It is not rare in the animal kingdom for males to “detest” other males of the same species, to rival each other and even to fight until one of them retreats. Reasons usually include defense of a territory and contest for food or females. An encounter between two males may end up in a tragedy with one or both of them severely wounded. It is therefore wise to avoid violent clashes and find other ways to show strength. In such cases there is a winner and a loser in the end, but none of them is wounded.
To display their strength, animals use various behaviors or morphological characteristics which imply a trait such as size, whole-organism performance and body condition. Characteristics or behaviors that are always associated with a second attribute are called reliable signals and have an impact on the behavior of the opponents.
Studies have highlighted various features used by animals to solve conflicts such as size, physical condition and physical performance. But there is no much information about how the whole-organism performance, the size of the reliable signal and the condition combined together, affect the outcome of a fight. It is difficult to determine whether it is the indicator or its associated signal that determines the outcome of a fight between two males without first uncoupling them.
Researchers wanted to show how an experimental procedure can disconnect reliable signals from their associated characteristics, in order to find out how animals solve their disputes. Justin P. Henningsen and Duncan J. Irschick, examined the effect of a signal, the performance capacity and the body condition in domination ability of male green anole (Anolis carolinensis). The anoles extend their dewlap by the movement of a set of bones, the hyoid apparatus. They use this signal during courtship with females or to establish territory among other males or during interactions with potential predators.
The experiment was conducted on a large number of sexually mature anoles from the Savannah River area of South Carolina in USA. Several measurements and comparisons were made that revealed correlations between body size and individual’s dewlap size, bite force and mass. In order to remove the effect of body size, the anoles were divided into equal-size couples. Attention was paid to the members of each pair not to «know» each other from before. This was achieved by collecting them from areas distant from each other. Matched this way, positive correlations were found between relative size of the dewlap and relative bite force, between relative bite force and body condition and between relative size of dewlap and relative body mass.
One member of each pair underwent a surgery to have its dewlap size reduced while the other member underwent a fake surgery. The operation was aimed at reducing the size of the dewlap without completely destroying the ceratobranchial cartilages of the hyoid apparatus. In this way the anole could still extend its dewlap, but the dewlap skin could not be fully expanded. Animals which underwent fake surgeries had their cartilages probed and then left unimpaired.
After recovery from surgery, the pairs were at first allowed to contact each other visually for 10 minutes and then freely inside a common area for 1 hour. The researchers recorded the head-bobs, the dewlap display bouts, the chases, the bitings, the turnings of their bodies perpendicular to the opponent, so called lateral displays, and the retreats i.e. when an animal moves away from the opponent. The anoles whose dewlap had been reduced had no problem carrying out head-bobs, push-ups, dewlap displays, lateral displays chases, bites and retreats at the same frequency with the rest. In other words, the surgery downsizing of the dewlap didn’t influence the behavior of the animals.
The results showed that the reduction in the size of dewlap didn’t play a role in the outcome of the interaction between the anoles. The expectation of the researchers that animals with smaller dewlaps would lose more fights was not supported by the results of the experiment. On the contrary, what was important in order to win was the bite force capacity. The bigger the capacity the greater the chance of winning. Anoles with better body condition had higher biting force capacity, while the body mass of the winners didn’t vary from that of the losers.
The researchers, based on the results of their experiment as well as previous research results, assumed that the size of dewlap is not taken into consideration when anoles are close to each other, but only when they are far apart. In close encounters, the anoles perhaps pay attention to other features and behaviors such as head size, push-ups, body condition etc.
Other possible explanation of the results where not fully supported by the outcomes of the experiment. Such explanations included the fact that the anoles were obliged to fight in the lab despite any evaluation of their opponent and assumptions that the dewlap size is important only during the initial stages of an encounter.
This experiment contributed to our understanding of anoles’ body language and signaling system, but we still have a long way to go. Specifically, their dewlap remains a mystery and we hope for further researches.
Henningsen, J. P. and Irschick, D. J. (2012), An experimental test of the effect of signal size and performance capacity on dominance in the green anole lizard. Functional Ecology, 26: 3–10. doi: 10.1111/j.1365-2435.2011.01893.x