richardson's ground squirrels are a native northern plains species found in Canada through foothills of the Rocky Mountains in southern Alberta, southern Saskatchewan, and southwest Manitoba. In the United States, the species range spans western Minnesota, North Dakota, north-eastern South Dakota and northern Montana. As Richardson's ground squirrels are non-migratory, their range is seasonally stable. (Michener and Koeppl, 1985; Swenson, 1981)

Habitat

Richardson’s ground squirrels inhabit grasslands, fields, rolling hills and farmlands. The species is semi-fossorial, digging extensive burrow networks, consisting of chambers, passages and numerous entrances. This species thrives in anthropogenic altered habitats such as pastures and ploughed fields, favoring regions with short grass (< 15 cm) for den site entrances. Squirrels also build burrows preferentially in brown soil zones on elevated microhabitats. (Michener and Koeppl, 1985; Proulx, et al., 2012)

  • Range elevation
    0 to 3747 m
    0.00 to 12293.31 ft
  • Average elevation
    Unknown m
    ft

Physical Description

Richardson’s ground squirrels are stout-bodied squirrels with relatively short, dense pelage. The dorsum is light pinkish or cinnamon buff with a dappled posterior and light, countershaded ventrum. The fronts of forelegs, neck, and the sides of the head are a darker cinnamon buff in colour in contrast to the light buff eye ring. The tail is also countershaded with black on the dorsal surface and a cinnamon or clay ventrum, edged with pinkish buff. The animal's ventrum bares five pairs of evenly spaced mammae from the axillary to inguinal region. (Bintz and Mackin, 1980; Bintz and Rosebury, 1978; Michener and Koeppl, 1985; Moore, 1961)

Mean, minimum and maximum skull measurements for each sex (in mm) are as follows: greatest skull length is 46.5 (ranging from 45.1 to 48.4) for females and 47.7 (ranging from 47.3 to 48.0) for males. Palatilar length is 23.7 (22.5 to 24.0) for females and 23.7 (23.0 to 25.0) for males. Zygomatic breadth is 30.4 (29.5 to 24.0) for females and 31.9 (30.5 to 33.5) for males. Cranial breadth is 19.6 (19.0 to 20.9) for females and 20.2 (19.9 to 20.8) for males. Interorbital breadth is 9.5 (8.8 to 10.0) for females and 9.9 (9.5 to 10.6) for males. Postorbital constriction is 11.1 (10.8 to 11.5) for females and 11.4 (10.8 to 11.9) for males. Nasal length is 16.9 (16.0 to 18.1) for females and 17.4 (17.0 to 18.0) for males. Maxillary toothrow length is 10.4 (10.0 to 10.9) for females and 10.4 (10.2 to 10.7) for males.

The dental formula for S. richardsonii is: I 1/1 C 0/0 P 2/1 M 3/3 = 22. The upper cheek teeth are slightly posteriorly convergent. (Michener and Koeppl, 1985)

The body mass of Richardson’s ground squirrels depends on sex, location, age, and season. They are a hibernating species, and seasonal torpor significantly depletes white and brown adipose deposits. Upon emergence, yearling females (1 year of age) weigh a mean 200 g, compared to the mean 250 g adult females (older than 1 year). Between emergence and insemination, females gain a mean of 23 g and they gain an additional 139 g over the course of pregnancy. Females lose an average of 54 g upon parturition, at which time their brown adipose deposits are at a seasonal low until juveniles are weaned. Reproductive females gain brown adipose at 40 days postpartum until hibernation, with pre-hibernation weights ranging from 350 g to 435 g. Without the cost of gestation and lactation, non-reproductive females begin gaining weight earlier in spring compared to reproductive females. (Michener and Koeppl, 1985; Michener, 1974a; Michener, 1978; Michener, 1983a; Michener, 1984a; Michener, 1984b)

Males sampled in central and southern Alberta weigh 360 g to 425 g upon spring emergence. Males sampled at the foothills of Alberta and Saskatchewan weigh 300 g after hibernation. Mass at emergence is age-dependent with older males weighing more than younger males. Mass is gained until the mating season (several weeks after emergence), at which time males lose 8 to 18% of their pre-breeding mass. When most females have mated, males begin to gain mass until hibernation, with pre-hibernation weights ranging from 450 g to 500 g in central Alberta and southern Saskatchewan, as compared to pre-hibernation weights in Southern Alberta, which span 500 to 600 g. While males are 25% heavier than females, they store less fat than females. In females, fat contributes 24% of body mass, as compared to 10% in males. (Michener and Koeppl, 1985; Michener, 1974a; Michener, 1978; Michener, 1984a; Michener, 1984b)

At 28 to 30 days of age, juvenile squirrels (younger than 1 year of age) have an average mass of 76 g, with minimum weights of 45 g and maximum weights of 120 g. Juveniles gain steadily from the time of emergence until hibernation, with mean pre-hibernation weights of 365 g for females and 459 g for males. (Bintz and Strand, 1983; Dolman and Michener, 1983)

  • Sexual Dimorphism
  • male larger
  • Range mass
    200 to 500 g
    7.05 to 17.62 oz
  • Average mass
    360 g
    12.69 oz
  • Range length
    277 to 306 mm
    10.91 to 12.05 in
  • Average length
    285.4 mm
    11.24 in
  • Range basal metabolic rate
    3.61 (high) cm3.O2/g/hr
  • Average basal metabolic rate
    1.67 cm3.O2/g/hr

Reproduction

Secondary sex ratios are 1:1 and become increasingly female biased between the juvenile and yearling year. Adult males emerge from hibernation before females, making the population sex ratio largely dependent on the number of females emerging above ground. The population sex ratio is usually female-biased, due to sex-specific mortality, with approximately 2 to 10 females per male. Despite the female biased population sex ratio, the operational sex ratio during breeding is usually male biased. Female emergence is usually synchronous, and spatial dispersal is highly clumped. In these cases, dominant males attempt to establish and defend territories in areas occupied by newly emerged females while subordinate males are non-territorial and move over a wide range over the breeding period. Under these conditions, the mating system appears to be male defense polygyny. However, poor environmental conditions (such as extended snow cover) can prolong hibernation and result in asynchronous female emergence. Consequently, female distribution is sparse and unpredictable, resulting in a higher male-biased operational sex ratio. Under these conditions, males are non-territorial and disperse more widely. (Davis and Murie, 1985; Gedhir and Michener, 2014; Michener and Koeppl, 1985; Michener and Michener, 1971; Michener and Michener, 1977; Michener, 1983a; Michener, 2002; Nellis, 1969; Schmutz, et al., 1979; Sheppard, 1972)

Copulation typically occurs below ground, however when mates copulate above ground, the male mounts the female from behind. The pair rolls onto their side for coitus such that the male lays on his side while the female lays partially on her side with her head and shoulders raised. Copulation lasts 3 to 4 minutes, after which males groom their genital area. A copulatory plug (19.8 by 6.9 by 5.8 mm) forms within 1 hour of insemination and is lost after 15 to 17 hours, after which females may copulate with another mate. (Davis, 1982; Michener and Koeppl, 1985; Michener, 1983a; Michener, 1984b)

Males are most social during the mating period, initiating frequent aggressive acts with other males (occasionally resulting in injury) in addition to sex with females. Males' increased activity and social investment comes at the expense of foraging time, resulting weight loss over the intense 1.5 to 2 weeks of the mating period. In contrast, females are rarely injured during the mating period and gain weight steadily from emergence until parturition, with the exception of the day of estrus. (Davis and Murie, 1985; Michener and Koeppl, 1985; Michener, 1979a; Sheppard and Yoshida, 1971)

Beginning at 11 months of age, 90-100% of females produce one litter annually. Females mate 3 to 5 days after emergence from hibernation in spring. Emergence date depends largely upon a population's range and the local climate. In southern Alberta, peak breeding is in mid- to late March, as compared to central Alberta and Saskatchewan, where peak breeding occurs in mid-April. While the mating season is usually 3 to 5 weeks (as measured in southern Alberta), 50% of females are inseminated within a 3 to 9 day period. Testes descend upon emergence from hibernation and reach peak length at 12 days post-emergence, regressing back to the abdomen after 8 weeks. The vulva swells when females are in estrus. Whether ovulation is induced or spontaneous remains unknown. (Michener and Koeppl, 1985; Michener, 1980a; Michener, 1983a; Michener, 1984c; Michener, 1985; Nellis, 1969; Sheppard, 1972)

Following a 22.5 day gestation period, litters are born in an underground burrow. Mean litter size at birth is 4.9 to 8.3, however mean litter size at emergence is reduced (3.8 to 7.2). Litter size is dependent upon the mother's body mass and fat index (to a greater extent than the mother's age). Juveniles emerge from the burrow at 26 to 33 days of age (mean age at emergence is 29.2 days). Synchronous natal emergence occurs in late May to early June across the majority of the species' range (eastern North Dakota, southern Manitoba, southern Saskatchewan, central Alberta, the foothills of the Rocky Mountains). However, in southern Alberta, juveniles emerge in April to mid-May. (Dolman, 1980; Michener and Koeppl, 1985; Michener, 1974a; Michener, 1977a; Michener, 1979b; Michener, 1985; Nellis, 1969; Schmutz, et al., 1979; Schmutz, et al., 1980)

Richardson's ground squirrels have altricial young, with a mean neonate mass of 6.5 g (with minimum and maximum masses of 4.4 g and 9.6 g, respectively). Neonates are born naked, with fused digits, closed eyes and ears, and unerupted teeth. A fine pelage is developed at 4 to 7 days of age, followed by separation of foredigits at 13 days. Eyes and ears open at 19 to 21 days. Upper incisors erupt at 20 to 22 days, and juveniles are weaned at 25 days postpartum. Postnatal mass is gained over three linear phases. In the first phase, juveniles gain 13% of their body mass daily (days 1 to 10). In the second phase, juveniles amass 6% daily (days 11 to 45). Finally in the third phase, juveniles gain 1.5% of their body mass daily (days 45 to 70). (Dolman, 1980; Michener and Koeppl, 1985; Michener, 1983a)

Although poikilothermic at birth, juveniles can elevate body temperature 4 to 5˚C above ambient temperature for ≥1 hour when exposed to experimental ambient temperatures of 25 to 30˚C. At 29 to 32 days old, juveniles can maintain a body temperature of 38˚C over 2 hour experimental exposures to 25˚C. (Dolman, 1980; Michener and Koeppl, 1985; Michener, 1983a)

  • Breeding interval
    Richardson's ground squirrels breed once annually in spring.
  • Breeding season
    Mating begins 3-5 days after emergence from hibernation, between mid-March and mid-April, depending on location. The mating season spans 3-5 weeks, however most copulation occurs within a 3-9 day period.
  • Average number of offspring
    4-7
  • Average number of offspring
    6.06
    AnAge
  • Average gestation period
    22.5 days
  • Average gestation period
    24 days
    AnAge
  • Average weaning age
    25 days
  • Range time to independence
    26 to 33 days
  • Average time to independence
    29.2 days
  • Average age at sexual or reproductive maturity (female)
    11 months
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    335 days
    AnAge
  • Average age at sexual or reproductive maturity (male)
    11 months
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    335 days
    AnAge

Mothers exhibit parental care in the nest and (to a lesser extent) after natal emergence. Females nurse, groom and warm offspring for 24 days postpartum. When mothers change nest sites, they carry their pups to the new nest and, when pups are too large to carry, females lead them to the new nest site. Mothers retrieve displaced pups from the nest until 20 to 25 days postpartum. Similarly, experimental studies show that when deprived of their nest, mothers will still pick up her displaced pup and carry it in her mouth. In nature, lactating females retrieve pups placed at the burrow entrance and return them to the nest. Conversely, nuliparous yearlings are less likely to retrieve and more likely to attack unrelated neonates placed in their cage than females who have previously reared offspring. This species does not exhibit alloparenting behavior in nature. (Michener and Koeppl, 1985; Michener, 1971; Michener, 1973a; Michener, 1974b; Michener, 1977b; Michener, 1980b)

Males do not provision females or offspring, however alarm calling behavior is considered by some to be a form of paternal investment. Additionally, males occasionally chase weasels from nests. Experimentally, yearling males exhibit aspects of parental care: they may retrieve pups placed in their home cage. However, compared to mothers, their responses are slow, erratic and occasionally result in aggression. Males have also groomed pups in experimental studies and engaged in parental care behavior, including rearranging nest paper around pups and assuming nursing posture. However, such behavior has not been observed in the wild. (Davis and Murie, 1985; Davis, 1984a; Michener and Koeppl, 1985; Michener, 1973a)

  • Parental Investment
  • altricial
  • female parental care
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • protecting
      • female
  • post-independence association with parents
  • extended period of juvenile learning

Lifespan/Longevity

Mortality is much lower in female squirrels than in males. For female juveniles, survivorship to one year of age is 40-50%, as compared to a mere 10-20% in males. Survivorship to the subsequent year is 50% for females, versus 15-30% for males. The average lifespan for females is 2 to 4 years, while adult males generally live 1 to 3 years. The oldest observed individuals in the wild were 5 years (female) and 3 years (male) of age. (Michener and Koeppl, 1985; Michener, 1998)

  • Average lifespan
    Status: wild
    5 years
  • Average lifespan
    Status: wild
    1-4 years
  • Average lifespan
    Status: captivity
    2-4 years

Behavior

Richardson's ground squirrels are a hibernating species. Emergence time depends largely on soil and air temperature. In southern Alberta, emergence begins in late February and March, however in central Alberta, Saskatchewan, southern Manitoba and eastern North Dakota emergence takes place in late March and April. Emergence time is age and sex-dependent: adult females (and female yearlings) emerge two weeks later than males and male yearlings. Squirrels in each cohort emerge synchronously, with 75% of the cohort emerging over a 1 to 2 week period. Juvenile litters emerge a mean 56 days after female emergence. (Michener and Koeppl, 1985; Michener, 1973b; Michener, 1974a; Michener, 1977a; Michener, 1979b; Michener, 1979c; Michener, 1983a; Michener, 1984c; Michener, 1985; Schmutz, et al., 1979)

Individuals are only above ground for 3 to 3.5 months each year, but collectively, the colony has a 7 month active season (from March to September in central Alberta, the foothills of southern Alberta and southern Saskatchewan). This discrepancy is a result of the cohorts' sequential entry of into hibernation: adult males begin hibernation in June and early July, and adult females begin in July and early August. Juveniles are above ground without adults for about two months before females descend into the hibernaculum in late August to early September, followed by males in September to early October. This seasonal sequence optimizes adult males' energetic requirements for mating, adult females' energetic investment in gestation and lactation, and juveniles' growth and development. (Bintz and Strand, 1983; Michener and Koeppl, 1985; Michener, 1974a; Michener, 1979b; Michener, 1979c; Michener, 1984c)

Squirrels spend 84% of their lives sleeping underground (hibernation) between their active seasons, making their subterranean burrows a very important site for this species. Burrows are used as hibernacula and, during the active season, are the sites for copulation, rearing offspring, sleeping, and offer refuge from predators and inclement weather. Burrows extend 4 to 15 m from their opening, descend 1 to 2 m in depth and include a network of tunnels and chambers measuring 15 to 23 cm in diameter. Richardson's ground squirrels are diurnal, emerging from the burrow approximately 95 to 130 minutes after sunrise, and delaying emergence during cool inclement weather. (Michener and Koeppl, 1985; Michener, 2002)

Females dig tunnels and sleep in 6 to 11 different chambers over the 15 week active season, and a single chamber for hibernation. Over the first half of the active season, during mating, pregnancy and lactation, females use multiple sleep sites. Sites are used once, over multiple nights, before switching to a new sleep site. In the second half of the active season, females move back and forth between 5 to 6 sites. Interestingly, females who do not wean juveniles over the active season also undergo this abrupt shift in sleeping sites. Furthermore, females always sleep in their future parturition chamber for some or all of gestation and remain there for some or all of lactation. Females never sleep in their future hibernation chamber in the active season prior to hibernation, but 50% of females use the hibernaculum as a sleeping site immediately after their spring emergence. However, those who continue to use the hibernaculum as a sleep site only do so over 1 to 5 nights before seeking new sleep sites. (Michener and Koeppl, 1985; Michener, 2002)

Richardson's ground squirrels have over 35 classified and studied behaviors, most of which are associated with feeding, moving, vigilance, and interacting with conspecifics. Behavioral variation is largely contingent on sex, age, and timing in the annual cycle. Between spring emergence and mid-summer, females spend 50% of time above ground feeding. In the final 4 weeks before hibernation, females invest 80% of time above ground in foraging activity. Females are more social than males, investing 5 to 15% of their time above ground in social interactions with conspecifics. Social interaction is most common around the time of juvenile emergence. (Davis, 1984b; Michener and Koeppl, 1985; Michener and Sheppard, 1972; Michener, 1979a; Michener, 1980a; Michener, 1984b; Sheppard and Yoshida, 1971)

During the mating period, males spend less time feeding, instead dedicating a greater proportion of their time to movement and interactive behavior. Males' social interactions are most frequent during this season, at which time they initiate aggressive acts with other males as well as sex with females. Over the remainder of the active season, males reduce social behaviors and movement, investing 50 to 60% of time spent above ground in foraging activities. This proportion of time foraging increases to 80 to 90% in the final 6 weeks before hibernation. (Davis and Murie, 1985; Michener and Koeppl, 1985; Michener, 1979a; Michener, 1984b)

Juveniles spend 40 to 80% of time above ground feeding as they learn to forage and struggle to meet the high energetic costs of development and fat deposition in preparation for hibernation. (Bintz and Mackin, 1980; Michener and Koeppl, 1985; Michener, 1981)

Richardson's ground squirrels are ranked as either 2 or 3 on a 5 grade social scale, although the exact value remains contested. Mothers, daughters and sisters form life-long affiliative bonds, and female kin share space as adults, which is characteristic of more complex social systems. However, male juveniles and yearlings disperse individually, maintaining no associations with family members into adulthood. The social system is matrilineal and matrifocal, with female natal philopatry and male dispersal. (Armitage, 1981; Davis and Murie, 1985; Michener, 1983b; Michener, 1984c; Schmutz, et al., 1979)

Spatial patterns are nether spatially nor temporally homogenous for this species. Males disperse as juveniles or as yearlings and females remain spatially aggregated near the site of their natal burrow. Adult distribution is clustered over the mating season with densities for large (> 10 ha) sites ranging from 1.4 to 5.2 individuals/ha during peak mating season. Surveys on a 108 ha site found clusters of 5.3 adults/ha over a single 1.5 ha portion, while the overall density for the site was 1.5 adults/ha. Smaller study sites have found 12.0, 19.1 and even 27.0 adults/ha. (Michener and Koeppl, 1985; Michener and Michener, 1977; Michener, 1973b; Michener, 1979a; Michener, 1979b; Schmutz, et al., 1979; Yeaton, 1972)

Richardson's ground squirrels have three major phases in their spatial and social patterns after the mating period. For the 7 week pregnancy and lactation phase, females are evenly dispersed in core areas that overlap slightly with related neighbours. At this time, females exhibit site-dependent dominance and are aggressive to conspecifics, particularly males and distantly related or unrelated females. It is during this phase that each female excavates her burrow, makes a nest, and rears her litter in isolation. (Davis, 1984b; Michener and Koeppl, 1985; Michener and Michener, 1973; Michener, 1973c; Michener, 1979a; Michener, 1980a; Quanstrom, 1971)

The next phase is 4 weeks and begins at the time of natal emergence as juveniles gradually expand their range from the natal burrow. The spatial distribution is highly aggregated at this time, with densities as high as 56.3 juveniles/ha on a 2 ha site. Two weeks after emergence, juveniles begin to interact with other neighboring adults and juveniles from other litters. Social interactions between juveniles and their mother or littermates are amicable, however interactions with more distantly related and unrelated individuals tend to be antagonistic. (Davis, 1984b; Michener and Koeppl, 1985; Michener and Michener, 1973; Michener, 1973c; Michener, 1979a; Michener, 1980a; Quanstrom, 1971)

The third and final phase before hibernation is characterized by a reduction in adult squirrels' range and time spent above ground. Juveniles establish their distinct core area near their family, however they no longer share their burrow with littermates or their mother. Dispersing juvenile males leave during this phase, and interaction becomes increasingly infrequent (although interactions between siblings remain amicable) as spatial dispersion increases. (Davis, 1984b; Michener and Koeppl, 1985; Michener and Michener, 1977; Michener, 1973c; Michener, 1979a; Michener, 1980a; Quanstrom, 1971)

Home Range

The range and movements of male dispersal remain poorly understood, however movements of 3 to 10 km from the natal site have been observed. Female home ranges are usually smaller than males because of dispersal, however in years where female home ranges are larger, female movements can be more widespread than years where the range of male dispersal is constrained. Both males and females undergo seasonal variation in their home range size. Female home ranges are constrained near the burrow over the period of gestation and lactation, expand after natal emergence, and are once again reduced during the four weeks preceding hibernation. For males, their home range is smallest after the breeding season, increase in size in midsummer, and is once again reduced just prior to hibernation. (Michener and Koeppl, 1985; Michener, 1979a; Schmutz, et al., 1979; Yeaton, 1972)

Communication and Perception

This species exhibits complex, apparently altruistic alarm vocalizations. Alarm signals are multi-syllabic and use variation in acoustic structure and repetition to convey predator class and response urgency to listening colony members. Receivers perceive these calls, interpret the encoded information and respond accordingly. Callers communicate response urgency with syllabic frequency, intersyllabic latency (which is negatively correlated with predator distance), and rates of syllabic repetition. Receivers also integrate information from multiple callers to assess the certainty of a threat, response urgency, predator location, and even predator movements. Using this encoded information, receivers respond with vigilance proportional to the magnitude or certainty of the threat. Calls are generally considered to communicate response urgency, however aspects of alarm signals are functionally referential. Chirps are typically issued in response to airborne threats and elicit a behavioral response of running to the burrow. Conversely, whistles alert colony members to terrestrial predators and elicit upright, vigilant postures in receivers. (Davis, 1984b; Hare, 1998a; Hare, 2001; Koeppl, et al., 1978; Sloan and Hare, 2008; Sloan, et al., 2005; Thompson and Hare, 2010; Wentin, et al., 2001)

Juveniles differ somewhat from adults in their anti-predator vocalizations. Juvenile response to adult callers is independent of kinship, likely because all adults are successful models of predator detection and anti-predator response. Consequently, all calls issued by experienced callers must be met with the appropriate response in less experienced receivers. In contrast, adult receivers are more selective listeners and, while they do not ignore juvenile callers, they respond with greater vigilance in response to calls that communicate greater certainty and response urgency. Adults assess response urgency based on the number of callers as well as the acoustic structure of the trailing elements. This age-dependent response variation may be explained by greater predation pressure on juveniles. Although adults are also under heavy predation pressure, their predator evasion strategies improve with experience, affording them increased investment in foraging activities. Investment in foraging and vigilance is a constant trade-off, and optimizing this trade-off is especially important in hibernating species. (Hare and Warkentin, 2012; Lima and Dill, 1990; Sloan and Hare, 2006; Sloan and Hare, 2008; Sloan, et al., 2005; Swan and Hare, 2008a; Swan and Hare, 2008b)

Alarm vocalizations span a broad range of frequencies. While whistle calls have a fundamental frequency ca. 8kHz, ultrasonic calls (or whisper calls) have a fundamental frequency of 48 kHz. Consequently, they attenuate over a shorter distance (ca. 15m) than audible calls, which may carry through the entire colony. Given their constrained active space, whisper calls are advantageous to callers when conspecifics are clustered in close proximity and predators are outside the call's active space. This call is especially important at the time of natal emergence when juveniles (who can perceive adults' whisper calls) are most vulnerable and spatially clustered. (Hare, 1998a; Michener and Koeppl, 1985; Wilson and Hare, 2006)

Although less studied than vocal communication, olfaction is thought to be an important mechanism for kin discrimination in this species. Olfactory cues are likely observed early in development, before juveniles open their eyes. Upon natal emergence, when juveniles are first exposed to non-littermates, olfaction is linked to kin and neighbour recognition. While this mechanism remains poorly understood, it appears to play a central role in the subsequent development of social structure and territoriality. Anal glands deposit pheromones in fecal matter, further suggesting that olfaction plays an important role in territoriality, especially considering higher rates of defecation in territorial displays. (Davis, 1982; Hare, 1998b; Michener and Michener, 1973; Michener and Sheppard, 1972; Sheppard and Yoshida, 1971; Sleggs, 1926)

Food Habits

Richardson's ground squirrels have a varied, primarily herbivorous diet comprised of succulent vegetation such as leaves and flowers. As a hibernating species, they rely on the high fat and nutrient content of seeds to store adequate fat deposits for the winter. Squirrels living on agricultural land grow larger and experience heightened reproductive success due to the high proportion of forage grasses and legumes (such as Bromus inermis, Agropyron cristatum, Trifolium pratense, T. rapens, and Meliotus alba) and domesticated cereal grains (including wheat, barley and oats) in their diet. They also consume native prairie grasses such as Bouteloua gracilis, Amaranthus sp., and Astragalus spp., but they consume proportionally less of these species in their diet compared to squirrels inhabiting native grasslands. Although Richardson's ground squirrels consume a 90% herbivorous diet, they are somewhat opportunistic, occasionally consuming insects (which make up the majority of the remaining 10% of their diet) and scavenging road-killed conspecifics. (Michener and Koeppl, 1985; Sheppard, 1972)

  • Plant Foods
  • leaves
  • seeds, grains, and nuts
  • flowers

Predation

Richardson’s ground squirrels also face predation pressure from various avian species including buteos (Buteo jamaicensis, B. regalis, and B. swainsoni), harriers (Circus cyaneus), falcons (Falco mexicanus), eagles (Haliaeetus leucocephalus), owls (Nyctea scandiaca, and Bubo virginianus), and black-billed magpies (Pica pica). Richardson's ground squirrels are an important prey item for buteos, comprising 90%, 76% and 75% of prey biomass for ferruginous hawks, red tailed hawks, and Swainson's hawks, respectively. Males are generally more vulnerable to predation by buteos than females. Despite their diurnal lifestyle, Richardson's ground squirrels are common prey to nocturnal predators such as the great horned owl (Bubo virginianus). (Davis, 1984a; Goulden, 1975; McInvaille and Keith, 1974; Michener and Koeppl, 1985; Michener, 2001; Proulx, et al., 2012; Schmutz, et al., 1979; Schmutz, et al., 1980; Sheppard and Swanson, 1976)

Richardson's ground squirrels are an important prey item for numerous wild and domestic terrestrial predators including mustelids (Taxidea taxus, Mustela frenata and Neovison vison), canids (Canis latrans and Vulpes vulpes), and skunks (Mephitis mephitis). Domestic cats (Felis catus) and dogs (Canis familiaris) are also common predators of Richardson's ground squirrels, especially in suburban habitats. Long-tailed weasels can have substantial impacts on population size, as they can enter burrow and prey on unweaned juveniles, reducing juvenile populations by upwards of 50% in a single predation event. Weasels are common predators to adults as well, although adults may fight and chase weasels. Badgers may ambush squirrels above ground, but more commonly dig up squirrel burrows, significantly reducing population sizes. Consequently, squirrels are most vulnerable to predation by badgers after the onset of hibernation and before the ground freezes. (Byrne, et al., 1978; Davis, 1984a; Michener and Koeppl, 1985; Michener, 1979b; Schmutz, et al., 1979; Schwab, 1978; Sheppard and Swanson, 1976)

Richardson's ground squirrels respond to high predation pressure with social adaptations such as grouping and alarm communication (see Behavior and Communication). Alarm calls are used to alert receivers to the identity of the threat as well as communicate vigilance to conspecifics. Referential alarm calls can communicate predator class, allowing receivers to respond with specific anti-predator strategies. In response to "chirps," which communicate aerial threats, receivers run to the burrow, whereas "whistles," which communicate terrestrial threats, elicit vigilant postures as squirrels scan their environment. (Davis, 1984a; Michener and Koeppl, 1985; Sloan, et al., 2005)

Ecosystem Roles

Richardson's ground squirrels are an important prey items for a wide diversity of predatory species across the northern plains, including ferruginous and Swainson's hawks (see Predation). Richardson's ground squirrel abundance is positively correlated with overall hawk density. Squirrels are also an important forage species. In south-western Manitoba, the species is known to promote biodiversity of local grasses and legumes when their population size is moderate, largely because of their control of the invasive plant species Bromus inermis. (Newediuk, et al., 2015; Schmutz and Hungle, 1989)

In addition to their significance in grassland food webs, this species is a host to numerous parasitic species. Squirrels carry the tick Dermacentor andersoni, a vector of Rocky Mountain spotted fever and tularemia. Squirrels through Alberta are also frequently parasitized by the fleas Opiscrostis labis and Orospsylla rupestrs, both of which are vectors of plague. Infestation ranges between 20 to 30 fleas at emergence from hibernation, however levels are reduced to 1 or 2 fleas per individual by June until August. Mites and lice, such as Linognathoides laeviusculus (observed in southern Manitoba) are common ectoparasites. In a study in southern Manitoba, L. laeviusculus infested 63.6% of squirrels with a mean intensity of 37.2 with highest prevalence in April and mid-June, and mean intensity peaks in June. Juveniles have higher prevalence and mean intensity of ectoparasites than adults, and females generally carry more ectoparasites than males. (Brown and Roy, 1943; Hilton and Mahrt, 1971a; Hilton and Mahrt, 1971b; McGee, 1980; McLeod, 1933; Michener and Koeppl, 1985; Michener, 1979b; Waterman, et al., 2014; Yunik, et al., 2016)

Sex-biased parasitism in this species can be attributed to different behaviour between sexes. As the dispersing sex, males are exposed to a greater diversity of endoparasites than females. Additionally, males' energetic trade-off between immunity and increased reproductive investment results in heightened infection by coccidians, trypanosomes, cestodes, trematodes, nematodes, and acanthocephalans. Compared to males, females have a constrained range, reducing their risk of infection by endoparasites, however grooming behaviour results in higher prevalence of ectoparasites in females than males. (Hilton and Mahrt, 1971a; Hilton and Mahrt, 1971b; McGee, 1980; McLeod, 1933; Michener and Koeppl, 1985; Waterman, et al., 2014; Yunik, et al., 2016)

As a burrowing species, Richardson’s ground squirrels are also ecosystem engineers in the prairies. Uninhabited burrows provide habitat to mammals such as field mice and voles, as well as other vertebrates such as salamanders. Invertebrates, including bumblebees, also use burrows as nest sites, suggesting that squirrels’ burrowing activity can indirectly promote pollination by providing pollinators with nest sites. Burrows enlarged by animals such as badgers are also used as nest sites by burrowing owls. (Miller, et al., 1994)

Commensal/Parasitic Species

Economic Importance for Humans: Positive

Richardson's ground squirrels are an important species influencing forage plants. Moderately sized populations in southern Manitoba can alter the plant communities on farmland by reducing the biomass of Trifolium pretense and increasing the biomass of Elymus repens and Medicago lupulina. Both of the latter species can offer high quality protein forage for cattle and promote weight gain, provided squirrel populations are small enough to prevent a net decrease in forage biomass.

Richardson's ground squirrels are also convenient research subjects for a variety of lab and field studies, especially in studies of sociality. Their small body size, ubiquity, diurnal lifestyle and well-characterized behavior and life-history make this species an excellent subject for various field studies. (Michener and Koeppl, 1985; Newediuk, et al., 2015)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

Richardson's ground squirrels thrive in human-modified habitats such as cultivated fields and grazed pastures, and their high densities can make them an agriculture pest. Average sized (12 squirrels/ha) populations can consume ca. 90 pounds of forage per acre in a month, resulting in substantial economic losses. Furthermore, burrowing can cause significant damage to crops (namely wheat and oats), with burrow opening densities ranging from 142 to 162/ha. Squirrels can compete with grazing farm animals for forage, cause significant economic losses in equipment damage, and cause water loss in irrigated fields. Species control measures have been enacted in Canada for over a century, however rodenticides such as anticoagulants, strychnine and zinc phosphide are largely ineffective as squirrels can develop resistance to toxins.

Richardson's ground squirrels can also be vectors of diseases such as Alberta rocky mountain spotted fever, sylvatic plague and tularemia. ("Survey protocol for the Richardson's ground squirrel", 2003; Alsager and Yaremko, 1972; Brown and Roy, 1943; Cassola, 2016; Ling, et al., 2009; Michener and Koeppl, 1985)

Conservation Status

Richardson’s ground squirrels are listed in the IUCN Red List of Threatened Species as ‘Least Concern’ due to their widespread range (including several protected areas), high prevalence and absence of major threats. This species thrives on farmland and is therefor minimally, and often favorably, affected by agriculture. This species is not currently listed on the US Federal List, the State of Michigan list and has no special status under CITES. (Cassola, 2016)

Other Comments

Richardson's ground squirrels (Order: Rodentia, Family: Sciuridae), also called the flickertail, were formerly assigned to the genus Spermophilus. However, phylogenetic studies have since re-assigned them to the Holarctic ground squirrels genus, Urocitellus. (McLean, et al., 2016)

Contributors

Jillian Reimer (author), University of Manitoba, Jane Waterman (editor), University of Manitoba, Tanya Dewey (editor), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

agricultural

living in landscapes dominated by human agriculture.

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

carrion

flesh of dead animals.

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal

chemical

uses smells or other chemicals to communicate

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females

folivore

an animal that mainly eats leaves.

fossorial

Referring to a burrowing life-style or behavior, specialized for digging or burrowing.

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

insectivore

An animal that eats mainly insects or spiders.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

polygynous

having more than one female as a mate at one time

scavenger

an animal that mainly eats dead animals

scent marks

communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

stores or caches food

places a food item in a special place to be eaten later. Also called "hoarding"

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

ultrasound

uses sound above the range of human hearing for either navigation or communication or both

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

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