For JEB readers, Lancelot Hogben is one of the journal's founding fathers, but for others he may have inadvertently triggered an ecological disaster. His pregnancy test for humans led to the accidental release of African clawed frogs (Xenopus laevis), causing immense ecosystem damage as the voracious predators consume the young of native species, wiping out local populations. But Maitena Dumont from the Koret School of Veterinary Medicine (KSVM), Israel, and Anthony Herrel from the Muséum National d'Histoire Naturelle (MNHN), France, realised that the amphibians’ relentless march might offer the chance to find out more about how individuals at the forefront of an invasion could differ from those nearer the heart of the population. Animals at the head of an invasion mate with others nearby, potentially enhancing characteristics that make them more successful intruders. So how much of an impact might this phenomenon have had on the legs of unwelcome Xenopus leaping across France? Could the leg bones of the frogs leading the invasion be longer and stronger to help push them on, or might the pioneering frogs have developed more powerful muscles to propel them forward at the expense of leg bone growth?
‘Julien Courant [MNHN], Pablo Padilla [MNHN] and Anthony went to ponds near Chalonnes-sur-Loire to collect the frogs’, says Dumont, who began CT scanning the femurs of both male and female animals. These frogs ranged in age (and size) from 1 to 6 years and form the vanguard of the species invasion. The researchers then compared the leg bones with those of frogs residing near Thouars in western France, where the frogs are more established, to find out how well-built the bones were. In addition, Dumont transported the leg bones back to Israel, where she and Joshua Milgram (KSVM) tested the bones’ strength by attempting to snap them, recording the forces that they could withstand and how far they bent.
Having focused on the CT scans of the dense outer bone surrounding the inner marrow, it was clear to Dumont and her colleagues that this ‘cortical’ bone was stronger and thicker in the frogs residing at the heart of the invasive population. In contrast, the outer bone in the femurs of the frontline frogs infiltrating new territory was thinner, probably allowing them to be more flexible. It seems that being at different locations within the invasive population has impacted the robustness of the interlopers’ bones, ‘This might mean that the frogs use their energy for moving quickly at the expense of building strong bones’, says Dumont. And when the team compared the strength and structure of the female bones with those of the males, it was clear that the leg bones of the females – which have larger bodies – are also more robust than the male bones, reflecting their heavier build. The bendier leg bones of the frogs leading the invasion might also reflect that they are generally more active than the frogs in better established areas of the population, allowing them the flexibility to forge new territories where they are definitely unwelcome.