Like a Moth to Flame

How humans have accidentally hacked the light-loving instincts of animals (and how animal behavior can save them).

Arguably one of humans’ most conspicuous behavioral traits is our unparalleled predilection for engineering the habitats we settle upon. As we shape the world around us, some species may be drawn towards our settlements – into what they perceive to be good habitat – only to find an environment that is unsuitable to their survival or reproduction. This increasingly prevalent phenomenon is known to ecologists as an ecological trap.

Originally described in 1972, an ecological trap occurs when the cues animals use to assess habitat quality no longer serve as reliable indicators of a habitat’s true quality (Dwernychuck and Boag 1972). An ecological trap is typically “set” by altering some aspect of the environment to cause the cue-quality mismatch, either by changing the cue or by changing the habitat quality itself. A trap can also be set by introducing a novel cue similar enough to one in the natural environment that an animal’s sensory system is unable to distinguish between the new, artificial cue and the true, natural cue.

Artificial sources of light happen to be one of our concrete jungle’s most pernicious traps. For example, many species of aquatic insects have been found to lay their eggs on shiny surfaces like asphalt, glass, gravestones, and solar panels. Attraction to human-generated light sources is not only for the spineless – perhaps surprisingly, even vertebrates such as sea turtles navigate towards artificial sources of light. Hatchling sea turtles hapless enough to be born on a beach also colonized by humans will often misorient toward busy, dangerous boardwalks because they are attracted to the lights, completely botching their prime directive to get themselves to the ocean before a predator nabs them from the shore.

What makes these animals particularly susceptible to making these suboptimal choices? Is there anything we can do to disarm the traps we set, or make them less harmful?

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Image A shows a mass of mayflies around a street lamp, while B shows female mayflies landing on asphalt to lay eggs. Images from Szaz et al. (2015).

We can do SCIENCE (the Tinbergen way).

Enter Nikolaas Tinbergen, who in 1963, published his monumental work “On Aims and Methods in Ethology.” He argued that for any behavior to be understood in its entirety, it needed to be examined from four levels of analysis. Tinbergen’s framework elegantly organized the study of animal behavior by proposing we ask how things work (the “proximate” view) and why things work (the “ultimate” view). Proximate questions study the sensory-motor mechanisms underlying behavior (how stimuli are received and the physiological changes that occur) as well as how development (such as the brain and body) affects behavior. Ultimate questions study phylogeny (how evolutionary history shapes behavior) and function (what advantages the behavior confer to the animal’s survival and reproduction).

How can we apply this to phototaxis traps?

Surely there is more than meets the eye, especially when it comes to navigation. The act of moving towards a source of light involves all four of Tinbergen’s questions, and understanding all of them is important if we want to hack the ecological traps that involve phototaxis as a cue and minimize their impacts. We must consider the following:

How do the eyes and brain receive and process light, and which components of light are responsible for behaviors?

This sensory-motor approach investigates the components of the signal and how animals perceive them. Many taxa of aquatic insects, such as mayflies and caddisflies, lay eggs on solar panels because they are attracted to the horizontally polarized light reflected from these surfaces, which resemble the light reflections off of bodies of water that they prefer. Many species of sea turtle are preferentially receptive to short, ultraviolet wavelengths of light (Young et al 2012). Knowledge of how organisms respond to specific components of a cue is valuable information to understand the root cause of a problem. If we can engineer our artificial sources of light to be as uninvasive as possible to animals–say, filtering boardwalk lights to the yellow-red spectrum, or using opaque curtains to reduce the intensity of light coming from human structures– we may reduce the number of sea turtles who misorient toward the tiki bar instead of the sandbar.

How, and when, do juveniles and adults develop the structures needed for light perception?

This developmental approach helps us understand how do organisms grow the structures needed for a behavior, and can explain when they may be vulnerable to using cues incorrectly. For example, sea turtles do not have fully developed eyes at the time of hatching, and rely on very basic cues (overall intensity and wavelength of light) rather than sophisticated optical processing to navigate correctly to the ocean. This has helped explain why hatchlings can’t perceive the source of light as inappropriate to move towards.

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Upon hatching, turtles use light cues to navigate towards the ocean. Image from http://www.seaturtleop.com/

How ancient a behavior is a positive phototaxis? How do related species differ in their use of phototaxis to make life decisions?

Orientation toward light is one of the most phylogenetically conserved behaviors of all eukaryotes, and its adaptive use as a cue to navigate toward safety and reproductive opportunities appears to be retained across a wide range of taxa, from insects to vertebrates. The probability that a species will be “trapped” by artificial light sources is likely dependent on the evolution of other optical structures that increase the amount of information available to the animal, as well as the evolution of neural machinery to recognize, differentiate, and respond to that additional information. Because we know a lot about the proximate drivers of phototaxis, taking a comparative approach and contrasting the optical structures, development, and cognition of related taxa may shed light on a species’ vulnerability to light traps. Even closely related species could be different enough to respond differently to potential traps– so the phylogenetic approach also provides a word of caution not to generalize to entire groups based on the response of a single species.

Why does the use of light to navigate help animals survive?

The last, “functional” question of Tinbergen aims to explain the fitness benefits animals gain by behaving in a specific way. Aquatic insects gather around bodies of water to mate and lay their eggs, so honing in on a cue (in their case, polarized light) as an indicator for water has, over evolutionary time, been an extremely reliable predictor to help animals navigate to the necessary sites. For hatchling sea turtles, positive phototaxis – attraction to light – may have evolved as a means to navigate as quickly as possible from their nest site to the ocean and away from terrestrial predators. Early in their evolutionary history, hatchlings that followed the “follow the light” rule had higher survival and reproductive success. The fitness consequences of this adapted “rule” in a new, manipulated landscape can vary in their severity, depending on the type of ecological trap and what is at risk for the animal. For the mayflies or dragonflies laying their eggs on concrete, the consequences are severe– they blow their only chance of passing on their genes. Knowing the severity of a trap’s fitness consequences on “trapped” animals is also important in predicting whether it will result in problematic population-level declines.

Has Tinbergen saved the day?

Tinbergen’s framework allows researchers and management groups to address ecological traps at their various ecological components. By understanding that horizontally polarized light is a strong cue for navigation and that insects perceive it as a body of water in which they can lay eggs, researchers are modifying the design of solar panels to decrease their attractiveness to insects. Partitioning solar cells with white borders helped reduce mayfly attraction by altering their perception of the solar panel as a suitable ovipositing site to, instead, a seemingly fragmented habitat (Horvath et al. 2015). Similarly, by knowing how turtle hatchlings navigate, we can attempt to mitigate many of the man-made causes for maladaptive behavior when they emerge. Conservation efforts can address light sources by altering their intensity, distributions, and wavelengths that are known to influence turtle navigation behavior.

Author Mary Clapp is a 5th year Ph.D. candidate in the Ecology Graduate Group at UC Davis, and is interested in ecological responses to human-caused changes to the environment.

Author Tinh Ton studies the chemical mechanisms mediating predation and larval settlement, linking those demographic processes to community structure in tide pools. He is currently investigating how ocean acidification alters the roles of chemical cues in order to predict the consequences on biodiversity.

Works Cited

Dwernychuk, L. W., and D. A. Boag. 1972. Ducks nesting in association with gulls–an ecological trap? Canadian Journal of Zoology 50:559-563.

Szaz, D., Horvath, S., A. Barta, B.A. Robertson, A. Farkas, A. Egri, N. Tarjanyi, G. Racz, and G. Krista. 2015. Lamp-lit bridges as dual-traps for the night-swarming Mayfly, Ephoron virgo: interactions of polarized and unpolarized light pollution. PLOS One 10(3).

Tinbergen, N. 1963. On aims and methods of ethology. Journal of Animal Psychology, 20: 410-433. Doi: 10.1111 / j.1439-0310.1963.tb01161.x.

Young, M., M. Salmon, and R. Forward. 2012. Visual wavelength discrimination by the Loggerhead Turtle, Caretta caretta. The Biological Bulletin 222(1): 46-55.

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