The Evolution of Coloniality in White‐tailed and Black‐tailed Prairie Dogs (Sciuridae: Cynomys Leucurus and C. Ludovicianus)

Abstract

In a 6—yr study, I investigated possible selective bases for coloniality in two species of squirrels (Sciuridae): loosely colonial White—tailed Prairie Dogs (Cynomys leucurus) and densely colonial Black—tailed Prairie Dogs (C. ludovicianus). White—tail study sites were in Wyoming and Colorado, USA; Black—tail study sites were in Colorado and South Dakota. I examined three hypotheses that might explain the evolution of coloniality: (a) shortage of suitable habitat, (b) social facilitation of foraging, and (c) reduced predation. The apparent surplus of unused suitable habitat and the absence of isolated individuals both indicated that prairie dogs are not forced to live together because of habitat shortages. An analysis of prairie dog foraging patterns indicated that there is no social facilitation of foraging in terms of either (a) group hunting of either large or elusive prey, (b) the location of large, scattered food supplies, (c) modification of the soil in order to effect the growth of vegetation that is more favorable or more abundant than that which would otherwise result, or (d) group defense of foraging grounds. Three lines of evidence indicate that reduced predation may be the most important benefit of prairie dog coloniality. First, simulated predatory attacks by badgers (Taxidea taxus) indicated that individuals in large wards (subcolonies) detect predators more quickly than do individuals in smaller wards; further, Black—tails detect predators more quickly than do White—tails. Second, individuals in large wards devote proportionately less time to alertness (i.e., scanning for predators) than do individuals in smaller wards, and Black—tails are less vigilant than are White—tails. Third, breeding synchronization and center—edge differences in individual alertness both indicate the possible importance of self herd effects. Interspecific differences in ward size and ward density may ultimately result because White—tail habitats contain significantly more protective cover than do Black—tail habitats.

Attributes of Black-Tailed Prairie Dog Colonies in Northcentral Montana

“…Level terrain facilitates predator detection, and has been the most commonly cited explanation for the prevalence of colonies on small slopes (King 1955, Hoogland 1979Cable andTimm 1988, Hoogland et al 1988). However, the steeply sloped areas of south Phillips County are highly erodible (U.S. Bur.…”

“…However, extremely hot summers might influence prairie dogs to select cooler, northerly aspects. Although black-tailed prairie dogs do not hibernate and spend most of their waking hours aboveground (King 1955, Smith 1958, Hoogland 1979, the relatively constant -environment of prairie dog burrows provides relief from both summer heat and winter cold (Henderson et al 1974). Although colonies in south Phillips County exhibited predominantly southern aspects, this southern exposure appeared to be an artifact local terrain, rather than selection by prairie dogs.…”

“…Black-tailed prairie dogs live in complexes of colonies that form a mosaic of on-and off-colony habitats referred to as the prairie dog ecosystem (Hoogland 1981. Researchers have noted generalizations about prairie dog colonies.…”

“…Several colony characteristics, including slope, aspect, and soil types, presumably reflect prairie dog preference. Researchers state that prairie dog colonies commonly are located on level land (Koford 1958, Tilestone and Lechleitner 1966, Knowles 1982, because steep terrain obstructs their view of predators (King 1955, Hoogland 1979Hoogland et al 1988). In addition, larger colonies may have greater slopes than smaller colonies because as colonies expand they may spread into sub-optimal habitat with steeper slopes (Koford 1958).…”

Group living and the effects of spatial position in aggregations of Mytilus edulis

“…Most studies that have demonstrated negative effects imposed by group living have been on aggregations of "sessile" organisms such as colonies of nesting birds (Hoogland and Sherman 1976) and prairie dogs (Hoogland 1981), bryozoans (Buss 1981), and spiders (Buskirk 1975). In contrast, studies of group living in highly mobile organisms have mainly illustrated positive aspects of aggregation (in fish: Weihs 1973;Barlow 1974;Neill and Cullen 1974;Peterson 1976;Robertson etal.…”