Seasonal changes in food quality : a proximate cue for reproductive timing in marine iguanas

Abstract

We investigated the proximate environmental cues that influence the timing of reproduction in the seasonally and synchronously breeding Galápagos marine iguana (Amblyrhynchus cristatus). Marine iguana foraging patterns are closely linked to the tidal cycle, and they feed exclusively on macroalgae. The Galápagos Islands are characterized by seasonal currents that impact water temperature and ultimately algal abundance, which is known to affect iguana body condition and survival. In our study, marine iguanas preferred Gelidium sp., which had the highest nutritional quality (nitrogen content, carbon content, and C:N ratio) of the three species of macroalgae on which they feed. Nutritional quality in Gelidium changed predictably with increasing sea surface temperature, peaking around the median iguana copulation date. Similarly, foraging behavior changed around the time copulations began. To test if consistent relationships existed among food quality, water temperature, and breeding life history, we used seven years of data from this population and found that breeding occurred earlier in years when the mean December sea surface temperature was warmer. Although iguanas might cue directly on changes in sea surface temperature, given the frequency and intensity of El Niño events in the Galápagos, this hypothesis is unlikely. Rather, our results suggest that marine iguanas may use subtle changes in food quality (particularly energy content) associated with seasonal, but annually variable, changes in water temperature as a cue to initiate breeding. The excess available energy may be used to withstand the costly breeding period, as well as to provision yolk to improve hatchling growth and survival.