Tuesday, March 9, 2010

The Price of Parenthood

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.


Bryan Kolaczkowski said...

Great work, although I expect - as the authors suggest - that manipulation of hormone levels is probably the strongest effect of removing the ovaries.

This is probably the cause of the authors' observations, rather than the relief of the 'tremendous burden of reproduction.'

Just speculation, of course :)

Glor said...

Surely someone has asked this question in humans as well. Does having children lengthen or shorten human life span?

Liam Revell said...

Rich - I put this link in my post (in which they show that child bearing shortened life in a historical human society, but that the effect was dependent on socioeconomic status); however, I'll bet you're right and that there are lots of other examples.

Glor said...

Thanks Liam, I should have read your post more carefully.

Th.Sanger said...

These physical manipulations are used for many types of studies in biology. Due to the correlated physical and hormonal changes that accompany the removal of an organ I have always found myself a little skeptical about their results.

From an experimentalists point of view, I would much rather see hormone levels either manipulated in an experimental context without surgery, or controlled for in the case of surgical removal of various bits. Louis Guillette, Richard Jones, and others did a lot of work with hormone levels in A. carolinensis in the 1970's and 1980's. It should be possible, while not necessarily easy, to devise an experiment that incorporates these factors. Perhaps a reptilian birth-control injection can be developed that prevents anoles from developing eggs! (I'm only partially kidding on that.)