Does it pay to delay? Flesh flies show an adaptive delay in reproduction after reaching a nutritional threshold

Life-history plasticity has been observed in a wide variety of organisms. However, a critical question is whether this plasticity is adaptive, enhancing the organism’s fitness. Before they can reproduce, most insects must reach a nutritional threshold. Previous research has shown that the flesh fly Sarcophaga crassipalpis can reach their minimum nutritional reproductive threshold after a single protein meal. However, if flesh flies are provided with low quality resources, they will delay reproduction compared to individuals receiving continuous liver. To test if this delay could be adaptive, we provided three sequential resource pulses throughout the reproductive delay period. As expected, poorly fed flies delayed reproduction and flies that received a resource pulse during the delay period benefitted from the additional nutrients. Flies that received a resource pulse earlier in the delay period had a greater reproductive output and provisioned their eggs faster than flies that received a later pulse. In addition, flies that received the earlier pulse were able to incorporate more dietary carbon into their eggs and somatic tissue than those provided a later pulse. These results indicate that the reproductive delay in S. crassipalpis is consistent with adaptive post-threshold plasticity.