Category Archives: vision

What’s in a Look? For Wolves, Maybe Everything

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WolvesIt’s been said that the eyes are the windows to the soul. They allow us to communicate feelings across a room, direct the attention of others, and express emotion better than words ever could.  The importance of eye contact in non-human species is well known—we’ve all heard that you shouldn’t stare a bear or angry dog in the eyes—but we don’t know a whole lot about how gaze is used between individuals of the same species. Japanese researchers took on this topic in a recent PLOS ONE article, focusing specifically on how eye contact and communication is affected by eye visibility and facial patterning around the eyes of canids.

Their research observed 25 canid species, comparing variations in facial pattern and coloring to observations about their social behavior and evolutionary history. They found that canines may use facial markers to either highlight or de-emphasize their eyes. Species with more distinguishable eyes tended to live and hunt in groups, where gaze-communication facilitates the teamwork that is necessary to bring down large prey and stay safe. Those with camouflaged eyes were more likely to live alone or in pairs, where communication with other members of their species may not be needed in the same way.

facial area maps

Using photos of each species, the authors analyzed the contrast between five areas of the canine face: pupil, iris, eyelid margin, coat around the eyes, and facial area including the eyes, as shown in the figure above. They measured contrast assuming red-green colorblindness of the observer (fun fact: canids cannot see the full spectrum of color). Species were then grouped according to the visibility of their eyes, described in the figure below:

  • Group A contained species with easily visible pupils and eye placement
  • Group B contained species with camouflaged pupils but clearly defined eye placement
  • Group C contained species with fully camouflaged eyes and pupils

group types

The authors found the strongest correlation between eye visibility and living and hunting behavior. More species in Group A, like gray wolves, live and hunt in packs, whereas more species in Groups B and C, like the fennec fox and bush dog, live and hunt alone or in pairs. Species in Group A also spend significantly more time in “gazing postures,” with their sight and body directed at another animal, an action that accentuates their focused attention to other members of the group. The genetic similarity between species was not as useful in explaining these differences, with A-type faces found in 8 of 10 wolf-like species, and in 3 of 10 red fox-like species. The authors suggest that A-type markings developed independently once these groups had evolutionarily split.

Lighter iris coloring is thought to be an adaptation to ultraviolet light in many species, similar to variations in human skin pigmentation. To determine whether this adaptation could explain the variation seen in canid iris color, the researchers compared the eye coloring of three wolf subspecies from Group A originating from arctic, temperate, and subtropical regions, to see if any differences in their lighter coloring could be attributed to geographical origin. They found that iris color did not vary significantly between the subspecies, suggesting that it may have developed to facilitate communication and not as an adaptation to specific geographical locations.

When the authors reviewed social behaviors, they found a number of social species with B- and C-type faces, the groups normally found alone or in pairs. These species are known to use acoustic or other visual signals, like a howl or the flash of a white tail, to communicate with their comrades. This allows them to skirt one possible disadvantage of gaze-communication: when prey can also identify and follow a gaze, and realize they’ve been targeted.

Gaze communication may be an important tool for other canids, including our own companions, domestic dogs. Previous studies have shown that domestic dogs are more likely to make direct eye contact with humans than wolves raised in the same setting. This could mean that after thousands of years of cohabitation, dogs see us in socially useful ways that wolves never will. Luckily for us, that means we get to see this.

Citation: Ueda S, Kumagai G, Otaki Y, Yamaguchi S, Kohshima S (2014) A Comparison of Facial Color Pattern and Gazing Behavior in Canid Species Suggests Gaze Communication in Gray Wolves (Canis lupus). PLoS ONE 9(6): e98217. doi:10.1371/journal.pone.0098217

Images 2 and 3: Figures 1 and 2 from the article

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Burning the Candle at Both Ends: Intertidal Ant Species Can Work Night and Day

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journal.pone.0076015.g001

If you’ve ever experienced rush hour traffic, you know firsthand that most humans base our schedules roughly around the rise and setting of the Sun, during daylight hours. However, the Australian intertidal ant, Polyrhachis sokolova, must instead schedule its busy day of foraging in the mangrove forest according to the rise and fall of the tide. Low tide can occur day or night, and to function effectively in both the brightest and darkest conditions, these ants possess several useful eye structures—not unlike the pupils in our eyes, or night vision goggles—that help them adjust to different light levels so that they can find food.

AntFaceThere are thousands of ant species that can have a variety of habitats, morphologies (shapes), and navigation methods. Australian intertidal ants use vision to identify landmarks like trees, and celestial cues like the angle of starlight to find their way. Low tide, whenever that may be, is the best time for foraging, so these ants need to see in all light levels without the assistance of flashlights or sunhats. Exactly how they manage to adapt to such a wide range of light conditions was investigated and described in a recent PLOS ONE study.

To learn more, researchers made tiny casts of intertidal ants’ eyes using fingernail polish. They flattened the casts and examined them under a microscope. Ants have compound eyes, meaning that their eyes are made of many tiny facets, or eye units, compared to simple eyes like ours that only have one eye unit each. Researchers counted the number of facets in each compound eye and measured each one’s diameter. The eyes were cast at different times—10am and 10pm—to inspect how the eye structures changed in dark versus light conditions. The light sensitivity of the eyes was calculated based on this morphological data.

journal.pone.0076015.g003Intertidal ants’ compound eyes each have around 596 facets and are similar to the eyes of other ant species specifically adapted to darker conditions. Eyes that “see” in the dark tend to have larger lenses and be extremely sensitive to light to get the most out of the little available light. This night vision adaptation would typically limit an ant’s ability to function in daylight because bright light would overload the photoreceptors in these highly sensitive structures, but the researchers found  other mechanisms that protect these ants’ eyes, restricting the amount of light that can enter—like a pupil—by making the openings that allow light to pass smaller. This mechanism helps the ants adapt their night-vision eyes to bright daylight.  This type of pupil is seen in other nocturnal ants but had not been found previously in ants that forage during the day.

Finally, to assist in navigation, the researchers found yet another structure in the ants’ eyes: special light detectors that act like skylights and help determine direction by sensing the angles of light sources in the sky. Therefore, Australian intertidal ants do not have the very best day or night vision, but they instead sacrifice some of their ability to see well in each condition in order to see “adequately” in both.

Citation: Narendra A, Alkaladi A, Raderschall CA, Robson SKA, Ribi WA (2013) Compound Eye Adaptations for Diurnal and Nocturnal Lifestyle in the Intertidal Ant, Polyrhachis sokolova. PLoS ONE 8(10): e76015. doi:10.1371/journal.pone.0076015

Image Credits: Images are from Figures 1, 2, and 3 from the manuscript.