Basic Social Organization

Despite the fact that Belding’s ground squirrels live in colonies, there is very little social interaction within the S. beldingi population, outside of those between mother and offspring and the nepotistic relationships between mothers, daughter, litter mate sisters, and non-litter mate sisters (Sherman and Morton 1979). In fact, all other interactions are agonistic (Turner 1972). Adult male S. beldingi are nomadic between seasons and the adult male that copulates the most will move the farthest from the colony in which he mated. Females, on the other hand, settle near their kin (Sherman and Morton 1984).

Why Colonies?

Belding’s ground squirrels live in colonies because of selfish herd effect (the more squirrels there are in one colony, the less likely one individual squirrel will be picked off) and because of alarm calling (see alarm calling). Females also live in colonies because they live near their kin, who help them defend territories (see nepotism). Males benefit from the alarm calls and the selfish herd effect as well, thus they stay in the colonies, even though they do not necessarily interact with other individuals.

Mating System:

S. beldingi has an especially unique sexual cycle due to its habitat. Because it must hibernate to avoid extreme cold, it must be able to breed shortly after arousing from hibernation so that the young are born at time that is conducive to their survival and growth. Due to this fact, breeding occurs in late May and early June (Sherman and Morton 1984) when the weather becomes warmer and there is plenty of food (in the summer), and litters are born before the winter strikes (Morton and Gallup 1975).

The mating system of S. beldingi is promiscuous, meaning that both sexes mate with multiple individuals (Sherman and Morton 1979), and no bonds are formed between the male and the female. When males emerge from hibernation in April, they are ready to mate (Loehr and Risser1977). Females will mate within 4-7 days of emergence (Sherman and Morton 1984, Sherman and Morton 1979). Incredibly, females are only sexually receptive for three to 4.7 hours one afternoon each year (Hanken and Sherman 1977). When a female is receptive, several excited males will gather around her at once (Sherman and Morton 1979). They will threaten , chase, and even fight with each other to gain access to her. Fighting behavior includes grappling, kicking, scratching, or biting. Females will mate preferentially with the winner of these fights. Thus, larger, older, and stronger males mate more frequently and are thus more reproductively successful (Sherman and Morton 1984). Females will mate with as many as five males during the afternoon. Multiple mating includes assured pregnancy and increased genetic diversity (Sherman and Morton 1979).

Juvenile male S. beldingi will not be able to mate until they are two years old, possibly because they are using this energy to increase their growth rate, ensuring a greater reproductive success in the next season (Morton and Gallup 1975). Juvenile females, on the other hand, are able to mate after their first hibernation, ensuring that every possible female is pregnant in each breeding season (Morton and Gallup 1975). Yearling females go into estrous later than the adult females, probably because they emerge later and are smaller in size. In addition, the litters of yearling females are smaller than those of adult females (Morton and Gallup 1975).

Pregnancy and Weaning:

Females give birth to only one litter per year (Morton and Gallup 1975). When they are pregnant, females will dig nesting burrows with at least two openings (to evade predators) and will create nests in them made out of grass and grass roots. These burrows are about eight to fifteen feet long, and are about one to two feet underground (Sherman and Morton 1979, Turner 1972). The female will create a new burrow or move to an abandoned burrow if her first one is disturbed by weather or by predators. They will establish a nesting-only territory around this burrow, which they defend against unrelated conspecifics by attacking and chasing intruders. These territories are defended until the young are weaned (Sherman and Morton 1979, Sherman 1981b). Young males and non-resident females are chased away from the nesting territories most frequently (See infanticide!) (Sherman 1981b). In fact, pregnant females show the most aggression of all of the S. beldingi (Turner 1972).

Gestation in S. beldingi lasts from 23 to 31 days and the young are born in late June and early July (Sherman and Morton 1979, Turner 1972). Litter size ranges from three to eight pups, depending on the age of the mother (One year olds have 3-4 young, two to five-year olds have 6-8 young, and six to eight-year-olds have 3-4 young (Sherman 1981b, Sherman and Morton 1979)). Female fecundity peaks at age four (Sherman and Morton 1984). Most males will disperse directly after mating, thus they do not interact with their offspring. As a result, females are the sole caregivers of the young (Sherman 1981 b). The pups are raised in the nesting burrow with only the mother present (Sherman 1981b). After a few weeks underground, the juveniles are ready to face the world. They emerge from their burrows in late July or early August (at higher elevations), and are weaned when they are 27 days old (Holekamp 1986). According to Turner, the mother will coax the young out of the burrow. On the first day out of the burrow, the young will spend most of their time posting by the hole, and will not wander far. By the third day out of the burrow, the young will wander about and explore. If the young wander too far, the mother will chase them back to the burrow. She allows them to wander farther and farther as they age, and finally the juveniles disperse (1972).

Infanticide:

Infanticide, the killing of conspecific infants, is a darker side to the nature of S. beldingi. In a study done by Paul Sherman at Tioga Pass in 1981, 8% of all juveniles (up to 28 days of age) were killed by other Belding’s ground squirrels. Sherman observed these gruesome incidences, and he describes the intruding squirrel dragging a squealing, squirming juvenile out of the nest burrow, and promptly killing it by biting its head. The killer will then occasionally eat the carcass. Interestingly, only the non-related individuals drag nursing infants out of the nesting burrow and kill them, but all ages, sexes, and kinship classes occasionally assail juveniles that have been weaned. Adult females and one year old males are the most frequent killers. Significantly, none of the adult females that kill reside in the area where the infanticide occurred. On the other hand, young males often attack young that live near the burrow they inhabit. These males usually only kill one juvenile per incident. The young male will carry the body with it and eat it, and even occasionally defend it. Sherman hypothesizes that males will do this to grow stronger and gain weight at a higher rate in order to win more females during the next breeding season. Females, on the other hand, only commit infanticide if they have lost young in another area and have emigrated (which will often occur if females do not have relatives in the area when their litters are killed). Females will never kill relatives, they seldom eat their victims, and they will not fight for the body as often as males. Because females kill more female young, Sherman suggests that females practice infanticide to eliminate competition for nest burrows. Sherman supports his hypotheses by determining that the frequency of infanticide is not related to population density, not all ages and sexes kill equally, and related young aren't killed. (Sherman 1981 b)

Juveniles:

Upon emerging from the burrow, juveniles are introduced into a whole new world. The bright light and the warmth of the summer sun great them, as well as tons of new and exciting stimuli. The first few days they are above ground, the juveniles will remain entirely with their mothers and siblings (Holekamp 1986). Juveniles will develop preferences for littermates (Holmes 1994) and the ability to respond to alarm calls (Mateo 1996) early in their life. They spend time playing with each other, practicing for when they will defend their own females or territories. Play behavior includes pushing with the nose, pouncing, chasing, wrestling with forepaws, nibbling, and clinging to each other (Holmes 1994). Because they emerge late in the season, juveniles begin to build up fat reserves for hibernation later that adults, and must go into hibernation later (late September). Before hibernation, female juveniles will usually settle near their relatives, but males will disperse after weaning, possibly to prevent inbreeding. (Sherman and Morton 1979).

Dispersal:

Juvenile male S. beldingi disperse from their natal burrows soon after they are weaned and will disperse again after they breed successfully. As a result, males face extreme selection pressures and there are fewer males in the population than females (Holekamp 1986). Females are much less likely to disperse. Some female S. beldingi disperse, but when they do, they move to improve access to food or space for maternal territories, both of which are important for reproduction (Nunes et. al 1997). Male-biased natal dispersal is found in other sciurids. Natal dispersal is prompted by an ontogenetic change in the squirrel, which is sex-linked. This change occurs when males have stored enough fat or they have a minimum ideal body weight (heavier males disperse). This change could prompt many different behavioral changes. For example, Holekamp found that males showed weaker fear responses than females, and this response reached a low before and during dispersal. In addition, juvenile male S. beldingi are much more active than juvenile female S. beldingi (Holekamp 1986). Males also disperse after mating and will settle elsewhere, and thus do not interact with their offspring. These males will stay in these new nesting areas until the breeding season of the following year (Sherman 1981b).

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