Invasive marsh frogs (Pelophylax ridibundus) at the edge of a pond in southern France. Photo credit: Fabien Pille.
Invasive marsh frogs (Pelophylax ridibundus) at the edge of a pond in southern France. Photo credit: Fabien Pille.
Most animals get along just fine in their natural homes. While they may do better in some years than others, they generally remain in harmony with their surroundings. However, when transported to a new location, some creatures are in nirvana. With nothing to keep them in check, they thrive and take over. But what gives some invasive species the upper hand while others fail to flourish when introduced elsewhere? Pablo Padilla and Mathieu Denoël (from the University of Liege, Belgium) with Anthony Herrel (National Museum of Natural History, France) wondered whether the natural variation in some physical features from individual to individual could help invaders to stake a claim on new territory. To test their idea, the trio turned to one particularly successful invader, the marsh frog (Pelophylax ridibundus), which is currently marching west across Europe from its eastern home.
‘Marsh frogs originate from the Balkans and because of massive introduction events, mainly for human consumption or university usage, their distribution has expanded across Belgium, Switzerland and France’, says Padilla, who travelled with colleague Fabien Pille to the Larzac Plateau in southern France to collect the amphibians. ‘Most could be picked up easily by hand at night from ponds because of their naivety towards predators and because they get dazzled by our headlamps’, Padilla recalls. After transporting the frogs back to Paris, Padilla measured the animals’ oxygen consumption to calculate their metabolic rate. Then he spooked the frogs into jumping several times, recording the force of the strongest leap and the distance covered, to get a sense of their overall power, in addition to measuring their endurance by getting them to jump repeatedly until they could leap no more. Finally, Padilla measured the length of each frog's leg, arm and hand bones, as well as the mass of key organs, including the heart, liver, kidneys and three of the largest leg muscles. Once he'd measured all of the frog's body dimensions, Padilla began the painstaking task of looking for relationships between these measurements and the amphibians’ metabolism, endurance and power in the hope of identifying the key physical features that make a successful invader.
‘The natural differences that exist in the metabolism or jump force between one individual and another can be rather well explained by their overall size and by the weight of their internal organs or muscles’, says Padilla. And, when he took a closer look at the correlation between metabolic rate and the size of different organs, it was clear from the size of the kidney – which is key for their survival – that it is one of the main organs contributing to the body's energy consumption: frogs with larger kidneys have higher metabolic rates. It also turned out that the frogs with the largest testes and ovaries tended to have the highest metabolic rates, which also makes sense, as they will be most likely to populate newly occupied regions. And when Padilla compared the size of the frogs’ leg muscles with their ability to jump, he was surprised that their glutes (gluteus maximus or buttock muscles) seemed to provide the bulk of their power, rather than their sizeable lower leg muscles (gastrocnemius or calf muscle), making them better prepared physically for invasion.
So powerful kidneys, gonads and glutes are the key physical characteristics that make for successful frog invaders and Padilla says, ‘These features may produce invaders that are more specialised in later generations’. However, these features only account for half of the animals’ success and Padilla suspects that personality also contributes to the frogs’ ability to succeed in uncharted territory.
