Researchers discovered this by accident.
the tree skink (Egernia striolata) is usually monogamous. Females only mate with their mate. Together they have offspring. And they lived happily ever after. At least we thought so. Because in reality, female tree skinks seem to cheat things a little every now and then and give birth to offspring that are not their partner’s. And they get away with that too.
Because their partners seem to know nothing. And we humans probably wouldn’t have noticed either, were it not for the fact that researchers separated a number of female tree skinks from their partners for an investigation into a completely different aspect of their lives. And then – to their great surprise – to witness a year later how these females, still separated from their partners, give birth to offspring. Barely recovered from the shock, an investigation followed which also showed that the steady partner of the females was not the father.
Storage of sperm
How is that possible? The females had not had any contact with their mate or males for a year and still gave birth to offspring. The researchers can only explain this in one way: the tree skink is used to store sperm. That can be read in the magazine Journal of Heredity.
The females would store the sperm of males they mated with before and who are not her regular mate for later. This allows her to fertilize her eggs later not only with sperm from her regular partner, but also with sperm that is still stored. And that in turn enables her to bring genetically more diverse offspring into the world and thus increase the survival chances of that offspring.
Long-term storage is important
Mating with another male is very risky for a female tree skink who already has a permanent partner. After all, she can be caught. Or her offspring can be rejected by the regular partner. “Being able to store sperm obtained during encounters with other males for a long time is likely to be very beneficial for females,” said study researcher Julia Riley.
According to researcher Martin Whiting, the research shows very nicely that female tree skinks have great control over their reproduction. But the study also raises many questions. “Future research should show how widespread sperm storage is among lizards, how the female’s reproductive organs manage to keep the sperm alive for that long, and under what conditions the females use the sperm of other males.”
In anticipation of an answer to those follow-up questions, this study has changed our view of the tree skink forever. “We now know that female tree skinks, already famous for their family life, are probably one step ahead of their male partners.”