Crotalus scutulatus and C. viridis are two species of rattlesnakes commonly found in eastern North America.

They are known to hybridize in a small area of southwestern New Mexico, however no hybridization appears to occur in a similar, wider zone of sympatry in West Texas. It is thought that, due to Palaeolithic climate cycles, species distributions have been affected in such a way that one contact zone is older than the other. It is hypothesised that the West Texas zone is older due to the two species having evolved pre-zygotic isolating mechanisms to prevent hybridization from occurring, as the young would be less fit for the environment. There is no current data to suggest the pre-zygotic isolating mechanisms are evolving in the snakes in the New Mexico.

High quality locality data was gathered from HerpMapper and iNaturalist. ArcMap and MaxEnt modelling software was then used to create present-day niche models and project the snakes’ potential distributions in the past.

These distributions were then compared to determine the probability that the two species were in contact during the last glacial maximum. The results were inconclusive as the past distributions were not as expected. There was no range overlap shown in the past distribution models. It is suggested that models are re-run using bioclimatic variables from different sources to if the results can be replicated.