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.