Adaptations of Aegean Wall Lizards to the Trade-Off Between Natural and Sexual Selection

Date of Publication: July 28th, 2014

Source: Behavioral Ecology

Summary: A study was conducted to investigate whether or not the coloration of Aegean wall lizards has adapted to accommodate the trade-off that exists between highly conspicuous sexual signals and detection avoidance by predators.

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There currently exists a potentially threatening trade-off between natural and sexual selection in animals. Many animals use sexual signals to attract mates and ward off competition, but all animals are in need of camouflage from predators. This study by Marshall and Stevens of the University of Cambridge's Department of Zoology examines how animals -- in particular, the Aegean wall lizard -- have adapted for this trade-off. Research has shown that conspicuous sexual signals are located on the surfaces of the animals that are in view of their potential mates and rivals (conspecifics), while out of view of predators; at the same time, camouflage is found heavily on the surfaces of animals that are more exposed to their predators' sights. This phenomenon is known as "signal partitioning". The authors of this journal article hypothesized that Aegean wall lizards exemplify signal partitioning; specifically, sexually competing males would be more conspicuous than females to both potential mates/rivals and predators, with camouflaged backs and brightly colored sides, while females would be camouflaged entirely (known as sexual dichromatism).

To test their hypotheses, Marshall and Stevens measured the conspicuousness of 83 wall lizards from three different islands as compared to their natural backgrounds using digital imaging of three regions of the lizards' bodies: ventrolateral flanks, upper backs, and lower backs (see left). They then tested for differences in the perceptions of avian predators and conspecifics of the lizards' coloration and conspicuousness. This was done by converting the pictures of the lizards that were taken on-site into images that would be seen if the camera was actually an avian predator and if the camera was a conspecific. To determine how well lizards blended into their environment/how conspicuous they were from the points of view of an avian predator and a conspecific, the researchers quantified the contrasts between the examined body parts of the photographed lizards and their background environment.

There were several notable results of the performed experiment. For one, the two parts of the lizards' bodies (the part visible to avian predators and the part visible to conspecifics) were distinct in coloration. Moreover, the coloration of the ventrolateral flanks was more distinct from the lizards' backgrounds than the coloration of the upper and lower backs, which supported the initial part of the researcher's hypothesis. In particular, Marshall and Stevens found that males were more conspicuous in coloration on the upper back and flanks, but not on the lower back, suggesting that the flanks and upper back are sexually dichromatic signals in wall lizards. In further support of their hypothesis, the researchers also found that females were inconspicuous throughout all three observed regions of the body, while males had significantly more conspicuous ventrolateral flanks than upper and lower backs.

In conclusion, Marshall and Stevens found much support for their hypothesis concerning the adaptation of Aegean wall lizards to the trade-off between natural and sexual selection. Their findings can be generalized to many animals, in which this signal partitioning adaptation exists, pinpointing one of the great phenomena of evolutionary history.

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Citation:

Marshall, K.L.A. and Stevens, M. 2014. Wall lizards display conspicuous signals to conspecifics and reduce detection by avian predators. Behavioral Ecology 25(6), 1325–1337