HABITAT DIFFERENCES IN MASS-SPECIFIC LITTER SIZES OF HISPID COTTON RATS

Litter size and body size of Sigmodon hispidus are known to covary pos itively both among and within some populations. In captivity, litter s ize varies with quality of diet. In the field, however, variation in m aternal size can obscure nutritional effects on litter size. We reexam ine litter sizes from sites in central Oklahoma varying in primary pro ductivity due to application of herbicides and controlled burning. Mas s-specific litter size of cotton rats showed no significant response t o burning. In areas treated with tebuthiuron, litters were larger and population densities and primary productivity of monocots were higher than in areas receiving triclopyr. Slopes of the relationship between litter size, a component of fitness, and maternal mass did not differ significantly between habitats. Thus, percentage increase in litter si ze in the better habitat was greater for smaller breeding females. Thi s implies that gains in fitness related to habitat productivity are si ze-specific in these populations.