Any parent will tell you that reproduction is costly. There are rising health care expenses, child care costs for working parents, expensive sports or extracurricular activities, and, eventually, college enrollment and tuition. From an evolutionary perspective, the only relevant costs of reproduction are those that depress survivorship and as a consequence decrease the future opportunity for subsequent reproductive output (and, in fact, such costs have been found in humans).
A recent study in the pages of 'Evolution' has demonstrated a very high toll of reproduction, indeed. By stymieing reproduction in female Brown Anoles (Anolis sagrei, pictured right) through surgical removal of the ovaries, Bob Cox and Ryan Calsbeek at Dartmouth University have found that female interannual survival increases nearly threefold (relative to females manipulated only with a control "sham" surgery; solid bars, right). In addition to the survival advantage of non-reproduction, ovariectomized females also exhibited higher growth than control females.
Although the result is consistent with abundant life-history theory predicting a trade-off between reproduction and survival, the proximate mechanism of increased growth and survival of non-reproductive adult female anoles remains unclear. In performance trials, females whose egg burden has been surgically relieved improved dramatically in both stamina and sprint speed, suggesting that ovariectomized females might be better equipped to avoid predatory attack. However, in results presented in this year's Society for Integrative and Compative Biology meeting (and discussed in a previous blog post), Bob found that experimental manipulation of predation regime had little effect on the survival probability of sham and ovariectomized females. Perhaps ovariectomized lizards are simply better able to allocate sparse resources to fat reserves, and thus exhibit improved survival during food scarcity. Furthermore, Cox and Calsbeek acknowledge that ovariectomy removes not only the physical burden of reproductive investment, but also the source of steroid hormones - which could also affect growth and survival in lizards.
No doubt these important questions regarding proximate causes for the relationship between reproduction and survival in female anoles will be the subject of future studies.
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