Abstract - Agression, Ecotparasitism, and Other Possible Costs of Prairie Dog (Sciuridae, Cynomys spp.) Coloniality

Hoogland, John L. 1979. Aggression, Ecotparasitism, and Other Possible Costs of Prairie Dog (Sciuridae, Cynomys spp.) Coloniality. In Brill, E.J. (ed.). Behavior. 35 p.

Summary

There are no automatic or universal benefits of coloniality. Two costs of coloniality, however, are probably inevitable: increased competition (for food, mates, nest sites, etc.) and increases transmisison of diseases and extoparasites (Alexander, 1974). Other possible costs that are not automatic include increased probability of misdirected parental care resulting from either mixing of unrelated young, cuckoldry, or intraspecific brood paratism; increases conspicuousness and increased attractiveness of predators; increased probability of indirect, deleterious consequences of nearby conspecific activity; and increased probability of having offspring killed or maimed by marauding conspecifics (Hoogland & Sherman, 1976).

Previous investigations have concentrated on the benefits of coloniality, with little of no attention to the costs (but see Hoogland & Sherman, 1976). My purpose in this report is to examine possible costs of coloniality for two species of prairie dogs: loosely colonial White-tailed Prairie Dogs (C. leucurus) and densely colonial Black-tailed Dogs (C. ludovicianus). Elsewhere I have examined possible benefits of prairie dog coloniality (Hoogland, 1978a, 1979b).

Prairie dogs are large, diurnal, colonial rodents of the squirrel family (Sciuridae). Five species, all of the genus Cynomys, are presently recognized (Pizzimenti, 1975), and all of them are probably in danger of extinction. The typical prairie dog colony is usually subdivided into two or more wards (King, 1955), or subcolonies, by a small stream, a row of trees, a hill, etc. Residents of one ward can sometimes see or hear residents of an adjacent ward, but movements and communications between wards are uncommon. Some of my investigations involved wards, whereas others involve entire colonies. The following brief descriptions of White-tailed and Black-tails are based mainly on reports by King (1955), Tileston & Lechleitner (1966), Clark (1977), and Hoogland (1977, 1978a).

White-tailed Prairie Dogs live in medium to tall grass prairies at altitudes of 1900-2600 m, and are found only in parts of Colorado, Montana, Utah, and Wyoming. Ward densities from studies of marked individuals range from 1.47 to 5.65 adults per hectare (ha), with a mean ± SD of 3.20 ± 1.40 (Hoogland, 1978a). Both males and females usually reach sexual maturity as yearlings, but only yearling females usually breed; males are considrerably larger than females at all stages beyond weaning. White-tails hibernate during the winter months, and breed in March and April. The usual litter size is 5-7, and the first emergences of weaned juveniles from their natal burrows occur in May and June. At no time of the year are there identifiable White-tail harems.

Black-tailed Prairie Dogs live in short grass prairies at altitudes of 900-1600 m, and are found in a narrow western belt that extends from southern Canada to central Texas. Ward densities from studies of marked individuals range from 7.52 to 32.7 adults and yearlings per ha, with a mean ± SD of 14.8 ± 9.67 (Hoogland, 1978a). Males and females of northern latitudes, at least, do not breed until at least two years old; adult males are only slightly larger than adult females. Black-tails do not hibernate, and breed in February and March. The usual litter size is 3-4, and the first emergences of weaned juveniles from their natal burrows occur in May and June. Black-tails are organized into harems known as coteries (King, 1955): the typical coterie contains a single adult male and 1-4 adult females, along with yearlings and young of the year. Coterie members defend a well-defined coterie territory from conspecifics, and restrict essentially all of their activities therein.

In addition to the higer Black-tail ward and colony densities, absolute sizes of wards and colonies with respect to both number of residents and physical area occupied are also higher for Black-tails than for White-tails. There is no indication that gross social structure varies with ward/colony density or absolute size for either species; for example, Black-tails are organized into coteries in wards/colonies of all densities and absolute sizes.

In the course of the 4-yr study, I never detected either a White-tail or Black-tail living solitarily (Hoogland, 1978a). Thus, I was unable to compare noncolonial and colonial prairie dogs with respect to the costs of coloniality. I hypothesized that the costs should be greater (a) for individuals of large wards than for individuals of smaller wards and (b) for Black-tails (large, densely populated wards)than for White-tails (small, sparsly populated wards). Whenever possible, I therefore attempted to investigate costs both intra- and inter-specifically. I realize, of course, that factors other than ward/colony density and absolute size affect the costs of prairie dog colonialty. Because such latter factors probably vary more between species than within species, I consider my intraspecific comparisons to be more valuable than my interspecific comparisons.

My investigations of the various costs of prairie dog coloniality are presented in the sections that follow. For additional references and a more detailed treatment of this subject, see Hoogland (1977, Chapter I).