28 December 2009

The secret to long life? Don’t have kids (if you're a lizard)

ResearchBlogging.orgIt is a sad but true fact that you cannot have it all. This has been known in evolution for a long time, where people often talk about trade-offs.

In a new paper, Cox and Calsbeek test the trade-offs between survival and reproduction experimentally, using female anoles.

Female anoles (Anolis sagrei; pictured) lay one egg at a time, though they do so throughout the breeding season. Cox and Calsbeek captured almost 400 animals, and performed sterilizing surgery on half the females, and sham surgery (incision but no other manipulations) Then, they let the animals return to the wild and tried to recapture as many as they could later. The statistics showed a significant difference in recapture rates a year later. The sterilized animals were more likely to survive the breeding season. Even after the season ended, the females were about twice as likely to make it over winter, which is surprising because the females aren't egg-laying then.

The females put on about twice as much mass over a breeding season. Admittedly, it’s 0.3 of a gram versus 0.15 of a gram in mass growth. This does not sound like a lot, but these are small animals, and the difference is statistically significant.

What accounts for these differences? There could be many, but one element is that eggs are heavy. Cox and Calsbeek also did some other surgical experiments where they didn't sterilize females, but simply removed eggs they had, and compared their locomotor performance to those with sham surgery. Again, those with no eggs ran faster and longer, which suggests an element is that having eggs makes it harder to escape from predators.

The authors' discussion is very good about pointing out the shortcomings of this experiment. For one, sterilization removes the ovaries, which produce a lot of hormones. What effect might hormonal changes alone have on survival and the like? Not clear. It's also not clear if the differences in running speed is really related to predation.


Cox, R., & Calsbeek, R. (2010). Severe costs of reproduction persist in lizards despite the evolution of a single-egg clutch Evolution DOI: 10.1111/j.1558-5646.2009.00906.x

Photo by Ezra S F on Flickr, used under a Creative Commons license.

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