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Intense female-female aggression in the Gila monster (Heloderma suspectum)

Lappin, A. Kristopher

Biological Sciences Department

California State Polytechnic University

Pomona, California USA

Peterson, Karl H.

Department of Herpetology

Houston Zoo

Houston, Texas USA

Powell, Anthony R.

Department of Integrative Biology

University of South Florida

Tampa, Florida USA

Taylor, John D.

Biological Sciences Department

California State Polytechnic University

Pomona, California USA

Alexander, Jennifer R.

Biology Department

Rio Hondo College

Whittier, California USA

Schuett, Gordon W.

Chiricahua Desert Museum

Rodeo, New Mexico USA

Department of Biology | Neuroscience Institute

Georgia State University

Atlanta, Georgia USA

Since Darwin, research on aggression in animals has been dominated by studies of male-male interactions. However, female aggression is receiving increasing attention, with studies of lizards proving particularly fruitful. Using a captive colony of Gila monsters (Heloderma suspectum), we observed significant and complex female-female aggression, and based on four unique dyadic trials, we developed a qualitative ethogram of aggressive behaviors. The prevalence and intensity of the aggressive acts we observed were unexpected and included sustained biting, envenomation, and lateral rotation during sustained bites (i.e., rolling of body while holding opponent with jaws). Given the prominent role of biting during interactions, we conducted bite-force experiments on a separate group of captive subjects. Based on these data, we postulate that osteoderms (bony deposits in the skin) may offer some degree of protection during intense aggressive interactions, thus mitigating potentially serious injury during female- female fights. In contrast to the rather extreme behavior we observed during female-female fights, male- male contests in H. suspectum are far less violent and highly ritualized, with biting rarely reported. Female-female aggression in other lizards has been shown to have a role in territoriality, courtship tactics, and nest and offspring guarding. The fact that H. suspectum is a nest-raiding specialist raises the possibility that aggression by nesting females towards marauding conspecifics, including other females, may function to prevent nest cannibalism. Future research on aggression in both sexes of Gila monsters, as well as bite-force experiments on wild subjects, is warranted to test these and other hypotheses in the laboratory and natural (field) conditions.


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