Age Structure of Invasive Green Anole Populations near Japan

Figure from a new paper by Yasumiba et al. illustrating how LAGs in the cross sections of bones can be used to infer lizard age.

Anolis carolinensis is a disruptive invasive species in the Osagawara Islands near Japan, a UNESCO World Natural Heritage site. It was first recorded on the island of Chichi-jima in the 1960’s and has since spread to surrounding islands. A recent post on Anole Annals describes efforts to improve the effectiveness of adhesive lizard traps on the islands by using cricket bait.

A new paper by Yasumiba et al. improves our understanding of these invasive A. carolinensis by quantifying their longevity and growth rates using skeletochronology. The authors viewed through a microscope femur bone cross sections from 126 lizards captured on Chichi-jima. Lines of arrested growth (LAGs, also known as Harris lines), which represent areas of high bone density that can form during periods of relative inactivity, were counted for each individual and used as a proxy for age (they typically form during the winter). The authors acknowledge that LAGs can occasionally be removed when portion of a bone is resorbed, resulting in an underestimate of lizard age. Nonetheless, they find that some individuals from these wild populations are at least five years old, with an average age around two years. Five years approaches some of longest recorded lifespans for captive A. carolinensis – seven years and two months for a lizard taken from its native North American range, and six years and four months for an individual from an invasive population Japan – suggesting that A. carolinensis on Chichi-jima experience low mortality risk. Indeed, their reputation as ecological pests in the Osagawaras suggests as much. The authors also document a precipitous decline in growth rate at age two.

Most anoles presumed to under high predation pressure, such as those in mainland Central and South America, die long before they’ve reached two years of age. In fact, a study in the Bahamas including Anolis smaragdinus, which is closely related to A. carolinensis, did not find a single A. smaragdinus that survived up to two years. Although this finding may be due to small sample size or high rates of lizard movement in and out of study areas, island anoles are thought to experience relatively low predation pressure, making the long lives of green anoles on Chichi-jima seem even more impressive.

Yasumiba et al. state that “our knowledge of the age structure of the Ogasawara populations of this lizard has been poor,” and their study definitely begins to address this issue. Future investigations into other life history traits, such as age at first reproduction, would permit intriguing comparisons with green anoles in North America and the Caribbean, in addition to supplementing our knowledge of these intriguing introduced populations.

Yasumiba et al. 2016. Minimum Longevity and Growth of the Invasive Green Anole, Anolis carolinensis, in Chichi-jima of the Ogasawara Islands, Japan. Current Herpetology 35(2): 101–105.