Comparative aggression in Sceloporus virgatus, S. undulatus consobrinus, and S. u. tristichus (Sauria: iguanidae).

Aggressive behaviour in both sexes of Sceloporus virgatus, S. undulatus consobrinus, and S. u. tristichus were compared by introducing a conspecific to a resident lizard in the field. Males of S. u. consobrinus exhibited high intensity aggression more commonly than did males of the other forms. Of the females, S. u. tristichus showed the greatest propensity for aggressive behaviour; S. u. consobrinus showed the least. In males, degree of aggression was correlated positively with degree of sexual dichromatism. The high degree of aggression in male S. u. consobrinus may be selectively advantageous because of (1) low density increasing space between females, (2) low density reducing selection against a relatively high frequency of violent fighting, and (3) high predation favouring lizards that do not leave their sites upon intrusion of another male. High aggression in female S. u. tristichus may be selectively advantageous because of (1) higher density leading to reduced food supply and fewer egg-laying sites, and (2) lower predation reducing selection against mobility and fighting in females.