Cemental Deposition, Tooth Succession, and Horn Development as Criteria of Age in Dall Sheep

Abstract

Patterns of cementum, tooth succession, and external and internal horn annuli were studied in 129 specimens of Dall sheep (Ovis dalli). Fluoromicroscopy of annually produced cemental layers may be used to estimate age of all sheep older than 1 year. During the first 4 years of life age can be determined from the sequence of incisor replacement. Counts of annual horn increments provide a means of estimating age, but may be difficult to distinguish in older animals, particularly females. The purpose of this study was to find reliable methods for estimating the age of Dall sheep. I have investigated growth patterns of horns, replacement and attrition of teeth, pattern of deposition of dental cementum, and have compared age-related characteristics of bighorn sheep (Ovis canadensis) and Dall sheep. The validity of horn-segment counts as indicators of age in bighorn sheep was established by Geist (1966). The sequence of eruption and replacement of teeth in Dall sheep has not been described, except in a very general way by Murie (1944). Techniques for the study of annual increments in cementum were first described by Laws (1952, 1953). Similar methods have been applied to North American cervids by Sergeant and Pimlott (1959), McEwan (1963), Low and Cowan (1963), Gilbert (1966), Ransom (1966), and to bovids by Armstrong (1965), and Novakowski (1965). These earlier studies have shown that cementum is deposited on the roots of the teeth in the form of alternating layers, of which one translucent and one opaque layer represent 1 year's deposit. I am indebted to R. L. Rausch, Arctic Health Research Laboratory, College, Alaska, for his continuing assistance and en1 Present address: Alaska Department of Fish and Game, Anchorage. 552 couragement, especially for donating the bulk of the specimen material used during this study, and for his critical review of the manuscript. P. L. Wright, R. S. Hoffmann, and W. B. Rowan, Department of Zoology, U iversity of Montana, offered many helpful suggestions during the research and preparation of the manuscript. R. Baker, Stella Duncan Institute, University of Montana, assisted in fluoromicroscopy and photomicrography. Radiograms were prepared by B. Cerino, Health Service, University of Montana. Several specimens were provided by biologists of the Alaska Department of Fish and Game, and the University of Alberta. To these persons, as well as to others not individually named, I express my sincere thanks. MATERIALS AND METHODS All but 11 of the specimens utilized were obtained from Nunamiut Eskimo hunters from the central Brooks Range, Arctic Alaska. The skulls or mandibles and horns from 118 Dall sheep harvested for food were saved and carefully labeled as to sex, date, and location between 1950 and 1966, an average of 7 per year. Dall sheep do not play an important role in the economy of the Nunamiut, but they are hunted when caribou are scarse (Rausch 1951). Ten more specimens, including one of knownage, from various locations in south central Alaska were provided by biologists of the This content downloaded from 207.46.13.131 on Sat, 15 Oct 2016 04:14:32 UTC All use subject to http://about.jstor.org/terms TOOTH AND HORN CRITERIA FOR AGING DALL SHEEP * Hemming 553 Alaska Department of Fish and Game, and the skull of a 5-day-old lamb was obtained from the University of Alberta. Bones were cleaned by means of dermestid beetles and bleached in hydrogen peroxide. After cleaning, all bones and horns were dried thoroughly before being measured. Published birth dates for Dall sheep fall between 5 May and 28 June (Sheldon 1930: 366, Dixon 1938:216, Murie 1944:89, Rausch 1951:193). Since the frequency distribution of births is not known, the median date, 1