It is well understood that a distinct relationship exists between temperature and most biological processes. This is especially evident in ectothermic (sometimes called cold-blooded) animals, whose body temperature closely tracks the temperature of their environment. Interesting trends have come from laboratory studies where temperature can be held constant and controlled tightly. For example, elevating the temperature of a lizard's nest can speed up embryonic development, but whether this is ecologically relevant is not known.

In the wild, temperature can fluctuate drastically, which may require other mechanisms to be employed for staying warm or keeping cool and may increase or decrease the cost of certain biological processes. Based on past observations of eastern fence lizards whose nest temperatures were found to fluctuate by 20°C on a sunny day, Christopher Oufiero and Michael Angilletta from Indiana State University set out to model the effects of temperature fluctuations on the energetic cost of development in this lizard species.

They performed their research in the laboratory where they hormonally induced female lizards to synchronously lay eggs so that the all of the embryos were at the same stage as the duo tracked their development. Then Oufiero and Angilletta exposed the newly laid eggs to either a warm or a cool temperature cycling regime that was meant to mimic the lizards' natural habitat. As the eggs developed, the team measured each egg's oxygen consumption rate at various times to track their metabolic rate. Finally, the pair used mathematical models based on the metabolic rate measurements that determined the energetic cost of development for each embryo.

The team found that embryos that developed in warm conditions developed faster while maintaining high metabolic rates. Conversely, the embryos that developed in cool conditions developed slowly while maintaining low metabolic rates. However, upon hatching all of the lizards had developed to the same extent, regardless of their thermal history, and had expended the same amount of energy to get to that point. The team also found that the average metabolic rates for eastern fence lizards exposed to cycling thermal regimes were similar to those of embryos of a similar species held at a constant temperature.

Collectively, Oufiero and Angilletta's findings suggest that while temperature can clearly speed up or slow down the process, the same amount of energy input is required regardless. Understanding the effects of thermal cycling on the timing of development is ecologically relevant. Predictions can be made as to how human-related changes in environmental conditions may affect population growth or distribution of organisms. Additionally, Oufiero and Angilletta's findings provide a basis from which questions significant to evolutionary processes can now be addressed. Isolating the energetic cost of development to the embryo allows researchers to investigate the role of a parent or another outside source in the process. For example, in species where a parent creates and/or guards a nest, this additional invested energy can further speed up or slow down the hatching process. Ultimately, however, these lizards are not taking any short cuts. They are getting out of it exactly what they put in!

C. E.
M. J.
Energetics of lizard embryos at fluctuating temperatures
Physiol. Biochem. Zool.