Boy or Girl?: A Climate-Fated Adventure In Lizards

Posted by Alex H.

Niveoscincus ocellatus (Photo credit)

Ever caught yourself wondering what it might be like if you could know the sex of your future children? Several methods allow for this to be determined with modest accuracy in the late stages of a human pregnancy, but what about early pregnancy? Before pregnancy? In other animals?

Sometimes the answers to these questions can be predicted based on the context in which they are asked. For example, I came across this article this article a few months ago, and would like to share its findings (full paper here; additional source). Essentially, the researchers found that climate, combined with genetics, was a main determinant of what sex-determining system emerged in a species of lizard.

But isn't sex in vertebrates determined by chromosomes? Not always. For instance, in many (not all) reptiles, sex is influenced by environmental factors, often temperature (i.e., temperature-dependent sex determination, or TSD). But how would a knowledge of TSD help you predict the sex your offspring? Well consider the example offered by our study in question.

In these experiments, an international cohort of herpetologists collected and observed female Spotted Skinks, Niveoscincus ocellatus from two types of habitats in Tasmania: highland and lowland. The habitats differ in altitude and therefore atmospheric oxygen, UV irradiation, moisture, and temperature, among others. Female N. ocellatus from the lowland habitat, when raised in a lab setting, varied in the ratios of male/female lizards based on the amount of sunlight they were exposed to (4 hr or 10 hr; sunlight functioning as a natural variable in temperature, UV irradiation, and light). Surprisingly, although the same species, female N. ocellatus from the highland habitat did not demonstrate this discrepency between sunlight exposure times.

What does this mean? The authors of this paper predict that in N. ocellatus, the lowland-dwelling lizards are more likely to give birth to female lizards than male lizards when temperature is higher. Lowland female lizards born earlier (as a result of warm temperatures earlier in the year) have a reproductive advantage over lowland female lizards born earlier in the year. Neither highland lizards or male lowland lizards experience a similar boost from being born earlier (resulting in more females in lowland species in warmer years). For the highland lizards, the overall temperature is colder and there is not a significant reproductive advantage to being born earlier in the year. Additionally, the paper revealed a possible mechanism for species divergence (i.e., cladogenesis) - that of environmentally-sensitive evolution of sex-determination systems.

Remember our question about predicting the sex of future offspring?  The authors also suggest that if the warming trend continues in the species' habitat, more females will be expected. Hypothetically, lowland N. ocellatus mothers can plan on more female offspring in their future (assuming it remains an evolutionary stable strategy for the species). Greeting card companies for newborns take note!

It a Gril! (via Cake Wrecks)