A test of the reproductive cost hypothesis for sexual size dimorphism in Yarrow's spiny lizard Sceloporus jarrovii
Article first published online: 10 OCT 2006
Journal of Animal Ecology
Volume 75, Issue 6, pages 1361–1369, November 2006
How to Cite
COX, R. M. (2006), A test of the reproductive cost hypothesis for sexual size dimorphism in Yarrow's spiny lizard Sceloporus jarrovii. Journal of Animal Ecology, 75: 1361–1369. doi: 10.1111/j.1365-2656.2006.01160.x
- Issue published online: 10 OCT 2006
- Article first published online: 10 OCT 2006
- Received 3 April 2006; accepted 24 July 2006
- body size;
- cost of reproduction;
- growth rate;
- 1Trade-offs between reproduction and growth are central assumptions of life-history theory, but their implications for sexual size dimorphism (SSD) are poorly understood.
- 2Adult male Yarrow's spiny lizards Sceloporus jarrovii average 10% larger than adult females. In a low-altitude (1700 m) population, this SSD develops because males grow more quickly than females during the first year of life, particularly during the first female reproductive season. This study tests the hypothesis that SSD develops because female growth is constrained by energetic costs of reproduction.
- 3To test for a growth cost of reproduction, I compared growth rates of free-living females that differed, either naturally or experimentally, in reproductive status. Females that naturally delayed reproduction until their second year grew more quickly than females that reproduced as yearlings, and ovariectomized yearlings grew more quickly and to larger sizes than reproductive controls.
- 4To determine whether SSD develops in the absence of this inferred reproductive cost, I also studied a high-altitude (2500 m) population in which all females delay reproduction until their second year. Sex differences in growth trajectories were similar to those observed at low altitude, such that males averaged 10% larger than females even prior to female reproduction.
- 5Although female growth may be constrained by reproduction, multiple lines of evidence indicate that this cost is insufficient to explain the full magnitude of SSD in S. jarrovii. First, differences in growth of reproductive and nonreproductive females are not observed until the final month of gestation, by which time SSD is already well developed. Second, the growth benefit accruing from experimental inhibition of reproduction accounts for only 32% of the natural sex difference in body size. Finally, SSD develops well in advance of female reproduction in a high-altitude population with delayed maturation.