Some Factors Affecting Survival of Desert Mule Deer Fawns

Abstract

This 9-year study found a strong association (n = 0.94) between aggregate rainfall on the Three Bar Wildlife Area during October-April and forage yield from forbs and small shrubs as measured in April. Survival of mule deer fawns (Odocoileus hemionus crooki) as reflected in the ratio of fawns per 100 does during January was also strongly associated with winter forb yield (r = 0.83) and October-April rainfall (r = 0.64) of the winter-spring period preceding the fawning period. A small herd of deer within a 244-ha predator-proof enclosure on the same area had significantly higher average January ratios of fawns:100 does than the herd outside of the enclosure. The ratios (1972-78) were not associated with forb production. These results suggest that forage condition, except in years with extreme winter drought, is probably not the factor that is limiting fawn survival in the herd outside of the enclosure to its present low levels. J. WILDL. MANAGE. 43(3):657-665 From 1958 through 1961 high populations of mule deer in much of Arizona had damaged considerable habitat, and either-sex hunts were held to reduce deer numbers. Subsequently, during the early and middle 1960's, mule deer herds throughout Arizona suffered dramatic declines associated with decreased or static productivity. These declines greatly exceeded what would have been desirable from the standpoint of allowing range recovery. In response to the decline, studies were begun to learn more about the dynamics of mule deer populations. Much of this research effort was on the Three Bar Wildlife Area, a desertscrub range in east central Arizona. One study explored the population history of this mule deer herd under a system of any-deer hunting (Smith et al. 1969) followed by a period of buck-only hunting (LeCount 1974, unpubl. Rep. Proj. W-78-R, WP2/J3, Ariz.). The present study was begun in 1968 to determine the cause of the low fawn survival (28 fawns:100 does) characteristic during late winter on the Three Bar Wildlife Area from 1962 to 1969. Low fawn survival was also associated with declining mule deer populations in other areas. The proportion of fawns surviving until January is the net result of fawn natality and mortality. Mule deer does examined throughout the western states had high fetal rates during recent years, generally in excess of 1.50 fetuses per adult female (Table 1). The sample of does from the Three Bar is small yet provides some indication that fetal rate there during the period of low recruitment was probably not responsible for the low survival. The present study was designed to examine the relationships among seasonal rainfall, vegetative production, and fawn survival. If significant correlations could be demonstrated they might provide a basis for further study of causal relationships. Perhaps even more important was the need to provide the wildlife manager with a means for predicting fawn survival whether or not the mechanism of survival was well described. This study was supported by Arizona Federal Aid in Wildlife Restoration Project W-78-R.