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Theory predicts the evolution of alternative male social tactics when intense competition coupled with the superior competitive ability of some individuals limits access to reproductive opportunities by others. How selection has shaped alternative social tactics may be especially interesting in long-lived species where size among sexually mature males varies markedly. We conducted experimental studies on long-lived eastern Australian water dragons living where competition was intense to test the hypotheses that mature males adopt alternative social tactics that are plastic, and that large size and body condition determine resource-holding potential. Approximately one-half of mature males (N = 14) defended territories using high rates of patrol and advertisement display, whereas 16 smaller mature males having lower body condition indices utilized non-territorial social tactics. Although territorial males were larger in absolute size and head dimensions, their heads were not allometrically larger. Territorial males advertised very frequently using displays involving stereotypical movements of the head and dewlap. More aggressive displays were given infrequently during baseline social conditions, but increased during periods of social instability. Female home ranges overlapped those of several territorial and non-territorial males, but females interacted more frequently with territorial males. The extreme plasticity of social tactics in this species that are dependent on body size was confirmed by two instances when relatively large non-territorial males spontaneously evicted territory owners, and by marked shifts in tactics by non-territorial males in response to temporary experimental removals of territory owners, followed (usually) by their expulsion when original owners were reinstated. The high level of social plasticity in this population where same-sex competitors are densely concentrated in preferred habitat suggests that chronic high energetic costs of defense may select for males to cycle between territorial and non-territorial social tactics depending upon their changing energetic status and their current capacity for competition with rivals.

The incidence of autism and cognitive disability was assessed in the biological siblings of 29 autistic probands subdivided on the basis of IQ. A significant clustering of autism and nonspecific intellectual retardation was found in the siblings of severely retarded autistic probands which was not present in the siblings of our higher-functioning autistic sample. These findings suggest that there may be etiological differences in autism, depending on the degree of associated mental retardation.

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Summary 1.A combination field and laboratory study tested the hypothesis that a herbivorous lizard, Cnemidophorus murinus , extends activity at high body temperatures to digest plant material. 2. Body temperatures ( T b ) of active lizards averaged 37·2 ° C and were no higher than those of insect-eating Cnemidophorus . 3. Near constant T b was maintained by behavioural means even though habitat temperatures varied greatly. 4. Field-based behavioural data show that C. murinus extend activity by shifting among microhabitats to increase duration of time available at high T b for digestion. 5. Laboratory studies revealed large variation in selected temperatures ( T sel ), but the overall average was 35·9 ° C, which is lower than field T b . 6. By carefully selecting microhabitats and extending activity, C. murinus maintains constant high body temperatures to digest plant material without risking potentially lethal overheating.