Ovarian dynamics, egg size, and egg number in relation to temperature and mating status in a butterfly

Although the temperature-size rule, that is, an increase in egg (and body) size at lower temperatures, applies almost universally to ectotherms, the developmental mechanisms underlying this consistent pattern of phenotypic plasticity are hitherto unknown. By investigating ovarian dynamics and reproductive output in the tropical butterfly Bicyclus anynana (Butler) (Lepidoptera: Nymphalidae: Satyrinae) in relation to oviposition temperature and mating status, we tested the relevance of several competing hypotheses for temperature-mediated variation in egg size and number. As expected, females ovipositing at a lower temperature laid fewer but larger eggs than those ovipositing at a higher temperature. Despite pronounced differences in egg-laying rates, oocyte numbers were equal across temperatures at any given time, while oocyte size increased at the lower temperature. In contrast, there were greatly reduced oocyte numbers in mated compared to virgin females. Our results indicated that temperature-mediated plasticity in egg size cannot be explained by reduced costs of somatic maintenance at lower temperatures, enabling the allocation of more resources to reproduction (reproductive investment was higher at the higher temperature). Furthermore, there was no indication for delayed oviposition (no accumulation of oocytes at the lower temperature, in contrast to virgin females). Rather, low temperatures greatly reduced the oocyte production (i.e., differentiation) rate and prolonged egg-maturation time, causing low egg-laying rates. Our data thus suggested that oocyte growth is less sensitive to temperature than oocyte production, resulting in a lower number of larger eggs at lower temperatures.