Question

You are an ecologist, working on some frog social behavior research on a remote island.  While out hiking one afternoon, you discover a new species of purple flies with females that lay their eggs inside coconut palm tree fruits (coconuts).  While the coconut is not a good source of nutrients for these flies (they feed on dead animals), the offspring are well protected and are seldom consumed by predators.  After the eggs hatch, only the females possess the mouthparts needed to chew through the woody covering of the coconut seed and escape.  All of the males die in the nest.  However, because the flies cannot digest and obtain nutrients from the woody seed coat, the young females must obtain the strength necessary for eating their way out of the coconut by ingesting spermatophores (small packets containing sperm and nutrients) that each male releases in the nest.  The spermatophore also fertilizes the female that eats it.  Once a male releases his spermatophore, however, he dies.  While the spermatophores contain nutrients, the likelihood that a female will escape the nest is highly positively correlated with the number of spermatophores that she ingests, due to the energy needed.

In this fly species, would you predict a sex ratio bias? Explain why or why not. Would this mating system be classified as promiscuity, polygyny, polyandry, or monogamy?  Explain your classification.

Answer 1

Yes, It can be predicted based on the given social behavior that there is sex ratio bias in this fly species as the males are not able to survive and escape the coconut seed while the females can.

This is an example of Polyandry as the majority of females mate with more than one male by ingesting their spermatophores in order to obtain nutrients to get enough strength required to digest the coconut seed and escape. In this case, females compete for the males and are more larger and with prominent features. By mating with multiple males, the female is ialso nsured to fertilize all of her eggs.

This cannot be Polygyny as the male dies as soon as it releases the spermatophore and its unable to mate with multiple female flies. Also, It is not a clear case of Promiscuity because it is not performed by both the sexes equally. There is bias involved. As multiple sexual partners are involved, It is not a case of monogamy.