Does Summer Range Quality Influence Sex Ratios among Mule Deer Fawns in Utah?

Abstract

Larger ratios of males to females were found among fawns from herd units where forage production on the summer ranges was low and the forage base was dominated by browse. Summer ranges where forage production was high and domhted by forbs produced even or female dominated sex ratios. The number of deer harvested per unit area was correlated with sex ratio of the fawn crop: harvests were lower where males were significantly overrep resented in the fawn crop and higher where sex ratios were even or female biased. We previously reported the comparative productivity of mule deer on 2 southeastern Utah mountains that differed markedly in summer range condition but had winter ranges of comparable quality (Pederson and Harper 1978, 1979). The La Sal Mountain mule deer herd was shown to consistently produce about 40% more fawns per doe than the Henry Mountain herd. Deer from the La Sals were also larger than animals of the same age and sex from the Henrys (Pederson and Harper 1978). We could detect no difference in either disease incidence or predation between the 2 herds, but the La Sal Mountain summer range was in good to excellent condition, and the forage base for deer was dominated by forbs. In contrast, the Henry Mountains summer range was in poor to fair condition, and the forage base was heavily dominated by shrubs (Pederson and Harper 1978). Since considerable evidence suggests that nutrition may influence the sex ratio of offspring born to a variety of cervid (deer) species (Julander et al. 1961; Verme 1965, 1967, 1969; Bannikov 1967; Robinette et al. 1973, 1977; McCullough 1979), we expanded our previous study to include a comparison of sex ratios in mule deer herds on 7 ranges known to vary among themselves in respect to vegetative condition. Sex ratio statistics have many implications for management. Medin and Anderson (1979) found that a 4% shift toward females in the ratio resulted in a 16% increase in fawn production. If range condition can be shown to be correlated with herd sex ratios, managers may be able to manipulate sex ratios through management of the forage base. Managers may also be better able to anticipate herd growth potential and prescribe more suitable management and harvest programs through a knowledge of range conditions coupled with inexpensive demographic studies.