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 pre dicts greater optimal offspring size when offspring survival is lower. Using simple demographic reasoning, we argue that optimal offspring s ize is larger when developmental time is longer (e.g., at lower temper atures). Earlier start of reproduction at higher temperature shortens the exposure of organisms to mortality factors. Then, a reduction in m ortality will have a smaller effect on survival to reproduction and, t hus, 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 si ze at birth for semelparic reproduction. For iteroparic reproduction w e consider two numerical examples: an artificial data set and real dat a on obligate asexual and cyclic parthenogenetic clones of Daphnia. Bo th examples support the prediction that at moderate mortalities the '' many small offspring'' strategy is favored at higher temperatures, whi le the opposite strategy is favored at lower temperatures.