My thesis work focuses on sexual selection that occurs during and after sex (postcopulatory sexual selection). I am specifically interested in how social and physical environments drive the evolution of postcopulatory sexual traits, such as sperm and genitalia, in lizards. Although lizards are numerous and diverse, less is known about sexual selection evolution and plasticity in this group than any other major taxonomic group. While precopulatory sexual selection in squamates (lizards and snakes) has been a focus of many studies, postcopulatory sexual selection has received comparatively little attention.
As part of my doctoral work, I have also done research to show that as geckos get bigger, the tissues that attach to their adhesive toe pads get stiffer. This stiffness is crucial in allowing the heavier animals to move safely along a variety of surfaces. On the theme of animal movement, I studied how the flexibility of the physical structures in a jumping lizard's environment affected how they got around. I discovered that green anoles crawl around on everything in their environment (including extremely floppy leaves), but choose relatively more stable structures to jump from. Details and pubs below.
The role of habitat heterogeneity in postcopulatory sexual selection in Anolis carolinensis
The degree of habitat heterogeneity (continuous vegetation versus clumped vegetation) has been shown to affect male density and male-male competition. I am interested in whether these habitat effects also affect postcopulatory sexually selected traits (e.g., testis size, sperm traits, male and female genital morphology). Is there a link between habitat structure and sexual trait characteristics?
Answering this question can require studying lizards in the wild. If you're curious what that's like, check out my field blog, Adventures Down South! See what happens when the data necessary for a dissertation chapter are at the mercy of mother nature.
Postcopulatory selection and genital evolution in Uta stansburiana
Uta stansburiana, the side-blotched lizard, exhibits alternative mating strategy polymorphism. In some populations this species has three genetically-determined male mating strategies. Males with orange throats (shown in the photo) control large territories with many females. Blue-throated males (also shown) closely guard females and cooperatively defend smaller territories. The third type, yellow-throated males, sneak on to other males’ territories to copulate with ungaureded females.
While these three types of males are behaviorally and morphologically dissimilar, particularly when it comes to mating strategy, it is unclear if they also vary in any postcopulatory sexually selected traits. I am testing the hypothesis that heritable differences in morphology and behavior in a population of polymorphic U. stansburiana and high levels of male-male competition result in morph-specific differences in hemipenis shape, in collaboration with Ammon Corl and the Sinervo lab.
The effects of system and substrate compliance on adhesion in geckos
The amazing adhesive ability of geckos has fascinated scientists for many decades and has more recently resulted in many attempts to create man-made materials that adhere to surfaces as well the adhesive pads found on the toes and sometimes tails of these arboreal lizards. While most of the research conducted to date has focused on the anatomical structures at the contact surface that allow adhesion, such as setae, results from several studies suggest that these structures alone do not sufficiently explain the adhesive capabilities of these animals. We are currently working to understand how the whole system of the gecko contributes to the amazing adhesive ability of these animals. In addition, we are also examining the effects of perch flexibility or softness on a gecko's ability to adhere.
Gilman C.A., Imburgia M.J., Bartlett M.D., King D.R., Crosby A.J., Irschick D.J. (2015). Geckos as Springs: Mechanics Explain Across-Species Scaling of Adhesion. PLoS ONE 10(9): e0134604. doi:10.1371/journal.pone.0134604
Also, in a study of how effective synthetic adhesives could be in adhering to surfaces with different textures (below), I assisted the engineers in testing the geckos' ability to adhere to different materials, so they could compare those data to the adhesive ability of the synthetic adhesive they had created. My collaborators demonstrated that the synthetic adhesives they fabricated had greater adhesive force capacity than the live Tokay geckos, and were able to support high loads on a range of surfaces.
King D.R., Bartlett M.D., Gilman C.A., Irschick D.J., and A.J. Crosby. 2014. Creating Gecko-like Adhesives for "Real World" Surfaces. Advanced Materials 26:4345-4351. doi: 10.1002/adma.201306259
The effects of perch stability on jumping performance and kinematics in green anole lizards (Anolis carolinensis)
One of my interests in lizard adaptation is how animals physically deal with the challenges of their environment. Arboreal (tree-dwelling) anoles often rely on jumping as a means to evade predators, pursue prey, defend territories, and simply move around their habitats. These lizards must jump from both stable perch sites, such as trunks and branches, as well as from more precarious perch sites like narrow branches, twigs, and leaves. Although jump kinematics have been examined in these lizards using stable jump sites, there is has been little focus on how perch instability affects jumping. I conducted a lab study to understand how flexible perches affect jump performance and kinematics in Anolis carolinensis. What I found is that as they jump, the force of the jump causes the flexible perch to be pushed away from them. Since they don't stay on the perch as the perch recoils (think of a human on a diving board), part of their jump energy is absorbed by the perch, leaving less energy for the jump itself. As a result, the lizards are unable to jump as far or as fast as they can from more stable perches. But what does this mean for lizards in their natural habitats that jump from flexible perches regularly?
Gilman C.A., Bartlett M.D., Gillis G.B., and D. J. Irschick. 2012. Total recoil: Perch compliance alters jumping performance and kinematics in green anole lizards (Anolis carolinensis). The Journal of Experimental Biology 215:220-226. doi: 10.1242/eb.061838
The supplemental video for this paper is on the left, and can also be found on the JEB website.
To address the questions raised by the results of our lab study, we continued our examination of the effects of perch flexibility on jumping and perch choice in green anoles in the wild. Read about the challenges we faced in the field study here, and for a summary of our results, check out my post in Anole Annals!
Gilman C.A., and D. J. Irschick. 2013. The foils of flexion: the effects of compliance on lizard locomotion and perch choice in the wild. Functional Ecology. doi: 10.1111/1365-2435.12063
Master's and Undergraduate Research
My primary Master's research focused on changes in respiratory physiology during reproduction in egg-laying lizards. Because lizards have no diaphragm and the abdominal and thoracic cavities are continuous, increases in egg size during reproduction decrease the available space for lung expansion during respiration. In this study I investigated how total lung volume, tidal volume, breathing frequency, minute volume, and carbon dioxide production change throughout the reproductive cycle in the egg-laying species Crotaphytus collaris and Gambelia wislizenii.
Gilman C.A., Candelaria G., Gershman B., Norenberg J.P., and B.O. Wolf. (2013). Changes in respiratory physiology associated with reduced lung volume during gravidity in Crotaphytus collaris and Gambelia wislizenii. Journal of Herpetology 47:262-269.
One of my undergraduate projects was a study of the effectiveness of portable ultrasound imaging as a nondestructive method for quantifying reproductive output in small lizards. This method allows for a reexamination of individuals over intra-annual and inter-annual time scales and insight into the effects of varying environmental conditions on reproductive output. Ultrasonography works reasonably well on a range of species with varying morphologies and life histories. I validated the use of this method on nine species, both live-bearing and egg-laying: Aspidoscelis tesselata, Aspidoscelis tigris, Cophosaurus texanus, Crotaphytus collaris, Holbrookia maculata, Phrynosoma cornutum, Phrynosoma hernandesii, Sceloporus poinsettii, and Uta stansburiana.
I have found this method extremely valuable and I continue to use it to estimate reproductive stage in the lizards I currently work with.
Gilman C.A., and B.O. Wolf. 2007. Use of portable ultrasonography as a nondestructive method for estimating reproductive effort in lizards. The Journal of Experimental Biology 210:1859-1867. doi: 10.1242/eb.001875
My first undergraduate project was a study of the effect of temperature on the behavior of a common desert grasshopper, Trimerotropis pallidipennis, on the Sevilleta Wildlife Refuge. This species preferentially consumes detritus, but because of the need to behaviorally thermoregulate, was unable to feed for several hours during the hottest portion of the day. During this time it sought out cooler microsites up on vegetation.
Gilman C.A., Toolson, E.C., and B.O. Wolf. 2008. Effects of temperature on behavior of Trimerotropis pallidipennis (Orthoptera, Acrididae). The Southwestern Naturalist 52:162-168.