Are the horns of Jackson’s chameleons genetically and behaviorally homologous to those of mammals?

Methods

Summary

To test our molecular and behavioral hypothesis, first, we will use PCR primers designed for mammalian horn genes to search for homologous loci in the Jackson’s chameleon, particularly a gene involved in the hormonal pathway, relaxin-like family peptide (RXFP2), which has been linked to regulation and development of horns in soay sheep. Using molecular techniques, we will sequence the candidate gene in the three-horned Jackson’s chameleon (Trioceros jacksonii), and look for similarities among taxa and differences between populations and sexes of Jackson’s. We hypothesize that horns have evolved in a vertebrate common ancestor and that this is therefore an extremely ancient trait. Second,  we will stage non-lethal contests between male Jackson’s to examine the role of horns in intra-sexual selection. We will set up staged behavioral contests among males with different horn/SVL ratios, to test the idea that longer horns are the best predictor of contest success. Preliminary behavioral trials have shown that paired adult male chameleons, placed within 1 m of one another, will readily engage in fighting using the horns, until one chameleon is dislodged from the perch.  Once this occurs, the dislodged male consistently, immediately undergoes color change from a bright green/yellow dominant color to a clearly darker tone.  In addition, the behavior of the loser is unambiguously submissive, characterized by refusal to engage in further combat. Thus contest outcome is easily scored as win or lose. We propose that horn length will be an important predictor of contest success.  

Challenges

Our first critical challenge will be to amplify a region of DNA that has not yet been characterized in the chameleon genome. Though we will be designing primers based on similar regions in anoles and geckos, the process of adjusting PCR parameters until the region is amplified may be long. Another critical challenge will be obtaining the high sample size needed for sufficient statistical power during our staged contests. It will be challenging to collect and maintain the high number of males we will need all at once. To overcome this problem, we will limit the number of animals we will keep at a given time to ten and ensure that all chameleons fight one another so that we maximize the number of encounters. 

Pre Analysis Plan

To determine the character that best predicts contest success in staged fights, we will perform a logistic regression, including all parameters: body size (SVL), horn length, and percent of previous fights won.