Sul Ross State University

Diets of Desert Bighorn Sheep at Elephant Mountain Wildlife Management Area, Brewster County, Texas

by Clay E. Brewer (Texas Parks and Wildlife Department) and Louis A. Harveson

We estimated food habits of an introduced population of desert bighorn sheep (Ovis canadensis spp.) at Elephant Mountain WMA (Brewster County, Texas) by comparing dietary differences between:

1. sexes
2. seasons
3. wet and dry periods
4. critical biological periods
5. bighorn populations located in the Baylor, Beach, Elephant and Sierra Diablo mountains of Texas.

Diet composition was determined through microhistological analysis of 432 fecal pellet groups (209 M, 209 F, 14 lambs), collected every two weeks between September 1998 and August 2000. Ninety-four dietary items were identified, with overall bighorn diets consisting of 50 percent browse, 35 percent forbs, 11 percent grasses and 4 percent succulents. 

Comparison of dietary proportions between sexes indicated a significant difference only for the combined (1998-2000) winter seasons (G3 = 8.772, P = 0.032). Shannon-Wiener diversity indices of the taxa consumed by each sex did not differ (t = -1.182, P > 0.05).

Diet proportions were consistent between years, seasons, and critical biological periods.

Mountain range comparisons reflected significant differences in male bighorn diets between Elephant and Beach Mountains (G3 = 10.154, P = 0.017) throughout the study.

Seasonal comparisons between mountain ranges indicated no significant differences between sexes. Seasonal differences were indicated among male diet proportions between Elephant Mountain and Beach Mountain during fall (G3 = 7.930, P = 0.047) and spring (G3 = 12.752, P = 0.005), Baylor Mountain during winter (G3 = 26.810, P < 0.001), and the Sierra Diablo Mountains during spring (G3 = 13.507, P = 0.004) and summer (G3 = 19.233, P < 0.001).

Differences were reflected among female diets between Elephant Mountain and Beach Mountain during winter (G3 = 16.559, P = 0.001) and the Sierra Diablo Mountains in fall (G3 = 18.674, P = 0.003), winter (G3 = 30.050, P < 0.001), and summer (G3 = 24.622, P < 0.001).

Dietary overlap was greatest among the Elephant Mountain and the Sierra Diablo Mountain populations (57.6%) followed by Baylor Mountain (55.7 percent) and Beach Mountain (52.0 percent). The highest average overlap was reflected in the browse component (50 percent), followed by forbs (22 percent), grasses (11 percent) and succulents (3 percent).

Restoration and management of desert bighorn sheep in Texas must consider the important influence of diet on reproduction and maintenance of viable populations. Management strategies should include: determining diets of all existing free-ranging bighorn populations; evaluation of the forage component prior to desert bighorn introductions; and implementation of techniques for evaluating habitat manipulations and monitoring forage use, status, condition and trends, including key indicator species. 

This project was funded by Texas Parks and Wildlife Department.