Three beach locations in San Cristóbal, Galápagos Islands, Ecuador were examined to observe aggressive behavior of Grapsus grapsus. Sally lightfoot crabs play a significant role in the intertidal ecosystem, so understanding factors that influence population dynamics is necessary to ensure equilibrium. With regards to age, it is hypothesized that variation in aggressive behavior is present between age groups of Sally lightfoot crabs. It is predicted that intermediate-aged crabs will display aggressive acts most often. Also, crab distribution by age varies with distance from water, and it is predicted that larger crabs will be more prevalent in the wet zones. Further, we hypothesize that aggression in Sally lightfoot crabs varies with level of activity by outside factors, and it is predicted the crabs inhabiting beaches of higher activity levels will display more acts of aggression. Three beach locations were analyzed one hour before and one hour after low tide for aggressive behavior such as chasing or physical contact between two or more crabs. A Chi-square test was used to determine significance of data collected. The majority of acts of aggression were initiated by intermediate crabs (58%). Juvenile and intermediate crabs were most often found in the zone of observation furthest from the water, while more adults were found in the moist zone. Across all age categories, the number of acts of aggression increased in beaches with more activity from outside factors. Age, food availability, and hormonal changes are all possible contributors to aggressive behavior in the G. grapsus
