Island Syndrome is a well known change in genetic patterns that occurs as a species evolves on an isolated island habitat. This change can happen to birds blown out to sea by storms or to small mammals and reptiles that float out on debris. Island Syndrome occurs because, on most islands, the species that landed has escaped its main predators, and islands typically have enough resources to support a high, stable population. Therefore, this syndrome is characterized by an increase in energy allocation to growth, as opposed to reproduction, since mates are guaranteed to be plentiful. Organisms grow bigger, live longer, and are less aggressive because resources are easy to find, meaning there is little competition within the species.
However, rarely, this stability is not always present on isolated islands, and the new species will either find another strong predator, limited resources, or a harsh climate, all of which can create high fluctuations in the population density. This is the case with the Licosa Island Wall Lizard (Podarcis sicula klemmeri) found on Licosa Island off the coast of Italy, which was studied by members of the Department of Earth Science and the Department of Structural and Functional Biology in the University of Naples. This lizard was a recent descendent of the Italian Wall Lizard (Podarcis sicula) and had recorded population density swings of up to 40%. This means that the total numbers of lizards on the island can drop by nearly half, from year to year. The members of the research team theorized an opposing hypothesis for the Island Syndrome in situations where population density is highly variable, which they called Reversed Island Syndrome (RIS). It was thought that this syndrome would be characterized by higher reproductive energy demands, since the survival of a mate at any one time would not be as secure. One of the most distinct traits of the Licosa Lizard was its bright blue color, which the team traced back to a gene that controls a specific set of traits that could theoretically be involved in the behavioral patterns of RIS. The physical and behavioral traits coded for by this gene were as follows: more aggressive behavior, extreme sexual dimorphism (difference in body size), and high sexual competition.
All of these behaviors were found in the Licosa Lizards, both when observed on the island and when they were raised in captivity. Through many series of tests and observations, it was clear that the large majority of these traits were much more apparent in the Licosa Island Lizards than in their mainland relatives. It was also apparent that many of these traits were driven by the fact that the Licosa Lizards expended much more energy on reproduction; for example, Licosa females are much smaller than Licosa males and mainland lizards because they put more energy into developing their eggs and young. Since these behaviors were characteristic of RIS, a wider survey of other research was done, and it appears that other reptile species that could possibly be influenced by RIS are often a black or blue color, meaning that the specific pigmentation gene could be a co-factor for RIS.
This does not mean, however, that all of the traits specified, such as coloring, must be present in order for RIS to be in effect. In addition, even though a reptile has a specific coloring it may not be under the influence of RIS. Instead the main driving force behind the presence of RIS within an island population is a highly variable population density and the possibility of a lack of mates at any one time that can trigger a selection for more sexually aggressive characteristics. This study has been one of the first steps for confirming this hypothesis and its result are an influential step towards confirming the effects of RIS on reptiles and other species.
~Written By: Rachel Taupier
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