Ectotherms, Temperature, and Trade-offs: Size and Number of Eggs in a Carabid Beetle

We studied the allocation of total egg mass to size and number in the carabid beetle Notiophilus biguttatus F. at several temperature and day length regimes. Eggs increase in number and decrease in size with increasing (constant) temperature. Day length interacts with temperature: at short day the effect of temperature on size and number of eggs is weaker than at long day. In diurnally fluctuating temperature regimes, egg size is affected disproportionately by the high temperature period. All treatments, however, are similar in affecting number and size of eggs in an opposite direction. Consequently, egg size is explained to a high degree by egg production rate. The relationship between size and number of eggs among treatments is furthermore characterized by a decrease in egg size with an increase in total egg mass production. Within treatments, rate of egg production and egg size are negatively correlated among females in the low-temperature groups but not in the high-temperature groups; the correlations among females are also characterized by a decrease in egg size, with an increase in total egg mass production. Hence, possible trade-offs between size and number of eggs are masked by phenotypic variation in reproductive effort. The observations enable us to propose a simple conceptual model that explains the within-treatment correlation by the same causal factor as the negative relationship among treatment means.