Physiological determinants of Ficus fruit temperature and implications for survival of pollinator wasp species: comparative physiology through an energy budget approach

Abstract

Figs are completely dependent for pollen dispersal on species-specific fig-pollinating wasps that develop within developing fig fruits. These wasps are very sensitive to heat and die at temperatures only a few degrees above ambient. Such temperatures are expected and observed in objects exposed to full sunlight, as fig fruits frequently are. In detailed field and experimental studies of 11 species of Panamanian figs with fruit ranging in size from 5 mm to 50 mm in diameter, we found that both the relative and absolute contribution of transpiration to maintaining non-lethal fruit temperatures increased with fruit size. Small and large fruits reached temperatures of 3 and 8°C, respectively, above air temperature in full sunlight when transpiration was prevented by grease. The temperature reached by large, nontranspiring fruits was sufficient to kill their pollinators. Control fruits which transpired reached temperatures of 2–3°C above air temperature in sunlight, regardless of size. An analysis of the solar energy budget of fruit revealed that large fruits must transpire to maintain tolerable temperatures for the wasps because heat diffusion from fruit to air was too low to balance net radiation in sunlight. By contrast, small fruits do not need to transpire to maintain tolerable temperatures for the pollinators.