Horn growth and population quality in Dall sheep.

Measures of horn growth of Dall sheep (Ovis dalli dalli) are compared with physical, demographic, and meteorological measurements. Horn growth is not correlated with body size in either ewes or rams. Rams showing rapid early growth of horn show slower subsequent growth. Gradual wear influences the measured length of the lamb annulus in both ewes and rams. Horn growth is a function of precipitation, implicating primary production, and shows marked differences between years. These differences appear to be correlated with recruitment. Horn growth of rams is more strongly affected by differences in environmental conditions between years than is horn growth of ewes. In good years, growth is enhanced uniformly over all ram age-classes; in poor years growth in younger age-classes is depressed more severely than is growth in older age-classes. Condition of the dam can affect horn growth of rams for a period of at least 5 years following birth. Useful indices of quality can be constructed from simple measurements such as length of annuli provided the lamb annulus is ignored and affects of calendar year are acknowledged. Measures presented argue for the importance of range quality in the production of trophy animals. J. WILDL. MANAGE. 42(4):764-775 Twici, huntsman to King Edward II, remarked, "The head grows according to the pasture, good or otherwise" (Dryden 1908). Since then, relationships between bioenergetic regime and antler growth have been documented for many species (Mouquet 1921, French et al. 1959, Long et al. 1959, Chapman 1975). Nievergelt (1966) noted correlations between precipitation, thus primary production, and horn growth of alpine ibex (Capra ibex). Heimer and Smith (1975) demonstrated a broad correlation (r = 0.43) between measurements of horn quality and percent glacial cover (assumed to be an index of range quality) for Dall sheep in Alaska. Other data relating horn growth to range quality for native bovids are scarce. Nievergelt (1966) noted that demographic characteristics and morphological variations were related within herds of alpine ibex. In particular, animals with large horn size, large body size, rapid growth, short life expectancy, and high fecundity occurred in expanding ibex herds, while smaller horn and body sizes, slower growth rates, longer life expectancies, and lower fecundities were found in stable or declining herds. Geist (1966, 1971) noted similar patterns in bighorn (0. canadensis canadensis) and Stone sheep (0. d. stonei) and expressed these relationships as an hypothesis of population quality. Together, these observations indicate relationships between quality of the horns, population, and range. Shackleton (1973, 1976) evaluated several postulates of the hypothesis expressed by Geist (1971), utilizing information from different populations of bighorn sheep. This paper concentrates on one attribute of population quality, horn growth, and examines that attribute for Dall sheep. It addresses differences between years within the same population and notes their implications to management. Objectives of the study were threefold 1) to examine hypotheses of population quality as they relate to horn growth of Dall sheep; 2) to document interannual differences in horn growth, and; 3) to discuss the implications of such dif'This study was supported by the Committee on Arctic and Alpine Research of the University of British Columbia and the Department of Indian and Northern Affairs of Canada. 764 J. Wildl. Manage. 42(4):1978 This content downloaded from 142.103. 160.110 on Mon, 23 Feb 2015 12:47:37 PM All use subject to JSTOR Terms and Conditions HORN GROWTH OF DALL SHEEP* Bunnell 765 ferences to the management of Dall