Fence lizards versus fire ants: Evolutionary fail?

As many know, this is the 150^th anniversary of the publication of On the Origin of Species. If I may be so bold, one of the things that might distinguish our thinking about evolution in the last 50 years from the first hundred years might be the speed at which natural selection can operate. For a long time, we thought of evolution taking long times: millions of years would be needed to see the gradual accumulation of changes. We learned in the past few decades that we can see the effects of selection over the course of a few decades.

There are a few fast changing situations that should press the fast forward button on natural selection. Invasions are one. That’s why they’re invasions, not slow expansions. Boronow and Langkilde look at how the invasion of red fire ants are affecting fence lizards.

The ants (Solenopsis invicta) are nasty little buggers. A dozen will kill a fence lizard in less than a minute. You’d think that would apply some pretty strong selection on the lizards if they have any traits in the population that provide even a little defense against the ants.

To test whether natural selection has started acting on the fence lizards (Sceloporus undulatus), they collected lizards from two locations: one was invaded by the ants 70 years ago, and the other has not been invaded yet. Then, they allowed some angry ants to bite restrained lizards, and measured the animals’ performance on several behavioural tasks, like biting, running, and so on. A control group of lizards where handled, but not bitten. They also looked at the effect of dilute venom on the lizards’ blood directly.

The bottom line?

There’s no effect.

The lizards from the region that had been putting up with ants for seven decades had the same behavioural responses to the ants as lizards from the region with no ants. No differences in the blood responses to venom, either, though the blood was affected by venom.

The authors suggest that the ant venom might have a “tipping point.” Less than a certain dose, and the lizard is fine. More than that dose, and you’ve got a scaly corpse. The range in between “fine” and “dead” could be minuscule, in which case, there may not be a lot of variation for natural selection to work on. Thus, if the lizards can keep the bites under the critical value, they suffer no fitness consequences.

Another issue is that the fence lizards do live with other fire ants, like Solenopsis xyloni. These have weaker venom, and they’re not as numerous as the red fire ants, but it might be that the fence lizards have already been pushed to have defenses against fire ants.

A third possibility is simply that there is no existing variation that gives some members of the population greater resistance than others. Seventy years, which is about 35 generations of lizards, is quite a while, but may not be long enough. Who knows when just the right mutation will give some lucky lizard – and its offspring – a selective advantage.

Reference

Boronow, K., & Langkilde, T. (2009). Sublethal effects of invasive fire ant venom on a native lizard Journal of Experimental Zoology Part A: Ecological Genetics and Physiology, 9999A DOI: 10.1002/jez.570

Lizard picture by J.N. Stewart on Flickr, used under a Creative Commons license.

Ant picture by AJC1 on Flickr, used under a Creative Commons license.