Adaptive Geometry of Burrow Spacing in Two Pocket Gopher Populations

Abstract

The configuration and spacing of the burrows of fossorial pocket gophers (Thomomys bottae) were analyzed at two sites in northern Arizona. The burrows represent a tremendous energetic investment and used for almost all activities of the rodents throughout their lives. Burrow length, home range areas, perimeter, and number of adjacent neighbors were significantly higher for reproductive males than for females or nonreproductive all in the population. Spacing within and between individual burrow systems was uniform for all sizes and bothe sexes. It appeares tha reproductive male pocket gophers relate to their neighbors in such a way as to increase the contact with potential mates, while all individuals, regardless of sex or reproductive condition, develop and maintain uniform intenal spacing distances that are perhaps related to resource management. Past studies have suggested that pocket gophers are polygynous. Our investigation indicates that the males practice serial monogamy, and we found four instances of males and females sharing a common, deep nest between their indivdiual burrows. This suggests a potential for parental care by the males. The rodents appear to be compensating for differencs in plant production by altering burrow length rather than altering spacing. Plant production was approximately twice as great at one site as at the other. At the side of higher production, the average burrow lengths were almost exactly one half those at the site of lower production. However, all values for spacing within and between individual burrow systems wer statistically indistinguishable between the two study sites. For both sites, the difference in burrow lengths appears to be attained by the addition of basic building units to a burrow, consisting of equal lengths and equal distances between branching points along a burrow, much like nodes and internodes along a plant stem.