The Decoy Effect

Dallas Miller | Assistant Editor of Natural Sciences

Over the summer, I had the opportunity to assist with a research project in Panama at the Smithsonian Tropical Research Institute (STRI), working with bats in a field station on the edge of the Soberanía National Rainforest Preserve located right along the Panama Canal. I helped Claire Hemingway, a graduate student in the Department of Ecology, Evolution and Behavior at the University of Texas, with a study that investigates the behavior of the fringe-lipped bat named Trachops full film Smurfs: The Lost Villa

Claire’s goal was to shed light on the mechanisms that bats use to evaluate and choose prey options when they are presented with various alternatives. During the course of this study, we were trying to answer the question, “do bats make rational decisions, or are they just as unpredictable as humans?” My job was to help her catch the bats and assist with feeding, transporting, and running the experiments in flight cages designed specifically for this bat research.

When I arrived in Panama at the beginning of August, it was the end of the field season, and most visiting scientists and students were returning to their universities in the U.S. or Europe. Claire had been working tirelessly all summer to complete her study, but she was still a few bats short of the sample size that she wanted. On most days, we set out in the late afternoon from Gamboa–one of the 15 stations across Panama where STRI scientists and interns reside–and ventured into the rainforest along Pipeline Road, a wide, muddy trail that was impassable without a four-wheel drive vehicle and reminiscent of something out of an Indiana Jones movie. We set up long nets of fine mesh, known as mist nets in the jungle along Pipeline Road and caught bats from around seven to ten o’clock at night. On the busiest nights, we were constantly occupied with untangling and releasing bats from the nets until we found a Trachops that Claire could use for her study. However, it often took a succession of long nights of mist-netting in the hot jungle, enduring insects swarming around our headlamps, getting bit by mosquitos, and avoiding bullet ants and snakes before having a successful experience. In addition to the difficulties of netting Trachops, unusual changes in El Niño caused this summer to be one of the worst droughts in Panama’s history. The drought affected the bats’ behavior, only making them harder to  be found. Seeing Claire stung by a scorpion that happened to be resting on a leaf that had fallen into one of the mist nets, I quickly learned that conducting bat research required a special kind of dedication and commitment, and I could only be amazed by her resilience when we were right back out in the jungle the next night. Finally, after successfully capturing a Trachops, we recorded pertinent data, inserted a PIT tag identification device under the bat’s fur, and drove back to Gamboa where we would spend the next three to five nights performing Claire’s experiment.

To give some background for Claire’s study, T. cirrhosus feed primarily upon insects such as katydids and frogs, including the male Physalaemus pustulosus, otherwise known as the Túngara frog. Male Túngara frogs produce a courting “whine-chuck” call which serves to attract female mates, but the courting call also makes it easier for the frogs to be hunted by bats. Model animals–in this case, bats–are assumed to possess absolute evaluation mechanisms, and they assign different and absolute “currencies” to their prey options, based on prey’s intrinsic properties, such as the frequency or amplitude of the frog calls. These currencies are believed to correlate with predators’ relative fitness, or survival, benefits as a result of the decision. In response to the frog calls, the currencies are expected to be the same regardless of how many various frogs are calling on a given night so that the bats could make a rational decision as to what type of frog calls they choose to approach and eat, since their preference should not be influenced by the presence of another option.

However, previous studies have shown that when hummingbirds are deciding between food options, they violate the assumptions of regularity, which are the underlying basis for rational decision-making. This indicates that comparative evaluative mechanisms may influence animal choice, similarly to humans human decision-making process. Therefore, while it is well-known that humans do not make rational decisions, it is still unclear whether bats, our close mammalian relatives, operate in a similar fashion. In order to answer this question, Claire designed her experiment to look for a phenomenon known as the “asymmetrically dominated alternative effect,” or the “decoy effect.” As the name implies, the decoy effect involves a decoy, which is defined as:

“an option that is dominated by at least one option in the set (designated the Target), but is not dominated by another of the options (the Competitor). An option A is defined as dominating an option B, if for every attribute of B the value for A is never less than the value for B, and if for at least one attribute, the value for A is greater than the value for B” (Bateson).

In the context of Claire’s experiment, each bat was first given the choice between a “Target” Túngara frog call with high amplitude and a “Competitor” Túngara call with high complexity. Claire played these calls in a random order from speakers connected to her computer in the flight cage. Each speaker had a pile of fish placed on top as a reward for the bat when it flew over to investigate the call, and Claire recorded which speaker the bat flew to. Then, each bat was presented with the same Target and Competitor calls, except this time with a decoy call that was lower in amplitude and complexity than the Target call and lower in amplitude than the Competitor call. The decoy effect occurs when individuals exhibit an increased preference for the Target call over the Competitor in the presence of the decoy call, compared to in the absence of the decoy call. This indicates that bats do not make rational decisions but, instead, rely on a type of comparative mechanism for choosing between frog calls.

To me, the flight cage experiment was a good example of a common theme in a behavioral biology research: long periods of animal training and observation that require sustained attention and focus, sometimes leading to long, stressful and tedious nights if the animal choose not to cooperate. Fortunately, the bats I helped with were very entertaining to observe and were incredibly smart with distinct personalities, making the experiments almost seem like a surreal reality TV show at times.

The results of Claire’s study may soon provide us with a deeper insight into the ways that animals make decisions, and I am very grateful to have played a small part in the discovery process. If you are interested in Claire’s research, you can send her an email at or go to the Ryan Lab website at for more information.

Bateson, M., Healy, S. D., & Hurly, T. A. (2002). Irrational choices in hummingbird foraging behaviour. Animal Behaviour, 63(3), 587–596. doi:10.1006/anbe.2001.1925

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