Interleukin-1 beta reduces daily activity level in male lizards, Sceloporus occidentalis.

In mammals, interleukin-1 (IL-1) mediates many of the behavioral consequences of pathogen infection. Other vertebrates show behavioral changes when infected, but the neuroendocrine bases of these changes are seldom known. Here we report that IL-1 beta alters the daily activity cycle of lizards (Sceloporus occidentalis) similar to that seen in lizards infected with malaria. To our knowledge, this is the first report of behavioral effects of interleukin in lower vertebrates. Male lizards were injected with human IL-1 beta (10 ng/g animal), saline, or nothing (control) and the activity level (proportion of lizards above ground) was monitored for 48 h. IL-1 beta-injected lizards showed a decrease in activity compared to saline-injected and control lizards within 2 h after treatment. Activity levels were equivalent among treatment groups during the middle of the day (1200-1600 h), but IL-1-treated animals were significantly less active in the morning (0800-1200 h) and evening (1600-2200 h). This decrease in activity is similar that seen in free-ranging lizards infected with malaria, supporting the hypothesis that IL-1 mediates the pathogen-induced alterations to activity behavior under natural conditions.