Female-female aggression in the Gila Monster (Heloderma suspectum)

Historically, the role of aggression in the social lives of animals
overwhelmingly focused on males. In recent years, however, female-female
aggression in vertebrates, particularly lizards, has received increasing
attention. This growing body of literature shows both similarities and
differences to aggressive behaviours between males. Here, we document
female-female aggression in captive Gila monsters (Heloderma suspectum).
Based on four unique dyadic trials (eight adult female subjects), we
developed a qualitative ethogram. Unexpected and most intriguing were the
prevalence and intensity of aggressive acts that included brief and
sustained biting, envenomation, and lateral rotation (i.e., rolling of
body while holding onto opponent with closed jaws). Given specific
behavioural acts (i.e., biting) and the results of bite-force experiments,
we postulate that osteoderms (bony deposits in the skin) offer some degree
of protection and reduce the likelihood of serious injury during
female-female fights. Male-male contests in H. suspectum, in contrast, are
more ritualized, and biting is rarely reported. Female-female aggression
in other lizards has a role in territoriality, courtship tactics, and nest
and offspring guarding. Future behavioural research on aggression in
female Gila monsters is warranted to test these and other hypotheses in
the laboratory and field.