To Blend in or Stand Out?
Body coloration of an animal can be useful for not only attracting prey, but also avoiding being eaten. One important question is whether or not this coloration can simultaneously serve both purposes? (Image Credit: Chen-Pan Liao, CC BY-SA 3.0).
Multifunctionality of an arthropod predator’s body coloration (2019) Liao et al., Functional Ecology, https://dx.doi.org/10.1111/1365-2435.13326
One topic that has interested ecologists for decades is that of animal body coloration, and what function that coloration can serve for the animal. Despite this fascination and the work that has been done to study this aspect of animal biology, the actual mechanisms driving the evolution and maintenance of body color are not well understood. Many different aspects of an organism’s life can shape and affect body color, such as avoiding predators, attracting mates, and whatever resources an organism has available to create specific colors. In addition, many of these aspects often compete with one another, such that a color that is good for attracting mates may also make you more easily-spotted by a predator.
Spiders provide an excellent system in which to study the evolutionary significance of body colors, as previous work has shown that body color affects mate attraction, predator avoidance, and prey attraction. The authors of today’s study wanted to know if these complex color patterns could serve more than one function in the spider’s life.
What They Did
The authors used both a field and lab experiment to understand what purposes, if any, the orange coloration on the front and back of Australasian coin spiders (Herennia multipuncta) serves. For the field experiment, they set up four types of “dummy” spiders with various combinations of colors on the trunks of trees. They then recorded all of the dummies to track what kind of interactions prey animals and predators had with them, and from these recordings they calculated the “prey attraction rate” and “predator attraction rate”.
For the lab experiment, individual spiders were collected and subjected to a “covering” or control treatment. Spiders in the “covered” treatment had the orange spots on their abdomen covered with black paint, as these spots are thought to startle and scare away predators. The control spiders had black paint added to the black area on their abdomen. The authors placed single lizards into the cages housing the individual spiders, and then calculated the lizard attack time, latency to attack, and the spiders push-up rate (behavior thought to scare the lizards away).
Did You Know: Chameleon Color Change
Many of us are familiar with the color-changing lizards known as chameleons, but it is actually a common misconception that these lizards change colors to blend in and avoid predators. It’s an understandable mistake to make, because it seems like they change to blend in with their environment, but they can run quite fast for their size, so any predator avoidance can be better accomplished by running away instead of staying in the same spot.
The actual reason for these color changes seem to be for social cues and temperature manipulation. Chameleons may get darker to scare off or intimidate a rival, while lighter colors could attract a mate. If a chameleon is cold, it may turn darker in order to absorb more solar energy and warm up, but turn lighter to cool down.
What They Found
For the field experiment, the authors found that the dummies with orange fronts attracted more prey animals than dummies with black or grey fronts during the day. At night, both the orange-fronted dummies and dummies with orange fronts and grey backs attracted more prey animals than the other dummies, with the orange-front grey-back dummy attracting the most prey. There were no significant differences in predator attraction rate for the different dummies, but all dummies were attacked more during the daytime than at night.
For the lab experiment, there were not any significant differences between the covered and control spiders in their push-up rate, meaning that the covering did not affect the spiders natural anti-predator behavior. Lizards stared at the control spiders longer than they did covered spiders, as well as waiting longer before they attacked the control spiders. Nonetheless, there was no difference in attack rates between the control and covered spiders.
While this is an exceptionally well-done experiment, with both field and lab components that included tests of all assumptions, the lab experiment placed single lizards in a cage with a single spider. Because the lizards were not offered a choice between control and covered spiders, it is hard to make conclusive statements about the functionality of the orange spots on the abdomen, especially since there was not a difference in attack rates between the covered and control spiders. The lack of difference may have been due to the fact that the lizard didn’t have any other option, and thus attacked and consumed whatever spider was available, even if it may not have in more natural circumstances.
Body color has long been thought to play important roles in the lives of many animals, and while that seems to be one of those “well of course it does” ideas, in science you still have to do the legwork and provide evidence before you can make those statements. This study was one of few studies to experimentally test this idea, and they showed that the same suite of colors can serve more than one function.
While these results certainly open the door for future work on body coloration and ideas about what purposes it serves, more studies with other spider species (or even other animals) need to be conducted to understand how common results like these are.