Scientists used to think that hermaphrodites, due to their low position in the evolutionary scale, did not have sufficiently developed sensory systems to assess the “quality” of their mates.

A new work has shown, however, that earthworms are able to detect the competition by fertilising the eggs that is going to find its sperm, tripling its volume when there is rivalry. This ability is even more refined as they are able to transfer more sperm to more fertile partners.

Hermaphrodites, organisms that have both female and male reproductive organs, such as earthworms, are denied the right to choose their partner. However, a study by researchers at the University of Vigo has shown that worms are capable of telling whether another worm is a virgin or not, and triple the volume of sperm transferred during copulation if they detect a fertilisation competition risk.

According to the study, which appears in the latest edition of the magazine Proceedings of Royal Society B-Biological Sciences, the partner’s assessment ability, which has often been considered incompatible with invertebrates, is a firm characteristic of worms when their sperm is competing for fertilisation. Authors of the study explained to SINC that “in high sperm competition situations, partner assessment is subject to strong selection in hermaphrodites, making these organisms very selective when choosing to whom and how much sperm to transfer”.

In total, scientists analysed 42 mature and virgin worms, allowing researchers to reach their conclusions. When worms detect a risk of their sperm competing with their rivals, these invertebrates are able to determine whether their partners have copulated previously, in which case they increase the volume of sperm donated. “This increase is even higher when worms mate with much larger partners, as they are more fertile”, explains Jorge Domínguez, one of the authors of the study.

Worms control their sperm volume

Thanks to the double mating experiment carried out by the scientists, the results show that worms have a refined control over the volume of sperm transferred during copulation according to the sex of the partner they are mating with. The advantage of donating such amounts of sperm is due to the highly competitive environment in which these hermaphrodites live.

Multiple mating is common amongst worms and the reason why they have developed specific strategies to deal with strong sperm competition in fertilisation. Researchers suggest that “sperm competition in fertilisation is an evolutionary force which has affected worm mating behaviour”.

Worm courting can last up to an hour during which time the organisms secrete large amounts of mucus and press against each other with short, repetitive rubbing actions for subsequent exchange of sperm. If there is no fertilisation competition, worms are prudent in how much sperm they release, even waiting to mate with high-quality partners. “Worms can control copulation time or, alternatively, can have mechanisms which prevent all their sperm being released in a single mating event”, stress the authors.

The results of the study conclude that the volume of sperm donated to worms that are not virgins has been more variable than that transferred to virgin partners. In this respect, researchers estimated that the volume transferred to larger size partners which had previously copulated was five times greater than that transferred to virgin worms.

Article: Velando A., Eiroa J., Domínguez J. “Brainless but not clueless: earthworms boost their ejaculates when they detect fecund non-virgin partners” Proceedings of The Royal Society B-Biological Sciences 275(1638): 1067-1072