Ain’t No Mountain High Enough .. To Stop Tree Squirrels From Hybridizing

The Chavez Lab will be going to the North Cascades of Washington this summer to do field work in the Tamiasciurus tree squirrel hybrid zone. We have been studying hybrid zone dynamics between Douglas squirrels (T. douglasii) and red squirrels (T. hudsonicus) for 10 years using mostly genetic and phenotypic data. Now is the time to start some observational field research to better document hybrid dysfunction and behavioral interactions between species and their hybrids.

This study contains a richness in questions as to the role that ecological divergence has in the maintenance of isolating barriers and ultimately speciation between these two species. These parapatric species, separated by an extreme change in habitat, meet each other in the different mountain ranges in the Pacific Northwest. Both species live primarily in coniferous forests and have diets and lifestyles that are specialized for feeding on seeds from conifer cones. In the North Cascades region, Douglas squirrels are mostly found on the west side of the Cascade Mountains in a mesic forest environment with a moderate coastal climate. Red squirrels on the other hand are mostly found in the rain-shadow of the Cascade Mountains on the eastside and live in a drier forest with a more seasonally variable climate. Due to the higher fire frequencies in the eastside forest communities, some of the conifer species that red squirrels depend on produce cones with very hard scales or are serotinous (only open during extreme heat from fires). As a result, red squirrels in this region have very strong jaw muscles and bite force in comparison with Douglas squirrels that only feed from trees that produce softer cones. There are many other environmental differences between the westside and eastside environments and thus strong potential for adaptive divergence between these species.

So, you may ask, what does all this ecology have to do with hybridization and speciation? Well, these species may be producing hybrids that have phenotypes that are not well adapted to either type of forest and thus are at a selective disadvantage. Our goal for this study is to examine more directly whether hybrids have lower fitness and dysfunctional traits that decrease their chances of surviving and reproducing. We plan to do this by live-trapping squirrels in a hybrid zone location where I know from previous genetic research that both parental species and hybrids occur. We expect all squirrel types to be living in close proximity with each other and thus we should have good opportunities to study behavioral interactions, as well as document differences in various performance behaviors, such as feeding, mating, vocalization, territorial defense, anti-predator defense, etc…

Stephanie Malinich with bird crownStephanie Malinich is going to be the lead field technician and she will supervise a crew of eager field assistants. Since this is our first field season, we expect a lot of surprises, hopefully more pleasant than difficult ones. This is an exciting time for our lab and we will update you on our findings on this blog later in the year.

 

Andreas ChavezAbout the Author: Andreas Chavez is Assistant Professor in EEOB as of Fall 2016. He is also Director of Mammals in the Tetrapod Collection at the Museum of Biological Diversity. This is his first blog post for the Chavez Lab on the MBD website.

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