The role of the Harderian gland in the chemical ecology of garter snakes
The Harderian gland is the largest cephalic gland in most groups of terrestrial vertebrates. Harderian gland secretions are diverse among different taxa and multiple lines of inquiry have resulted in many diverse putative functions being ascribed to it. Recent studies suggest that the Harderian glands of garter snakes are major components of the vomeronasal chemosensory system. The Harderian glands of squamate reptiles are physically connected to the vomeronasal organ via the nasolacrimal duct. The glands produce fluid which travels through this duct to fill the vomeronasal lumen. Protein components of these secretions are essential to vomeronasal chemosensory function, facilitating the solubilization and detection of sex pheromones and prey kairomones. These chemical signals are essential to mate recognition, mate selection, feeding, and mediation of annual cycles, and are critical components of individual fitness and the targets of natural and sexual selection.
Histological studies of the Harderian gland show sexually dimorphic structural changes that occur between the spring-mating and summer-feeding seasons indicating that the gland is hypertrophied during behavioral phases where the vomeronasal chemosensory system is most actively being used.
Analysis of the protein components of Harderian gland secretions of garter snakes via SDS-PAGE show several secreted proteins with sexually dimorphic expression patterns likely to be targets of selection and important to the biology and physiology of these snakes. RNA sequencing shows high expression of lipocalins – proteins which bind and solubilize lipids in aqueous solution likely acting as pheromone binding proteins facilitating the detection of nonpolar sexual attractiveness pheromone.
Further analyses of the protein components of Harderian gland secretions via mass spectrometry and high throughput RNA sequencing are currently underway to reveal the identity of these secreted proteins and further investigate their function within the vomeronasal chemosensory system.Vomeronasal/Harderian gland system of the garter snakeImage
HG=Harderian gland, LC=lacrimal canal, NC=nasal cavity, LD=lacrimal duct, VNO=vomeronasal organ
Pheromones in garter snakes
Dr. Robert Mason was the first scientist to isolate, identify, and synthesize a pheromone from a reptile, and his discovery fostered several research areas into the role of pheromones in the reproductive biology of garter snakes. Previous work by Dr. Michael LeMaster examined the intrasexual, seasonal, and population-level variation in the sexual attractiveness pheromone produced by female red-sided garter snakes. He has continued to collaborate with Dr. Mason and has involved several undergraduate students from Western Oregon University in projects that aim to describe species differences in pheromone composition from several garter snake species (e.g. Thamnophis ordinoides, Thamnophis radix). Recent projects by Honors students have investigated the effects that factors such as UV radiation and captivity have on pheromone profiles.
Thermal Biology of the Red-sided Garter Snake
The garter snakes that we study come from the Interlake region of Manitoba, which in the last 50 years has experienced an increased average annual temperature of nearly three times the global average. With a winter dormancy period of up to eight months, and a summer feeding period of as little as ten weeks, the Red-sided garter snake may be especially effected by these extreme environmental changes to the local climate. We are looking closely into the thermal biology of these snakes, using a multifaceted approach utilizing decades of historical measurements and samples along with modern molecular techniques that include genomic analysis of the population over the last 30 years, as well as investigating transcriptional response to thermal stresses on the modern population.
Comparative and experimental endocrinology
The red-sided garter snake shows strong patterns of seasonality, which are reflected in their behavior and circulating levels of sex and stress hormones. This species serves as a model dissociated breeder, meaning that their maximal mating behavior is uncoupled from gametogenesis and maximal sex steroid production. Several previous students in the Mason lab have explored different aspects of the endocrinology of this and sister taxa (T. s. concinnus, T. s. sirtalis).
Evolution of mating systems and strategies
Our focal species has a polygynandrous mating system that is excellent for investigating sperm competition, cryptic female choice, sperm storage, and sexual conflict, as numerous collaborative projects with Dr. Richard Shine have reported. Female red-sided garter snakes use stored sperm from multiple matings, and thus post-copulatory sexual selection and coevolution are likely important factors in this mating system. Previous collaborative work with Dr. Stevan Arnold and Dr. Suzanne Estes shed light on multiple paternity in this species. Dr. Chris Friesen, who completed his Ph.D. in the Mason lab, continues to work on the garter snake system and is exploring the consequences of limited female pre-copulatory choice. By looking at patterns of paternity, seasonality of mating, and mating frequency of both males and females, we hope to gain insight into the factors that drive sexual selection (e.g. Bateman's gradients) and sexual conflict in this system and how each sex has responded to resolve conflict. We have an ongoing collaboration with Dr. Patricia Brennan (University of Massachusetts Amherst) to assess the role of genital morphology in sexual conflict.
Metabolic cost of courtship and mating
In our system, the sex ratio is skewed, with males greatly outnumbering females during the spring mating season. Thus, males scramble to gain access to females, leading to mating balls of up to one hundred snakes. During the mating season, the snakes do not eat and engage in very robust, stereotypical courtship behavior that involves significant levels of activity. We have recently started a collaboration with Dr. Don Powers to assess the metabolic cost of courtship for these snakes. Mason lab alum, Dr. Chris Friesen (University of Sydney) is working with Dr. Powers on this topic using two separate approaches (doubly labeled water and respirometry) to address this question.
Reproductive implications of parasites
Garter snakes are host to several genera of endoparasites including trematodes, nematodes, and cestodes. Collaborations with Dr. Michael Kent (Oregon State University) and Dr. Vasyl Tkach (University of North Dakota) have allowed us to identify these parasites. The newest area of research in our lab, and the focus of work by recent graduate Dr. Emily Uhrig, investigates how infections by such parasites affect reproductive fitness in garter snakes. We are also investigating how immune activity can influence reproduction independent of parasite pathogenesis. As parasites and immune activity can potentially affect biological processes such as courtship and mating, gamete production, and pheromone production, and can also affect or be affected by endocrinology, this avenue of research interacts with many of the lab's other research areas.
If you are interested in becoming a part of the Mason lab, please contact Dr. Mason directly (masonr at science.oregonstate.edu) or one of his grad students for more information (Ehren Bentz, firstname.lastname@example.org, Leslie Blakemore, email@example.com or Dave Hubert, firstname.lastname@example.org).