How does current reproductive investment impact survival and future reproduction?


A central assumption of life-history theory is that investment in current reproduction reduces survival and future reproduction. But which aspects of reproduction render it costly? And is this trade-off structured by intrinsic sources of mortality, such as somatic maintenance, or by extrinsic factors, such as predation? In part, these questions remain largely unanswered due to a historical reliance on purely correlative data and breeding designs that are limited to laboratory model organisms. My collaborators and I are addressing these questions from an alternative, experimental approach that manipulates both the physiological basis of reproductive investment and the ecological context of this trade-off in wild populations.

Costs of reproduction

in females

A female brown anole, found dead with a large egg still visible in her abdomen. Reproduction can be costly.

Home       Research       Publications      Teaching       CV       Collaborators       Photos      Artwork

Related Papers


Cox, R.M., and R. Calsbeek. Severe costs of reproduction persist in Anolis lizards despite the evolution of a single-egg clutch. Evolution (in press).


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: 1361-136

Reproduction and survival. In the absence of reproductive investment, breeding-season survival (May-Sep) increases dramatically in female anoles (Fig. 2). This survival cost of reproduction is even more dramatic when measured from one year to the next. Brown anoles are effectively an annual species under natural levels of reproductive investment, with only 10% of adult females surviving to a second breeding season. However, more than 30% of females survive to a second breeding season when prevented from investing in reproduction. Thus, reproductive investment can have a substantial effect on population demography.

Costs of reproduction in anoles. As a graduate student, I developed a method of manipulating reproductive investment by removing the ovaries of female spiny lizards prior to reproduction. These initial experiments revealed such a dramatic trade-off between reproduction and growth that I extended this project to my current focal species, the brown anole. Anoles are unusual because they produce a single-egg clutch. Has this adaptation freed them from the costs of reduced locomotor performance and survival that typically accompany reproduction? Or do the cumulative energetic demands of successive clutches render reproduction costly nonetheless? Does reproduction also force trade-offs with maintenance and locomotor performance? And what can these trade-offs tell us about intrinsic versus extrinsic mortality (Fig. 1)?

Figure 1. Costs of reproduction in anoles. Green pathway indicates the hypothesized trade-off between reproduction and survival, a central assumption of life-history theory. Blue pathway indicates a hypothetical scenario by which intrinsic sources of mortality structure this trade-off. Orange pathway indicates a non-exclusive, alternative scenario by which extrinsic sources of mortality structure the trade-off. Manipulations of reproduction facilitate direct tests for downstream effects on survival, maintenance, and performance.

Figure 2. Eliminating reproductive investment via surgical ovariectomy (OVX) results in a dramatic increase in survival over the breeding season (May-Sep). The survival of females receiving a sham surgery in which the egg and ovaries are left intact (SHAM) is nearly identical to that of unmanipulated females (Natural) from a nearby reference site.

Other costs of reproduction. In addition to its effects on survival, reproductive investment also severely constrains growth (Fig. 3). This suggests that the energetic cost of reproduction may also impact energy allocation to components of somatic maintenance that may directly influence survival, such as immune function and DNA repair. The burden of even a single egg also significantly reduces stamina and sprint speed (Fig. 3). This reduction in performance may render gravid females more susceptible to predation. Our current research focuses on exploring these two pathways in greater detail to determine whether the survival cost of reproduction is driven by intrinsic (maintenance) or extrinsic (predation) sources of mortality.

Figure 3. Left panel: eliminating reproductive investment via surgical ovariectomy (OVX) results in a dramatic increase in growth over the breeding season. Right panel: surgical removal of an oviductal egg (EGGX) results in a significant increase in stamina and sprint speed. These treatment effects suggest alternative pathways that may link reproduction to survival via both intrinsic and extrinsic sources of mortality (see Figure 1).