Deception in animals and plants

Deception is widespread in the animal kingdom. Caterpillars may look like twigs and crabs may cover their shells with a mini-forest of algae, to fool their predators. Fake eyespots on moth wings or the rear ends of caterpillars deflect predatory attacks from real heads. A predator may simply hide itself, in ambush, conveying no apparent threat.

Sometimes the fakery involves decision-making on a more immediate and individual level. For instance, a defensive animal may puff itself up to look bigger and perhaps more dangerous than it really is. Or a chimpanzee, a titmouse, or a jay might emit a fake danger call when no predator is near, just to spook other animals away from a food source.

Deliberate deceitfulness is well-known among animals that cache food, particularly if another critter observes their caching behavior. The trickery goes beyond merely spacing the caches more widely, as chickadees do, or going behind some visual obstacle to make a cache. Gray squirrels make fake (empty) caches if they are watched by another squirrel. Ravens (and other corvids) are even more duplicitous, if they are observed by another raven: A bird with a food item caches the item but surreptitiously sneaks it out and goes away to stash it elsewhere, while the observing bird visits the now-empty cache site. And it matters just who the observer is: a stranger or known pilferer may be treated much more suspiciously than a mate or a buddy!

In the plant kingdom, a wide array of plants uses deceitful flowers to attract pollinators. A local example is the Calypso orchid, which looks and smells like food to a bee, but in fact offers no reward at all; thus it is visited mostly by naïve bees that lack experience.

There are almost thirty thousand species of orchids, mostly pollinated by bees and wasps (some by flies and other flower visitors) and researchers estimate that about a third of them use some form of floral deception to achieve pollination. Food deception, as in Calypso, is common, fooling various wasps and flies, for example. Sometimes an orchid flower looks or smells like a female wasp or bee, and males get revved up and try to copulate with the flower, accomplishing pollination in the process. Still others look like male bees, and when the flowers sway in the wind, they are attacked aggressively by other male bees. Some Old-World species emit the aroma of alarmed aphids, which attracts female aphid-eating hoverflies to a potential egg-laying site; but it’s a fake—if eggs are laid, there are no aphids for the larvae to eat. In the meantime, hoverflies can be good pollinators. And then there are orchids that resemble prey and are attacked by female wasps that are foraging food for their offspring.

Pollination by deception has evolved many times in the plant kingdom, occurring in many evolutionary lineages (but fewer in total than all the deceptive orchids). Here are some non-orchid examples. A South American species in the potato family attracts small flies with the odor of decaying carrion that smells like a place for these flies to lay their eggs, although it is not. A Middle Eastern member of the arum family emits the fragrance of dung and similarly deceives flies that are looking for a brood site. A Chinese member of the dogbane family uses a pitfall trap to capture certain little flies, using a scent that suggests a predator (such as a spider) has captured an insect. These flies are ‘kleptoparasites’, making their living by stealing prey from predators. They are drawn to the flower by the scent of the spider’s prey and fall into the trap. As they scrabble around in the trap, they pollinate the flower. In a few days, the trap collapses and allows the fly to escape and go to another plant with its load of pollen.

Fungus gnats are the typical pollinators of jack-in-the-pulpit, attracted by the smell of spongy, fungus-like tissue that seems to offer food for gnat larvae (but it’s a fake). Jack-in-the-pulpit plants bear many small flowers on a spike-like inflorescence that is surrounded by a vase of vegetative tissue (a type of pitfall trap). They usually produce male and female flowers on separate plants. Both draw in the gnats; in male flowers the gnats are temporarily trapped by tissues around the inflorescence but ultimately escape through an exit hole at the bottom of the trap. When they then visit a female flower, they carry pollen, achieve pollination, but cannot escape the trap.

That’s just a wee sample of the nefarious ways that flowering plants fool a variety of insects into pollinating the flowers. Clearly, if all the visiting insects were fooled, populations of the relevant insects would die out—they’d waste time and energy on profitless activity, or starve, or their larvae would die. So some pollinators learn to avoid the frauds (as apparently happens for Calypso) or some members of the species that can be fooled are not so foolish.

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Seeing UV

First, some basics: Vision depends on light, which comes in a spectrum of wavelengths, ranging from very long to very short. Vertebrate eyes have two kinds of light receptors in the retina at the back of the eye: Rods, which are sensitive at low light levels, and cones, which are stimulated at higher light levels and function in color vision.

Humans (and a few other mammals) have three types of cones; each type is receptive to a different range of wavelengths with peak sensitivity in the middle of the range. One type of cone deals with long wavelengths toward the red end of (what we call) the visible spectrum; other cones are sensitive to medium-long wavelengths in the middle part of the spectrum. The third type of cone is sensitive to short wavelengths, in the blue-violet end of the spectrum. Still shorter wavelengths, outside of the normal human visible spectrum, we call ultraviolet (UV). Humans and some other mammals have cones that are slightly sensitive to UV light, but the lenses filter it out.

However, lots of birds, fish, and reptiles have a fourth kind of cone that is UV-sensitive. Even a few mammals (e.g., some rodents and bats) can see UV light quite well. Furthermore, some mammals have lenses that don’t filter UV wavelengths, so they can use UV to some extent– examples include hedgehogs, dogs, cats, and ferrets, among others. Day-hunting snakes have lenses that block UV wavelengths, but night-hunting snakes have lenses that transmit UV. For these animals, just little extra light might enhance vision in some conditions.

I’d love to be able to present a survey of all the vertebrates, not only about who has UV vision, but also to find possible correlations of UV sensitivity with the ecology, behavior, and evolutionary history of the species. But such a systematic survey does not exist. Part of the problem lies in the complexity of what determines the sensitivity; several factors are involved. The animal must possess the visual receptor cells (typically cones). Those cones must also be functional; that is, they must not be turned off by genetic mutations. And the UV wavelength must actually reach the retina, not filtered out by lens, cornea, or other structures. Apparently only seldom have enough of those features been measured in enough animals allow a wide search for correlations with ecology, behavior, and evolutionary history.

There is still a further question: if an animal can see UV, how is it useful to the animal? This is often difficult to determine, and suggestions outnumber the answers. Here are a few bits and pieces:

UV sensitivity may be useful in foraging: Several studies have suggested that birds of prey that hunt small mammals may key in on trails left by the mammals as they scent-mark with reflective urine, although another study showed that vole urine is not very reflective in the UV range. It is possible that UV-sensitivity helps locate ripe fruits or insect prey because the UV reflectance of fruit and some insects differs from that of background leaves. But how often this works in the natural world is uncertain. Hummingbirds can see in the UV range. Many flowers either reflect or absorb UV, and hummers may use that ability to discriminate among flowers that they might visit and pollinate.

Among bats, a mutation causing loss of functional short-wave light sensitivity is found in nocturnal species that commonly roost in caves and echo-locate, using sonar to navigate and capture prey. Researchers suggest that perhaps using sonar pre-empts brain space otherwise used for UV perception. However, the correlation is not so clear, because the loss also occurs in fruit bats, which roost in trees and do not echolocate.

 

Decent data are more available for the use of UV reflectance and sensitivity in social situations in birds, fishes, and reptiles with good color vision. For example, male mountain bluebirds have more UV-reflective plumage than females, and males that reflect more UV are more successful in mating and siring offspring. Similarly, female sticklebacks and guppies perceive UV and prefer to associate with males that have good UV reflectance. Another study showed that lizards living in light, UV-rich habitats have social displays that convey signals in the UV range, while those in dark habitats do not.

I’ve left mention of amphibians to the end, because that story gets more complicated. It seems that many amphibians can see color in the dark. They have two kinds of rods (sensitive at low light levels) in addition to cones; some of those rods are UV sensitive. Could that be true of some other vertebrates too?

This leaves UV vision in insects and spiders and other invertebrates for another story (maybe).