Aggression, ectoparasitism, and other possible costs of prairie dog (Sciuridae, Cynomys spp.) coloniality.

In a 4-yr study, I investigated the costs of coloniality for two species of squirrels (Sciuridae) : loosely colonial White-tailed Prairie Dogs (Cynomys leucurus) and densely colonial Black-tailed Prairie Dogs (C. ludovicianus). Study sites were in Wyoming and Colorado (White-tails) and Colorado and South Dakota (Black-tails). By an examination of both intra- and interspecific effects, four costs were investigated: (1) increased aggression, (2) increased transmission of diseases and ectoparasites, (3) increased probability of misdirected parental care resulting from the mixing of unrelated young, and (4) increased conspicuousness to predators. The possibility of various miscellaneous costs was also investigated. I hypothesized that the costs of coloniality should be greater (a) for individuals of large wards (subcolonies) than for individuals of smaller wards and (b) for Black-tails than for White-tails. Isolated individuals of either species were never observed. To measure aggression, most of which was probably related to competition for mates and nesting burrows, I recorded fights, chases, and other hostile interactions. Three lines of circumstantial evidence indicated that this sort of aggression was deleterious to individual prairie dogs. For both White-tails and Black-tails, aggression per individual per h correlated positively with ward size. Aggression was not more pronounced for Black-tails than for White-tails, but interspecific comparisons were difficult because aggression in the two species was manifested differently. White-tails and Black-tails are both extremely susceptible to sylvatic plague, but intra-or interspecific investigations of the transmission of this disease were not possible. I measured ectoparasitism by sampling for fleas at burrow entrances and by counting fleas and lice on the adults and young themselves. Four lines of circumstantial evidence indicated that ectoparasites were deleterious to their prairie dog hosts. For both White-tails and Black-tails, the number of fleas per burrow entrance correlated positively with ward size. Counts from burrow entrances and from the animals themselves both indicated that ectoparasitism was probably more costly for Black-tails than for White-tails. Both White-tail and Black-tail young mingled regularly with young from different litters shortly after their first emergences from the natal burrows, with Black-tail young tending to mingle sooner. Experiments involving the transfer of colour-marked young into foster litters indicated that mixing of unrelated young, with the possible consequence of misdirected parental care, was probably not seriously deleterious for parents of either species. Vocal conspicuousness was measured by recording the rate of territorial calling, and visual conspicuousness was measured by counting numbers of visible mounds and numbers of visible prairie dogs. Both vocal and visual conspicuousness correlated positively with ward size for both White-tails and Black-tails. Further, both types of conspicuousness were more pronounced for Black-tails than for White-tails. Neither vocal nor visual conspicuousness could be measured on a per-individual basis, and an assessment of associated costs was therefore difficult. In summary, I conclude that there are probably several costs associated with prairie dog coloniality, that the severity of some of the costs correlates positively with colony or ward size for both White-tails and Black-tails, and that some of the costs are probably more pronounced for Black-tails than for White-tails.