To see, or not to see

Manuel recently published a paper in the Journal of Herpetology with Leo Fleishman and Maya Prebish, examining visual acuity and perception of Anolis dewlap patterns. Yesterday, a news feature came out about the article in El Neuvo Día, sharing the findings en español. Keep scrolling below for an English translation!

Especial El Nuevo Día

It’s common to assume that animals perceive their environment in the same way we do. However, how their experiences and ‘reality’ can be markedly different— and at times, completely unexpected.

Many animals use vision as the primary sense to perceive what is happening around them. The eye, the organ responsible for vision, exhibits a high diversity of forms and complexities resulting from natural selection. In fact, the eye has evolved at least 60 times independently, providing different solutions for how to perceive changes in light, images, and movement.

In the eye, the retina contains rods (photoreceptors that produce black-and-white images at night) and cones (photoreceptors that distinguish colors during the day). The composition of the retina, such as the numbers and types of cones and rods, may differ between species, the result of adaptive responses to the light conditions of the surrounding environment.

For example, nocturnal rodents have a larger number of cones, allowing them to perceive their surroundings better while scurrying around in low light conditions. Bees, on the other hand, have cones that can see the ultraviolet reflectance of flowers – wavelengths of light that are invisible to humans – which help them locate flowers for feeding and pollination. Some species of birds, reptiles, and fish likewise have cones to see ultraviolet light.

To investigate the diversity of visual systems and how it is tied to species interactions, Puerto Rican biologist Manuel Leal, a professor at the University of Missouri in Columbia, published an article in the Journal of Herpetology, describing the vision of Anolis lizards. Co-authors of this publication include Dr. Leo Fleishman and Maya Prebish, from Union College in New York.

The researchers collected information about the size and shape of the eye, as well as the density (quantity) of rods and cones in the retina of Anolis lizards. In anoles, the retina contains four types of cones, including a cone that allows them to perceive ultraviolet light.

In addition, they took into consideration that the lizards’ eyes are oriented to the sides of the head. This eye placement allows for a wider visual field but limits the perception of distance and perspective.

Also, they recorded and analyzed images of the lizards’ dewlaps, which are fan-shaped retractable flaps of skin that extend from the throat. These structures are deployed during social activities to broadcast their presence to mates, demonstrate their reproductive health and quality, and repel rivals or predators. Dewlaps are known to have remarkable variation in color and pattern, including patterns in the ultraviolet.

All of the data was collected and entered into Acuity View, a computer program that allows for simulations of how the anoles perceive different kinds of dewlaps, according to the unique properties of their eyes.

Several photos of the three most common dewlap patterns in the Anolis were obtained and modified to maximize their distinctiveness from a lizard’s visual system. These are (1) spotted dewlaps– dots of one color on top of a background of another color), (2) two-color dewlaps, and (3) solid single-color dewlaps. Visual simulations were then created in which lizards ‘observed’ the dewlaps at varying distances, in aims of characterizing their central and peripheral vision.

Fleishman, Prebish, and Leal concluded that dewlap patterns affect how clear they can be seen at different distances. At close distances for central vision, fine details of all dewlap patterns were perceivable. However, for central vision at greater distances (or for peripheral vision at any distance), only the dominant color of the patterned dewlaps could be detected. Solid dewlaps could be seen most clearly from all distances. This has important implications for how dewlaps have evolved, because the properties of visual signals thought to be dependent on detection and perception by the intended receiver(s) of the signal.

Differences in visual perception can influence how effective the different dewlap patterns are in communicating the health and reproductive quality of males. However, all patterns broadcast sufficient information for a lizard observer to identify the species identification of the lizard who is displaying the signal.

This study shows that it is a misconception that human vision can be used to accurately represent how other animals perceive their environment, including Anolis lizards. Moreover, the researchers demonstrated the importance of considering the physical characteristics of animals’ sensory systems in order to study and better understand their behavior and communication.

Author: Wilson González-Espada, a professor in physics and science education at Morehead State University, Kentucky, and a member of Ciencia Puerto Rico.

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