A Multi-Regional Evaluation of Nutritional Condition and Reproduction in Elk
Cook, Rachel Catherine
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We examined the role of nutrition on productivity in elk (Cervus elaphus) by estimating nutritional condition of 861 individual female elk in 2114 capture events from 21 herds across Washington, Oregon, Wyoming, Colorado, and South Dakota during 1998 through 2007. For each animal, we measured body fat of the ingesta-free body, body mass, thickness of the longissimus dorsi muscle, age, pregnancy status, and lactation status. When possible, estimates were obtained in both late winter-early spring (late February through early April) and autumn (November or early December) to estimate seasonal changes in animal condition. Pregnancy rates of females 2- to14-years old ranged from 68 to100% in coastal populations, 69 to 98% in Cascade populations, 84 to 94% in inland northwestern populations, and 78 to 93% in Rocky Mountain populations. Mean body mass of 242 calves from 3 populations was 75, 85, and 97 kg, 55 to 70% of potential mass for 6 month-old calves. Body fat levels of lactating females were consistently lower than their non-lactating counterparts in autumn, and herd averages ranged from 5.5% to 12.4%, a level only 30 to 75% of those documented for captive elk fed high quality diets during summer and autumn. Body fat levels were generally lowest in the coastal and inland northwest regions and highest along the west-slope of the northern Cascades. During winter, adult females in most herds lost an average of 30.7 kg or 13.4% (range: 5 to 62 kg or 2.6 to 25%) of their autumn mass, indicating nutritional deficiencies in winter. Spring body fat level was a function of previous-autumn body fat levels; we found no significant relationship between spring body fat or change in body fat over winter with winter weather, region, or herd location. Body fat levels of lactating females in autumn were related to herd location but not to previous-spring body fat levels; the level lactating females could achieve in late autumn significantly varied among herds. Summer ranges of only 3 herds sampled supported relatively high levels of autumn body fat (12% body fat) and, concurrently, high probability of pregnancy even of females that successfully raised a calf year after year.