A venomous tale: Vipers shape lizards' tail-shedding abilities
University ecologists and their colleagues have answered a question that has puzzled biologists for more than a century: What is the main factor that determines a lizard's ability to shed its tail when predators attack?
The answer: venom.
Tail-shedding, known to scientists as caudal autotomy, is a common anti-predator defense among lizards. When attacked, many lizards jettison the wriggling appendage and flee. The predator often feasts on the tail while the lucky lizard scurries to safety. Later, the lizard simply grows a new tail.
The ease with which lizards shed their tails varies from species to species and from place to place. For more than a century, biologists have suspected this variation is controlled mainly by predator pressure: As the number of local lizard-eaters rises, so does the need for this effective defense mechanism.
When lizards live alongside lots of creatures eager to devour them, they're more likely to develop the ability to shed their tails easily. The trait enables them to survive long enough to reproduce and pass their genes to the next generation.
However, tail loss carries long-term costs, including impaired mobility, lower social status and slower growth rates. So from an evolutionary perspective, it only makes sense to maintain tail-shedding ability if there are predators around.
A U-M-led team decided to test the long-held predator-pressure idea using a combination of laboratory and field measurements made in mainland Greece and multiple Aegean Sea islands inhabited by different combinations of predators.
Its conclusion? The predator-pressure hypothesis, while generally true, comes with an unexpected twist: Not all predators are created equal.
"The only predators that truly matter are vipers," says vertebrate ecologist Johannes Foufopoulos, co-author of a study published online this week in the journal "Evolution."
"In the Aegean, vipers are specialized lizard predators," says Foufopoulos, an assistant professor at the School of Natural Resources and Environment and the Department of Ecology and Evolutionary Biology. "So it makes sense, in retrospect, that the lizards' primary defense would be aimed against their main enemy, the viper. But no one had made this connection, until now."
This result is perhaps best explained by the peculiarities of viper attacks, Foufopoulos says. When non-venomous predators attack, tail-shedding is only useful in the relatively rare instances when the tail is firmly grasped by the predator.
But when a viper bares its fangs and strikes, even glancing contact with the lizard's tail can inject a lethal dose of venom. In that case, the ability to shed a tail within seconds before venom reaches the lizard's vital organs becomes a life-or-death matter.
"You lose your tail, but you come away with your life," Foufopoulos says. "And you can always grow another tail."
The team looked for correlations between autotomy rates and the presence or absence of various types of lizard predators at the study's 10 collecting sites. The autotomy rate measures the ease with which lizards shed their tails. The higher the rate, the easier the tail separates from the body.
The only strong signal that emerged from the study was the link to vipers.
The team found that viper-free islands are home to lizards that have largely lost the ability to shed their tails. Conversely, all the locations where vipers have survived contain lizards with high autotomy rates.
Understanding tail-shedding abilities among different lizard populations has important practical applications for conservation biologists. Because tail-shedding is an important defense against predators, the expression of this ability can help predict which lizard populations are most vulnerable to the accidental introduction of non-native predators.
As island extinctions in other parts of the Mediterranean already have shown, lizards that have lost this capacity cannot defend themselves and quickly succumb to invasive snakes.
Lead author of the "Evolution" paper is SNRE's Panagiotis Pafilis. The co-authors are Foufopoulos, Nikolaos Poulakakis of Yale University, Petros Lymberakis of the Natural History Museum of Crete and Efstratios Valakos of the University of Athens.
Funding for the study was provided by the School of Natural Resources and Environment and the Modern Greek Program.