Within ectotherms, increases in body size with latitude are thought to be the consequence of the effect of a decline in development temperature, which results in a larger final body size. In contrast, latitudinal declines in body size are usually ascribed to limited resources. It has been suggested that if generation time is similar to or constitutes a significant proportion of the growing season length, then season length is likely to have a considerable influence on body size because of constraints on resource availability, so resulting in a decline in body size with temperature (latitude). However, if generation time declines relative to season length, resources effectively become available for longer. Temperature influences on growth and differentiation are likely to be most significant, resulting in an increase in body size with latitude. Here, we test the hypothesis by comparing intraspecific altitudinal body size clines in a monophyletic group of weevils from two regions that differ substantially in seasonality. On the relatively aseasonal Marion Island, body size increases with altitude, whereas on the more seasonal Heard Island the opposite is found. In addition, overlapping generations on Marion Island indicate year-round resource availability, whereas more discrete generations on Heard Island indicate winter cessation of growth and development. Our data provide support for the hypothesis that the seasonality of resource availability has a major influence on body size clines. Furthermore, we argue that analysis of interspecific body size clines should be preceded by nested analyses of variance to determine the influence of clinal replacement of higher taxa on these patterns.