Kent N, Cristescu RH, Piza-Roca C, et al (2019) Maternal nesting behaviour in city dragons: a species with temperature-dependent sex determination. J Urban Ecol 5:1–11. doi: 10.1093/jue/juz005
Featured image: Eastern water dragon in the pool (photo by Esa; Wikimedia)
Heating up in the city
Living in the city is drastically different than living in natural environments. City habitats are full of roads, sidewalks and tall buildings. These man-made structures retain more heat than natural surfaces which leads to warmer temperatures inside of cities. For many plants and animals, small changes in temperature can be very challenging. Some species many not be able to live in those warmer habitats. The urban heat island effect is when cities habitats are warmer than their surrounding natural habitats.
This nest is too hot, this one is too cold, and this one is just right
Sex isn’t always determined by the chromosomes (like XY (male) and XX (female) in humans). For many animals, sex can be determined by environmental factors such as temperature. One of the most well-known examples of temperature dependent sex determination are sea turtles, which experience a stronger bias towards more females spawning from nests due to increased temperatures. Similarly, the warmer temperatures of cities could affect sex ratios of reptiles that rely on nest incubation temperatures for sex determination. Nicola Kent and colleagues set out to explore this question in Australia by examining populations of water dragons.
The game of eggs: a story of hot and cold nests
Eastern water dragons are a medium bodied lizard native to Australia (Fig 2). They occur in great abundances in urban areas (Fig 2) and the sex of their offspring is determined by the temperature of their nesting sites. The temperature in Parkland has risen 1 degree Celsius due to heat island effects. This temperature increase could lead to a higher frequency of eggs developing as females. In fact, if nest temperatures rose by 2 degrees Celsius, it could mean that no males would develop.
Is there hope for the mother of dragons?
Mothers can mitigate the effects of rising temperatures by carefully choosing where to lay their eggs. For example, Nicola Kent and colleagues found that dragons in Parkland lay their eggs deeper into the soil. By laying their eggs in deeper burrows, dragons can decrease the effect of the warmer soil (Fig 3). Nicola Kent and colleagues had expected to find that dragons would seek areas of greater forest cover to lay their eggs. Instead, they found that most eggs were laid in areas with less forest cover. The authors believe that digging deeper nesting sites more effectively regulated rising temperatures while decreasing the energy costs of traveling further to find areas with more forest cover.
Figure 3: Depth at which dragons lay their eggs at the urban site in Parkland versus other nearby natural sites. Eastern dragons at Parkland lay their eggs deeper to mitigate the effects of higher temperatures in the city.
Flexibility and survival
Balanced sex ratios are extremely important for maintaining a healthy population. Higher temperatures within cities could lead to water dragons having fewer, or in some extreme cases, no males. The authors of this study demonstrated how flexibility in behavior helps the dragons to mitigate the problem of rising temperatures and allows the species to persist in an otherwise unfavorable habitat. Other work has also shown that behavioral flexibility is an important component in adjusting to the rapidly changing urban environments. However, we should note that many species are unable to adapt, and thus we must continue to monitor urban sprawl and think of ways to mitigate the phenomena of urban heat islands.