Anolis scriptus
Hurricanes can be devastating events having long-last effects on ecosystems. For humans, livelihoods completely turned upside down, devastation to homes and businesses are more often than not extreme. Usually, the well-being of animals is a second thought.
Biologists at Washington University in St. Louis and Harvard have recently published a landmark study in the journal Nature looking at the physical characteristics seemingly dictating the difference between life and death of a tropical anole lizards living in Turks and Caicos. 2017 was busy hurricane season in the Atlantic region. Hurricane Harvey hit in August 2017, followed by Irma weeks later and the Maria in September. Damages were huge, largely due to intense winds reaching in excess of 170 mph.
Immediately prior to the arrival of hurricane Harvey the biologists finished a survey of Anolis scriptus, a small common lizard found throughout the cays of Turks and Caicos. A follow-up survey weeks after the hurricanes demonstrated, for the first time, hurricane-induced natural selection. Colin Donihue, a Harvard post-doctoral fellow said: “The prediction was that if we saw any changes, they would be changes in the features that help lizards hold on — they would be related to clinging ability,” Donihue said. “For example, the sticky toe pads on their fingers and toes, maybe they would be larger.”
So, following the hurricanes “We walked exactly the same transects we had the last time,” Donihue said. “There were definitely fewer lizards. We had to work harder to catch our sample size.” The research team caught 71 adult lizards in the initial survey and 93 during the revisit on two separate islands. They measured the forelimbs, hindlimbs, core body lengths and took pictures of their toe pads. These morphological measurements of the hurricane survivors were then compared to those of general lizard population.
The results showed parallel shifts in limb and toepad morphology between the pre- and post-hurricane populations on both islands. The survivors had significantly longer fore legs than the pre-hurricane population, while the femur, long bones in between their hips and knees on their back legs, were shorter. The survivor population also had on average larger toepads and smaller bodies.
The authors proposed two reasons for the shifts in phenotypes, but suggest that natural selection favored individuals able to survive the hurricanes. First, if the hurricanes caused direct selection, then after the hurricanes the survivors should show reduced trait variation. They tested this prediction by calculating the variance for each measurement. In support, the variance decreased among the survivors traits in almost all populations on both islands, a finding that unlikely occurred due to chance alone. Second, was that survivors had traits that allowed for a greater clinging. The authors confirmed an already documented positive relationship between toepad size and clinging capacity. Thus, the larger toepads of the surviving anoles supports the hypothesis that natural selection favored individuals with the greatest clinging capacity during high winds. Due to the posture assumed during high winds, Donihue and his colleagues guessed that longer hind limbs would present a larger surface area and increased the likelihood the anoles would get blown off its perch.
The authors acknowledge that there could be other selection pressures at work here, like food availability after the storm, vegetation structure and thermal micro-climates. However, whatever the selection forces were, they must have been strong to create significantly different mean morphological characteristics. This was likened to Darwin’s famous finches. The idea of phenotypic plasticity has been gaining evolutionary importance and is likely more prominent than we realize.
As the earth’s climate continues to change due to the anthropogenic impacts, the organisms within these affected ecosystems must continue to evolve. If we as humans cannot or are reluctant to change, we leave to the lizards with the largest toepads.
For this first-of-its-kind study please see the full text at:
https://www.nature.com/articles/s41586-018-0352-3#Sec2
Donihue, C. M., Herrel, A., Fabre, A. C., Kamath, A., Geneva, A. J., Schoener, T. W., ... & Losos, J. B. (2018). Hurricane-induced selection on the morphology of an island lizard. Nature, 1.
