UC Riverside

Defining a species is notoriously difficult, as often individuals in previously well-defined species groups are found to hybridize in some portion of their range. Because they represent a test of the barriers that lead to reproductive isolation between lineages, hybridization events are useful “natural laboratories” for developing a deeper understanding of how reproductive isolation is (or is not) maintained, and what traits either strengthen or weaken it. Rattlesnakes provide a unique opportunity to study interspecific hybridization due to the common occurrence of hybridization between lineages in nature, the seeming general weakness of prezygotic reproductive barriers, and the establishment of effective techniques for monitoring the behavior and ecology of free-ranging individuals. I used a naturally occurring hybrid zone between Mojave (Crotalus scutulatus) and Prairie (C. viridis) Rattlesnakes as a case study for examining potential extrinsic barriers to hybridization that could develop from interactions between behavioral expression and ecological conditions. I used a combination of radiotelemetry, fixed-field videography, and laboratory behavioral assays to compare the spatial ecology, hunting behavior, and differential expression of behavioral types and syndromes in individual snakes sampled from across this hybrid zone, and used genomic analyses to quantify the ancestry of individuals in my analyses. I found that parental and hybrid individuals were generally similar to one another in many behaviors and in the variability of behavioral traits, suggesting relatively conserved behavioral phenotypes within these lineages. However, I found some potentially important differences in certain aspects of spatial (movement frequency and core space use patchiness) and hunting (chemosensory behaviors and abandonment time of the hunting site) behaviors. I also found that Prairie Rattlesnakes were significantly more likely to rattle defensively than hybrid or Mojave Rattlesnakes, and, furthermore, hybrid rattlesnakes that had a greater portion of their genome derived from Prairie Rattlesnakes were more likely to rattle defensively. My research highlights the potential utility of using behavioral expression in free-ranging animals to identify differences between lineages that could impact further reproductive isolation and other evolutionary dynamics of hybrid zones.