Trade-offs in Gecko Design

Sounds glam, right? Gecko design?

At the 2012 SICB in Charleston, Skye presented research that shows how traits that improve bite force in geckos have negative impacts on the gecko's sprint speed. Meaning that males who are better fighters might also be less adept at escaping predators ...

Costly design indeed.

Let's learn more by having a look at Skye's abstract, with {comments in brackets from me}.

Trade-offs and compensatory traits: bite force and sprint speed pose conflicting demands on the design of male geckos (Hemidactylus frenatus)
by Skye Cameron, Melissa Wynn and Robbie Wilson

The evolution of exaggerated ornaments and armaments is driven by the benefits accrued to reproductive success and by the costs imposed on viability. {This means that} when traits are required to perform multiple functions that are important to both reproduction and viability, trade-offs can result in a compromised phenotype.

{Imagine, for example, a species of bird in which females are more likely to mate with males that have larger tails; but males with larger tails are more likely to be captured by predators. Both reproductive potential and survival are important to the male - so evolutionarily, the bird may end up compromising on tail length to make sure he both reproduces and survives.}


{Intuitively, we expect that exaggerated male traits (like super-long tails) would decrease locomotor capacity, resulting in lower survival rates due to predation.} Despite only mixed empirical support for such locomotor costs, recent studies suggest these costs may be masked as a result of the evolution of compensatory mechanisms that offset any detrimental effects.

{What are compensatory mechanisms? Imagine if that bird with the long tail-feathers developed longer wings, that enhanced its flying abilities. It might offset some of the survival costs of the long tail.}

In this study, {Skye} provides a comprehensive assessment of the importance of potential locomotor costs that are associated with improved male-male competitive ability by simultaneously testing for locomotor trade-offs and compensatory mechanisms. For males of the Asian house gecko (Hemidactylus frenatus), both fighting capacity and escape performance are likely to place conflicting demands on an individual’s phenotype.

Males that are highly territorial and aggressive are more likely to require greater investment in jaw size/strength in order to compete with rival males; {Skye} found that males with larger heads had stronger bites and showed greater prey-capture and fighting capacity. This performance trade-off was amplified for male geckoes with larger heads when {they were} sprinting up inclines.

{So, what does this mean? Geckoes with large heads are better at fighting and better at capturing prey, but may be worse at evading predators themselves. A compensatory mechanism would be something - like longer legs - that would enhance their ability to avoid predation.} {However, Skye} found little evidence for compensatory mechanisms that off-set the functional trade-off between bite force and sprint speed.

Ongoing work in this area includes testing the survival of male geckoes with different sized heads in controlled-but-natural conditions.

Run Gecko Run (Don't Ever Look Back)

How do scientists study running performance? Well, with humans we'd take subjects out onto a track and measure how fast they could sprint between two points.

With geckoes?

It's pretty much the same. Except we have
to design a track that encourages the gecko to move as quickly as it can in a straight line. (Because geckoes aren't as good at taking verbal directions as humans are ... )

Skye's current experiment is based around this running track. She puts a gecko into the near end (as shown in the top photo) and then chases it down the track with a foam paddle. Don't worry, the gecko's fine - the paddle is just to prevent it from stopping or turning around or anything that might disrupt measurements.

As the gecko runs down the track (away from us, in this picture), it passes the 4 light sensors that help Skye to accurately record the gecko's speed. Skye repeats the run 3 times more, so that she can get the gecko's fastest performance (best of 12 time splits - 3 trials x 4 sensors).

After measuring running performance, Skye takes the weight of the gecko to account for its body size in her calculations. This is because larger geckoes will be able to run faster independently of temperature.

Then, the gecko returns to its home in the lab.

Each gecko completes 8 running trials at temperatures between 15 and 38 degrees Celcius. Some populations won't ever have seen some of these temperature extremes in the wild (and others will have). 

Skye is predicting that where a gecko comes from will affect how it performs at these different temperatures. For example, at low temperatures geckoes from tropical North Queensland should perform more poorly than geckoes from Brisbane - because, in nature, Brisbane geckoes experience low temperatures every year (and tropical geckoes don't). 

Skye's still working hard measuring gecko running speeds ... so we can't tell you any results yet. But watch this space!

Why Temperature Matters to Geckoes

At the moment, Skye's doing a really cool experiment looking at how Asian house geckoes from different environments handle changes in temperature.

Why is this important? Well,
climates are changing. And scientists want to know how species will handle climate change - will they go extinct? will they do even better?

To answer these questions, we have to know more about how species perform across their natural range. Because animals that are already living close to their thermal limits might be at greater risk if things heat up, or even cool down. And because some animals have a greater capacity to rapidly adjust to temperature change (or acclimate) than others. And because most of the animals on the planet are what we call ectotherms, which (unlike humans) can't warm themselves up internally: their digestion, brainpower, muscle activity - everything - is dependent on the temperature in their environment.

So what's Skye doing? She's collected Asian house geckoes from across their latitudinal range in Australia - from Brisbane all the way up to Cape York. (ahem - more traveling??). Geckoes from all these different populations have grown up in quite different environments - but in general, temperatures get hotter and less variable as you head north from Brisbane.

Which leads us to some fundamental questions in thermal ecology: do animals in these different environments become 'experts' at performing under just those conditions? Or can they perform over a wide range of temperatures - just not very well? Is the pattern changeable (suggesting acclimation) or unchangeable (suggesting adaptation)?

Skye is testing these questions by looking at the running performance of geckoes from different populations  - across a range of temperatures. If you've ever watched a gecko on your window at night, you'll see why running is so important to them - it's how they catch prey, but also how they escape from predators and is a key factor in determining dominance of individuals.

So looking at running performance is a great way to assess how temperature affects the geckoes' ability to survive and make babies.

This is just a teaser, really. Letting you know why *some* people spend hours tending to and running geckoes in all sorts of temperatures. In the next post, I'll talk a bit more about the specifics of Skye's study - including her amazing experimental set-up!