The black spiny-tailed iguana

Inconspicuous lizards and the fascinating world of sexual dimorphism

Female black spiny-tailed iguana, Manuel Antonio National Park, Costa Rica, 2024. Photo by Amaranta Chavez.

I was walking along the beach adjacent to Manuel Antonio National Park and walked right past this iguana. Only after doing a double take did I realise it was in fact a living thing and not a pile of dried leaves and sticks. That’s something I love about nature in the tropics. You could be looking right at five of the weirdest and most wonderful creatures you’ve ever seen and not even realise it until it’s pointed out to you.

I had seen a few spiny-tailed iguanas before and noticed this one had a more slender upper body. For some animals, it’s easy to tell the difference between males and females simply by observation and for others it’s physically impossible.

Species that do have distinguishable males and females are known as being sexually dimorphic. Sexual dimorphism can be seen across the entire animal kingdom. Everything from insects and herpetofauna to birds and mammals. They all contain species that exhibit clear differences between sexes in terms of morphology, physiology or behaviour.

The most stand-out examples of sexual dimorphism I can think of are lions, orange-tip butterflies, elephant seals, and pheasants; my list could go on and on for birds (notably quetzals, which if you'd like to hear more about you can check out my previous article). I’m in the mood to go down a rabbit hole of googling surprising animals so if you’d like to share with me any examples of interesting sexual dimorphism drop a comment below or let others know in my subscribers chat.

Sexual dimorphism arises due to differences in selection pressures (forces that drive natural selection by making some traits more favourable than others). These pressures could be either:

• Intrasexual, e.g. males competing with each other to mate with females. This often drives larger body size to increase chances of winning fights with other males.

• Intersexual, e.g. females choosing males with characteristics that indicate good health and fertility. This often drives extravagant traits such as bright colours and courtship behaviour. Often times though, these traits that help them win over a female also increase the possibility of predation. This is known as the handicap principle and I suppose it supports Richard Dawkins’ ‘Selfish Gene’ quite well.

  

  The stark contrast between male and female peacocks and a prime example of the handicap principle (Credit: Getty Images/iStockphoto).

• Different ecological and reproductive roles. This is an example of a natural selection pressure which can lead to e.g. females prioritising traits that enhance reproduction and survival. At the same time, males may prioritise traits that help them secure a mate.

• Predation. This is another example of natural selection pressure. We already established that males are often more extravagant and therefore more likely to be targeted by predators than camouflaged females. But is it always this straight forward?

Back to the black spiny-tailed iguana. Females are smaller, slower and less aggressive than males. Both sexes have well-developed senses of smell, sight, and hearing, but they respond to predatory sounds in different ways… sex-dependent ways. A study in 2016 found that female black spiny-tailed iguanas responded significantly more to predatory sounds compared to males. The ‘response’ was either flee or freeze.

Contrary to our previous logic of larger and more extravagant males being a more obvious target to predators, this study indicated that female iguanas may experience a greater predation risk due to the lower energy cost for the predator since females are easier to pursue, carry, kill and digest. The spiny-tailed iguana has one hell of a tail slap, but they are also the fastest lizard in the world - at full sprint, they can run up to 21.5 mph- leaving them just behind Usain Bolt at the 2012 Olympics (23.1mph). The females seem to be aware of their slim chances of survival against predators like jaguars when facing them head-on (or rather, tail-on). Hence, they are more likely to freeze or flee.

From my research, it is unclear whether male black spiny-tailed iguanas do experience a lower predation rate than females. But whether male-specific traits are their hamartia or their saving grace is determined by complex overlapping genetic and ecological factors that differ between species.

Once sexual dimorphism has been established, differing phenotypes between the sexes can result in differing levels of predation pressure, as well as different types of predators in general. So sexual dimorphism is a bit cyclical in nature. Differences driven by differences that create more differences.