A figure from Eric Mueller’s poster showing the conserved pathway of how growth hormone may affect body size.
Anyone familiar with Anolis lizards is aware of the dramatic variation in body size. Think dwarf twig anole and crown giant. Although the ecological and evolutionary processes that can lead to such variation in body size have been studied, it is still unknown what physiological mechanism explains the variation we see today. Eric Mueller, a graduate student at Southern Illinois University – Edwardsville, presented a poster asking just that question. Specifically, do differences in circulating levels of plasma growth hormone regulate evolutionary changes in body size among anole species of differing size and morphology?
Growth hormone (GH) is secreted by the pituitary gland and has many functions in the body, including promoting muscle and bone growth and increasing protein synthesis (among many, many other things!). It seems a logical candidate mechanism to investigate when it comes to explaining variation in body size. Mueller looked at GH levels in three anoles of varying size: A. equestris, A. carolinensis, and A. sagrei. GH was higher in A. equestris and A. carolinensis than A. sagrei, supporting his hypothesis. However, there was no difference in GH levels between A. equestris and A. carolinensis despite dramatic differences in adult body size. Looking within species, GH levels were positively correlated with SVL only in A. equestris, and not the other two species.
Although differences in circulating GH may explain some size differences among anole species, as in other studies of anole hormones, things don’t seem to be simple. Mueller hypothesized that other aspects of the GH pathway may be more important. For example, GH receptors, Insulin-like Growth Factor (IGF) levels, and IGF-binding proteins should be examined for a full picture. The GH-IGF axis also interacts with other hormone pathways, such as testosterone, making this a very complex issue. Since endocrine systems are so multi-faceted, and multiple components have the possibility to evolve independently, there is lots of potential for future research that seeks to explain species differences in body size.