VILLANOVA, Pa. – Geckos are known for sticky toes that allow them to climb up walls and even hang upside down on ceilings. A new study shows that geckos have gained and lost these unique adhesive structures multiple times over the course of their long evolutionary history in response to habitat changes.
The findings are published in the most recent edition of PLoS ONE. Aaron Bauer, PhD, professor and Gerald M. Lemole Endowed Chair in Integrative Biology at Villanova University, is the study’s senior author. The research is part of a long-standing collaboration on gecko evolution among biologists at Villanova University, the University of Minnesota and the University of Calgary.
Geckos, a type of lizard, are found in tropical and semitropical regions around the world. About 60 percent of the approximately 1,400 gecko species have adhesive toepads. Remaining species lack the pads and are unable to climb smooth surfaces. Geckos with these toepads are able to exploit vertical habitats on rocks and boulders that many other kinds of lizards can’t easily get to. This advantage gives them access to food in these environments, such as moths and spiders. Climbing also helps geckos avoid predators.
The researchers found that sticky toes evolved independently in about 11 different gecko groups. In addition, they were lost in at least nine different gecko groups. The gain and subsequent loss of adhesive toepads seems associated with habitat changes; e.g., living on boulders and in trees versus living on the ground, often in sand dunes, where the feature could be a hindrance rather than an advantage. “The loss of adhesive pads in dune-dwelling species is an excellent example of natural selection in action,” says Bauer.
Repeated evolution is a key phenomenon in the study of evolutionary biology. A classic example is the independent evolution of wings in birds, bats and pterosaurs. It represents a shared solution that organisms arrived at separately to overcome common problems.
“Scientists have long thought that adhesive toepads originated just once in geckos, twice at the most,” added University of Minnesota postdoctoral researcher Tony Gamble, a coauthor of the study. “To discover that geckos evolved sticky toepads again and again is amazing.”
In order to understand how the toepads evolved, the research team produced the most complete gecko family tree ever constructed, including representatives of more than 100 genera (closely related groups of species) from around the world. This family tree can serve as the basis for answering many other questions, such as how and when did live birth, temperature-dependent sex determination, and night color vision evolve in geckos? The family tree will also allow the authors to revise gecko taxonomy to best reflect the group’s evolutionary history.
Gecko toepads adhere through a combination of weak intermolecular forces, called van der Waals forces, and frictional adhesion. Hundreds to hundreds of thousands of hair-like bristles, called setae, line the underside of a gecko’s toes. The large surface area created by this multitude of bristles generates enough weak intermolecular forces to support the whole animal.
The amazing clinging ability of Gecko toes has inspired engineers to develop biomimetic technologies ranging from dry adhesive bandages to climbing robots. “Gaining a better understanding of the complex evolutionary history of gecko toepads allows bio-inspired engineers to learn from these natural designs and develop new applications,” added co-author Anthony Russell, of the University of Calgary.
While scientists have a good understanding of how geckos stick at the microscopic level, they are just beginning to understand how geckos use their adhesive toepads to move around complex environments in the wild. Learning how gecko toepads have evolved to move in nature is an important step in developing robotic technologies that can do similar things. Examining the repeated evolution of gecko toepads will let scientists find common ways natural selection solved these problems and focus on the characteristics shared across different gecko species.
Authors of the study included scientists from Villanova University, University of Calgary, the University of Texas at El Paso and the University of Minnesota. Funding was provided by the National Science Foundation of the United States and the Natural Sciences and Engineering Research Council of Canada.
About Villanova University: Since 1842, Villanova University’s Augustinian Catholic intellectual tradition has been the cornerstone of an academic community in which students learn to think critically, act compassionately and succeed while serving others. There are more than 10,000 undergraduate, graduate and law students in the University's five colleges – the College of Liberal Arts and Sciences, the Villanova School of Business, the College of Engineering, the College of Nursing and the Villanova University School of Law. As students grow intellectually, Villanova prepares them to become ethical leaders who create positive change everywhere life takes them.