Hermaphroditism — a condition where an organism can produce both male and female gametes — is very common in plants and many animals. While the ability to self-fertilise seems like an evolutionary ‘cunning plan’, there are drawbacks. If hermaphrodites only self-fertilise, they run the risk of inbreeding depression, but where survival or mating opportunities are slim, then self-fertilisation is better than not reproducing at all.

Theory does a good job of predicting when hermaphroditism will benefit plants. Interestingly, animals have not received the same attention. George Jarvis and his PhD supervisors Craig White and Dustin Marshall have found that the same theories predicting hermaphroditism in plants can also be applied to animals.

Essentially, strong competition among siblings for resources or among gametes for fertilisation may drive the evolution of hermaphroditism in both plants and animals.

Highly competitive environments can tip the balance towards hermaphroditism, because hermaphrodites can reallocate resources from male to female function (or vice versa) to increase their competitive advantage.

George and his supervisors considered the accepted evolutionary drivers for hermaphroditism in plants and looked for analogues in marine animals.

In plants, species with limited seed dispersal can experience strong competition among siblings for resources, and are more likely to be hermaphrodites. In marine animals, larval dispersal provides an analogous model to test if the same forces are at play.

Larvae released into the plankton travel further than non-planktonic larvae and feeding larvae travel even further than their non-feeding counterparts. Plant theory predicts that hermaphrodites are more common in species where local competition is greatest; in marine animals then, hermaphroditism should be more common in species with non-planktonic larvae.

Similarly, plants pollinated by specialist insects are more likely to be hermaphrodites. In animals, internal fertilisers are akin to plant species with specialist pollinators; competition between gametes is fierce and so an extra guarantee in the form of hermaphroditism should be more common.

George and his supervisors found other analogues between plant and animal theory as well. Smaller plants and plants at higher latitudes are more likely to be hermaphrodites due to increased competition between gametes and rarer mating opportunities respectively. If plant theory holds, the same patterns between hermaphroditism, size and biogeography will be true for animals as well.

The research team set about compiling data for reproductive mode, larval development mode, fertilisation mode, latitude and adult size for 1,153 species of marine annelids, echinoderms and molluscs.

They found theory developed to understand hermaphroditism in plants, predicts many patterns of hermaphroditism in marine animals. Overall, hermaphroditism is generally associated with limited offspring dispersal, internal fertilisation and small body size.

But just to complicate things, George and his supervisors also found that in annelids and molluscs, species that are external fertilizers are more likely to be hermaphrodites when large, but in internal fertilizers the opposite is true.

George, Craig and Dustin feel their results support Darwin’s supposition from over a century ago: hermaphroditism evolves in response to an organism’s life history and ecology —  not the other way around.

This research was published in the journal Evolution.