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Dr. Nicola Mitchell answers your questions

September 2013

Nicki is an integrative biologist with a research profile across several fields in physiological ecology and conservation biology. Her major research theme is reproduction and climate where she examines the mechanisms by which the traits of offspring (including sex ratios in reptiles) are modified by the developmental environment, asking whether behavioural or physiological plasticity could be an adaptive response to environmental change.

Qs. 1 Judith asked,"I recently came across an article that mentioned that urban areas no longer provide a suitable habitat for skinks considering the effects of climate change by increased temperature levels. It recommended that they should start migrating back to the natural habitat to get used to the conditions there. How true is this?"

Nicki answered,"The story you are referring to is probably based on a 2011 study that reported high egg mortality in a species of skink from Taiwan that uses cracks in concrete walls for nesting. A link to the article and a commentary on the implications can be found here:

The implication of the story was that global warming has caused an increase in the temperatures of concrete walls, and these structures have shifted from being optimal environments for producing healthy young lizards, to suboptimal environments.

Female lizards are known to be highly selective when choosing an egg deposition site, and females that choose cooler sites (e.g. in more shade) may have greater reproductive success if air temperatures continue to warm. If female nesting preferences have a genetic basis (which appears to be true of several reptile species) then nesting behaviour can evolve fairly rapidly via natural selection, which should allow skinks to persist in urban areas.

That said, cooler nesting environments would be more typical in natural habitats that have lower thermal inertia than concrete, but I would argue that cool nesting environments could also be found in many urban areas. So I don't expect an exodus of skink species from urban areas just yet. However, one possibility not discussed in the article is the compound effect of global warming and the replacement of transpiring shade-producing vegetation with dark concrete structures that absorb heat (the 'heat island effect'). This is causing more marked warming in cities relative to non-urbanised areas, and perhaps the rate of warming will prove too rapid for some 'city skinks' to adapt? Time will tell."


Qs. 2

Is climate change is putting pressure on the population of reptiles especially when it comes determining the sex of their offspring?

Nicki answered, "Possibly... As you are obviously aware, the gender of the offspring of many reptiles is environmentally-determined rather than determined genetically. In most of these species incubation above a certain temperature threshold will produce female offspring. Hence under warmer air temperatures, incubation environments will heat up by a similar amount, favouring female-biased sex ratios in hatchlings. That said, oviparous reptiles (those that lay eggs) can select cooler nest sites by digging deeper nests for example, and even viviparous reptiles (those that give birth to live young) can thermoregulate to select cooler body temperatures and hence neutralise any feminising effect on their young. So, behavioural modifications can potentially offset any impact of climate change provided that behaviour is either flexible, or under some genetic control and therefore subject to natural selection. Moreover, moderate female biases in hatchling sex ratios are not necessarily much of a problem provided that fertile males continue to exist.

Having said all this, there are some species of reptile that produce male offspring rather than female offspring at the warmest nest temperatures. This pattern of sex determination is rare, but is best known in tuatara - an ancient lineage of reptiles represented by just a single species in New Zealand. I have done quite a bit of work on one small population of tuatara from North Brother Island, and shown that under maximum rates of climate warming the population would be at risk of demographic collapse and 'functional' extinction - meaning that only one sex (males) would remain.

Here is a link to earlier media coverage of some of this research if you'd like to read more: "