Evaluation of alternative hypotheses to explain temperature-induced life history shifts in Daphnia

Negative correlations between environmental temperature and body size are widespread in planktonic organisms, and ectotherms generally, but remain poorly understood. Here we evaluate experimentally two alternative hypotheses suggested to explain life history shifts induced by raised temperature using parthenogenetic clones from two Daphnia species. Explanation 1 proposes that the life history shifts could be adaptive if increased temperature is used as an indirect cue to indicate increased risk from size-selective predators. Explanation 2 proposes that at larger body size energy becomes more limiting as temperature increases because of a less favourable assimilation: metabolism balance. In a factorial laboratory experiment we examine the effects of three rearing temperatures on the growth and reproductive traits of Daphnia raised in water with fish kairomone, Chaoborus kairomone, or in uncontaminated water. None of the three predictions of explanation 1 were met by the data. In both D. pulex and D. curvirostris, and some other published studies, data suggested that at larger body sizes the sum of growth and reproduction was lower at high temperature, supporting our prediction from explanation 2. However, we propose a novel third explanation based on new evidence of temperature-dependence in both reproductive effort and cost in D. pulex.