Gender and tooth wear effects on diets of grazing goats

Diet selection by bucks and does and the diets of goats with unworn and worn tooth are described from microhistological analysis of fecal samples in a Chihuahuan desert range of northern Mexico. In the rainy season bucks had more (P < 0.05) shrubs in their diet than does (78 ± 37% versus 58 ± 35%), but in the dry season the diet of does contained a substantially higher (P < 0.05) proportion of shrub (85 ± 49%) than bucks (69 ± 23%). Forbs were eaten in larger (P < 0.01) amounts by does (38 ± 14% of the diet) during the rainy season compared to bucks (20 ± 10%). Bucks avoided grasses in the rainy season but they formed 15% of the buck diet in the dry season. Bucks also avoided succulent plants during drought conditions but they made up 14% of the does diet. Diets of does never contained more than 5% grasses. Dietary overlap was greater between goats with unworn and worn tooth (mean similarity index = 74), than between bucks and does (mean similarity index = 59). Goats with severe erosion of the incisors avoided grasses and consumed a higher (P < 0.01) proportion of tender-leaved shrubs such as Buddleja scordioides and Atriplex canescens than goats with unworn incisors. For all classes of goats, Larrea tridentata, the dominating species in the community, was the only plant selected at levels below its availability. These data are consistent with the sexual dimorphism-body size hypothesis regarding different diets between sexes, because females in general used higher proportion of tender-leaved shrubs, succulent plants and forbs, while males in general used more abundant but poorer quality and coarse forages. These results also indicate that goats with worn teeth adopt a diet which is most suited to the state of their dentition.