COX, R. M.*; JOHN-ALDER, H. B.; Rutgers University; Rutgers University: Does Female Reproductive Investment Constrain Growth and Promote Male-Larger Sexual Size Dimorphism in Yarrow�s Spiny Lizard, Sceloporus jarrovii?

Energetic trade-offs between reproduction and growth are central assumptions of life-history theory, but the implications of such trade-offs for sexual size dimorphism (SSD) are largely unexplored. We addressed this issue in Yarrow�s Spiny Lizard, Sceloporus jarrovii, in which adult males average 10% larger than females. We compared growth and SSD between two elevations at which females either (1) reproduce as yearlings (low elevation, 1700 m), or (2) delay reproduction until their second year (high elevation, 2500 m). Low-elevation males grew faster than females at most ages, but this growth difference was most pronounced during the first reproductive season and subsequent gestation period. SSD developed rapidly over this period, suggesting that female reproductive investment promotes male-larger SSD. However, sexual differences in growth were similar in our high-elevation population, where SSD was fully expressed prior to reproductive maturity. We also experimentally tested for a trade-off between reproduction and growth by ovariectomizing (OVX) low-elevation females to prevent reproductive investment. Non-reproductive OVX females grew significantly less than reproductive controls during the fall breeding season and winter gestation period, but significantly more than controls during the month prior to parturition. Thus, despite an apparent trade-off during the final month of gestation, treatment groups did not differ in size at parturition. Together, our descriptive and experimental results suggest that this inferred energetic cost of female reproductive investment cannot explain the pattern of male-larger SSD observed in this species. Supported by NSF IBN-0135167.