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

Posted on June 30, 2014 by Tara Garnett

It’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.

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

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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The Science of Snakeskin: Black Velvety Viper Scales May be Self-Cleaning

Posted on March 8, 2014 by Michelle Dohm

West African Gaboon viper

Whether you love them or hate them, snakes have long captivated our interest and imagination. They’ve spurred countless stories and fears, some of which may have even affected the course of human evolutionary history. We must admit, there is something a little other-worldly about their legless bodies, willingness to swallow and digest animals much bigger than them, and fangs and potentially fatal (or therapeutic?) venomous bites.

Not least of all, their scaly skin is quite mesmerizing and often laden with intricate and beautifully geometric patterns just perfect for camouflaging, regardless of whether they live high up in a tree, deep in murky waters, or on the forest floor. Snakeskin was the focus of recent research by the authors of this PLOS ONE study who sought to determine whether it has any special properties less obvious to the naked eye.

Please meet the West African Gaboon viper, Bitis gabonica rhinoceros (pictured above). Native to the rainforests and woodlands of West Africa, these large, white-brown-and-black snakes can be identified by large nasal horns and a single black triangle beneath each eye—nevermind that, because they also lay claim to titles for the longest fangs and most venom volume produced per bite. The pattern of their skin is intricate and excellent for camouflage, and the black sections have a particularly velvety appearance. These eye-catching characteristics intrigued zoology and biomechanics researchers from Germany, who decided to take a closer look.

In a previously published paper, the authors analyzed the Gaboon viper’s skin surface texture by using scanning electron microscopy (SEM), as well as its optical abilities by shining light on the snakeskin in different ways to see how it’s reflected, scattered, or transmitted. They found that only the black sections contained leaf-like microstructures streaked with what they call “nanoridges” on the snake scales, a pattern that has not been observed before on snakeskin. What’s more, the black skin reflects less than 11% of light shone on it—a lot less than other snakes—regardless of the angle of light applied. The authors concluded from the previous study that both of these factors may contribute to the viper’s velvet-like, ultra-black skin appearance.

Scanning electron microscopy (SEM) of viper scales

In their most recent PLOS ONE paper titled “Non-Contaminating Camouflage: Multifunctional Skin Microornamentation in the West African Gaboon Viper (Bitis rhinoceros),” the authors conducted wettability and contamination tests in hopes of further characterizing the viper skin’s properties, particularly when comparing the pale and black regions.

To test the wettability of the viper scales, the authors sprayed droplets of water, an iodide-containing compound (diiodomethane), and ethylene glycol on the different scale types shown above, on both a live and dead snake, and then measured the contact angle—the angle at which a liquid droplet meets a solid surface. This angle lets us know how water-friendly a surface is; in other words, the higher the contact angle, the less water-friendly the surface.

Contact angle (A) and snake skin with water droplet on light and dark areas (B)

As you can see in the graph above, the contact angle was different depending on the liquid applied and the type of scale; in particular, the contact angle on the black scales was significantly higher than the others, in a category that the authors refer to as “outstanding superhydrophobicity,” or really, really, really water-repelling. This type of water-repelling has been seen in geckos, but not snakes.

Water droplet appearance on live snake skin

The authors then took some of the snake carcass and dusted it with a sticky powder in a contamination chamber, after which they generated a fog for 30 minutes and took pictures.

Skin before dusting (A), skin under black light after dusting (B), skin under black light after fogging (C), section of SEM, showing light and dark skin (D)

After 30 minutes of fogging, the black areas were mostly free of the dusting powder, while the pale areas were still completely covered with dust. The powder itself was also water-repelling, and so the authors showed that despite this, the powder rolled off with the water rather than sticking to the black areas of snake skin. Therefore, as suggested by the authors, this could be a rather remarkable self-cleaning ability. The authors suspect that the “nanoridges,” or ridges arranged in parallel in the black regions, may allow liquid runoff better than on the paler areas of the snake.

How does this texture variation help the snake, you ask? The authors posit that all these properties basically contribute to a better form of camouflage. If the snake were completely covered in one color, it may stand out against a background of mixed colors (or “disruptive coloration”), like that of a forest floor. If the black regions have fairly different properties from the paler regions, mud, water, or other substances would rub off in these areas and continue to provide the light-dark color contrast and variation in light reflectivity that helps the snake do what it does best: slither around and blend in unnoticed.

Citations

Spinner M, Kovalev A, Gorb SN, Westhoff G (2013) Snake velvet black: Hierarchical micro- and nanostructure enhances dark colouration in Bitis rhinoceros. Scientific Reports 3: 1846. doi:10.1038/srep01846

Spinner M, Gorb SN, Balmert A, Bleckmann H, Westhoff G (2014) Non-Contaminating Camouflage: Multifunctional Skin Microornamentation in the West African Gaboon Viper (Bitis rhinoceros). PLoS ONE 9(3): e91087. doi:10.1371/journal.pone.0091087

Images

First image, public domain with credit to TimVickers

Remaining images from the PLOS ONE paper

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Ant-Mimicking Spider Relies on a “Double-Deception” Strategy to Fool Different Audiences

Posted on November 16, 2013 by Alex Theg

From snakes that look like they have two heads to color-shifting chameleons, deception is at the heart of many animals’ survival strategies. Both visual and chemical predator deterrence are well-documented phenomena in the animal world, but new research on ant-mimicking spiders, published in PLOS ONE, may be the first documented case of a species that uses visual deception to elude one group of predators, and chemical deception to escape another.

Ant mimicry, or myrmecomorphy, is a tactic used by numerous spider species, and with good reason, since many predators steer clear of preying on ants due to their aggressive tendencies and often unpleasant taste. Ant-mimicking spiders can have body shapes that closely resemble those of ants, as well as colored patches that look like ant eyes. Combine these characteristics with behaviors such as waving their front legs in the air to resemble probing ant antennae, and these spiders can successfully convince predators to look elsewhere for their next meal. The jumping spider Peckhamia picata is one such ant mimic whose visual signals are an effective deterrent for visually focused predators, such as other species of jumping spiders. The picture below shows a jumping spider on the left and the ant it imitates on the right.

The PLOS ONE study shows that the ant-mimicking spider can also elude predators that rely heavily on chemical signals to identify their prey. In the current study, the spiders successfully eluded spider-hunting mud-dauber wasps (pictured below), and received significantly less aggression from the ants they mimic than other non-mimicking jumping spiders. The researchers presented wasps with a choice between freshly killed ant-mimicking and non-mimicking spiders. In all of the trials conducted, the wasp probed both types of spiders with their antennae, but every time the wasps chose to sting and capture a spider (seven out of eight times), it chose the non-mimicking spider.The researchers also staged encounters between Camponotus ants and live ant-mimicking and non-mimicking spiders. After probing them with their antennae, the ants were significantly less likely to bite the ant-mimicking spiders than non-mimicking ones. These results demonstrate that the jumping spider has a remarkably effective ability to deceive potential predators who focus on chemical cues when selecting prey.

The researchers point out that the spider is not a chemical mimic of the ant species it emulates. Insects rely heavily on hydrocarbons secreted from their cuticles (the hard outer covering of invertebrates) to identify and signal one another. It turns out that ant-mimicking spiders have very low levels of these molecules, only a small fraction of the amount found in non-mimicking spiders and the ants themselves. While further research is required to fully explain the jumping spider’s chemical mechanism for predator evasion, a likely explanation is that the low level of these chemicals does not register as significant to a probing ant or wasp, and the chemical evasion is accomplished in this way.

This study may be the first to describe an animal using a “double-deception” strategy: visual tricks and a deceptive chemical signature, both intended for different audiences. The authors hypothesize that this kind of chemical deception is likely widespread among other visual mimics in the animal kingdom.

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Category Archives: camouflage

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

Ant-Mimicking Spider Relies on a “Double-Deception” Strategy to Fool Different Audiences