Empirical evidence for competition‐driven semelparity in wild medaka

Abstract

AbstractMetabolic theory predicts that maintenance rate increases faster with animal body size than food intake rate, such that the critical resource densityR* at which ingested energy exactly covers maintenance requirements increases with body size. Small‐sized (lowR*) juveniles may thus exclude their larger‐sized (highR*) parents in resource competition, resulting in apparent semelparous life histories and non‐overlapping generations. However, empirical support for such a competition‐driven semelparity (CDS) remains scarce. Here, we report a high consistency of cohort dynamics with CDS in wild medaka (Oryzias latipes). As predicted by the theory, there was a strong juvenile‐adult diet overlap, and all individuals died after reproduction as semelparous age‐1 adults, synchronous with a rapid somatic growth of age‐0 juveniles into the adult stage and with dropping abundances of zooplankton food resources. In addition to the theory, we found evidence for increased reproductive allocation under food stress, translating into immune depression and elevated parasite prevalence. Therefore, CDS in medaka emerges both from intercohort competitive exclusion and from food‐dependent energy reallocation from maintenance to reproduction, the later presumably representing an adaptive response to the former. The literature data show that the strengths of both intercohort competition and reproductive allocation increase at higher temperatures in many ectotherms, pointing to climate warming as a potentially powerful magnifier of CDS in the wild.