Everything is bigger in Texas: skull morphology in bull snakes across a desert-grassland gradient

Yeasha and Skakel are working with one of the largest snakes (4 to 8 feet) in North America, Pituophis catenifer. This snake is known by many common names across it’s very wide range that includes southern British Columbia down to Sonora, Mexico and California east to Indiana.

There are currently six recognized subspecies, but Dr. Ed Myers and his team are concerned with just two of them: the Sonoran gopher snake (var. affinis) and the bull snake (var. sayi). These two subspecies exhibit a considerable amount of morphological diversity, shaped by the distinctive biomes they inhabit: desert (var. affinis) and grassland (var. sayi). ​

In particular, Skakel and Yeasha are investigating how skull morphology differs between these two subspecies. An earlier study - using a very small sample size (n=4 individuals) - pointed to very clear differences in the shape of rostral bones (e.g. nose). The working hypothesis is that snakes that live in grasslands will have a wider, more robust head in a response to the challenges of digging in soils where movement is impeded by fine, dense mats of fibrous grass.

​To test this hypothesis, Yeasha and Skakel have already photographed over 100 snakes across the Chihuahuan desert and Texas grasslands and will soon be using geometric morphometrics to describe the shape of their snakes’ heads. Later comes mapping and correlating skull morphology with soil type data. But for now, it’s measuring and photographing a lot of big, dead snakes that still look very much alive when emerging from the formalin/ethanol bath.