Why larger offspring at lower temperatures? A demographic approach

Many exothermic animals produce larger offspring at lower temperatures, Consideration of the trade-off between offspring size and number predicts greater optimal offspring size when offspring survival is lower. Using simple demographic reasoning, we argue that optimal offspring size is larger when developmental time is longer (e.g., at lower temperatures). Earlier start of reproduction at higher temperature shortens the exposure of organisms to mortality factors. Then, a reduction in mortality will have a smaller effect on survival to reproduction and, thus, a smaller effect on fitness. In addition, the offspring produced earlier have a higher reproductive value. Thus, a reduction in clutch size, which allows a female to produce larger offspring, is both more costly and less rewarding at high temperature. We found the optimal size at birth for semelparic reproduction. For iteroparic reproduction we consider two numerical examples: an artificial data set and real data on obligate asexual and cyclic parthenogenetic clones of Daphnia. Both examples support the prediction that at moderate mortalities the ''many small offspring'' strategy is favored at higher temperatures, while the opposite strategy is favored at lower temperatures.