Date of Award

Spring 2002

Document Type

Restricted Thesis

Terms of Use

© 2002 Irene Garcia. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts


Biology Department

First Advisor

Rachel Merz


On sunny summer days up to half of the Uca pugnax visible on exposed marsh surface in Lewes,DE exhibit a stereotypical posture, "posing". The crabs stand on the tips of their legs, backs oriented to the sun. They use four or fewer legs to hold their bodies off the ground while extending their claws and remaining legs away from the body. They can pose motionless for up to two hours and while in this posture seem less sensitive to movement around them, lingering, visible on the surface while other crabs dart into their burrows. This behavior could serve to facilitate molting (Jansen, 1970), as a sexual display or thermoregulation. We collected 75 crabs that were either posing, active on the surface or in nearby burrows and monitored them in the lab for 4 weeks. Burrowing animals molted at the highest rate (8%, compared to 4% for posing or active crabs). Behavioral surveys performed in the field throughout the lunar cycle and seasons show that the posing frequency does not parallel that of the waving display but instead increases as ambient temperatures rise. In the field, we measured the temperatures of crabs that were either posing for at least two minutes, otherwise active on the surface or in adjacent burrows. The posing crabs were significantly warmer (Friedman two-way ANOVA, N = 40, P <.001). Thus, posing seems to provide a basking function. Higher body temperature could expedite digestion, gamete maturation or other physiological functions. However, these benefits must be considered in the context of the potential risk involved.