COX, Robert M; JOHN-ALDER, Henry B; Rutgers University: Energetic mechanisms for testosterone-mediated growth inhibition in male eastern fence lizards, Sceloporus undulatus

Conceptual energetic models predict that growth will be traded off against other important functions (i.e., maintenance, activity, reproduction) when energy is limited, but the implications of sexual differences in such trade-offs with regard to sexual size dimorphism (SSD) are poorly understood. In the eastern fence lizard, Sceloporus undulatus, female-larger adult SSD results from a reduction in juvenile male growth that occurs coincident with male maturational changes in aggressive behavior, movement, and daily activity. We have shown that treatment with exogenous testosterone (T) increases male activity and movement while inhibiting male growth. Further, surgically castrated animals grow to substantially larger adult size than intact animals or castrates receiving T supplementation. To determine whether this T-mediated growth inhibition arises from energetic trade-offs, we compared energy acquisition and respiratory energetics in T-implanted males versus placebo-implanted males and females under both field and lab conditions. Potential energetic growth constraints arising from T manipulation include (1) decreased energy acquisition, (2) increased respiratory costs of maintenance metabolism, and (3) increased respiratory costs of movement and activity. Treatment groups did not differ with regard to food consumption (lab), fecal dry mass (field and lab, highly correlated with food consumption in lab animals), or standard metabolic rate (lab), thus mechanisms (1) and (2) are not supported. We also provide an analysis of field activity energetics using the doubly labeled water method, and discuss these results in the context of energetic growth constraints and SSD in S. undulatus. Supported by NSF IBN0135167.