Rational decisions in frog-eating bats

The fringe-lipped (also called frog-eating) bat, Trachops cirrhosus. Photo by Christian Ziegler. 

This week's paper is lead-authored by Claire Hemingway who (like I was, in full disclosure) is a graduate student with Mike Ryan at UT Austin and Rachel Page at the Smithsonian Tropical Research Institute, both of whom are also authors on the paper. They address the concept of "rationality" in animal decision-making. Rationality in this case is used in the economic sense. In examining human decision-making the field of economics has established principles of rationality. One of the principles of rationality is independence from irrelevant alternatives. This principle is that preference for two alternatives should not change with the addition of other options. If you prefer A to B, the addition of C should not change your relative preferences for A and B. However, humans are highly irrational. This is demonstrated by the "decoy effect" that is frequently used in marketing. Hemingway and her colleagues give a nice example: if there are two cans of vegetables on a shelf one of which is expensive and organic (choice A) and the other of which is inexpensive and conventional (choice B) you may not have a strong preference between them. But if a third can is added that is expensive and conventional (choice C) this may cause you to now prefer the expensive organic over the inexpensive conventional. This irrationality has been demonstrated in many other animals, including hummingbirds, honeybees, and even slime moulds (such a cool paper!). Here it is tested in frog-eating bats. 

What they did

Frog-eating bats hunt frogs by listening for their calls (referred to as "eavesdropping"). In particular, a substantial literature has developed on bat response to calls of the túngara frog. Túngara frogs make simple calls (a "whine" alone) and complex calls (a whine plus "chuck" sounds added to it). Bats prefer complex calls, but they also prefer louder calls. The two options that Hemingway and her colleagues used, therefore, were a complex call (choice A) and a louder simple call (choice B). In the decoy effect test they added in a third stimulus, a simple call but even quieter than the complex call (choice C). This "decoy" should (if bats are irrational) increase the preference for A over B, because C is inferior to A in two different ways, versus inferior to B in only one way. 

What they found

When you pool all the bats together they had no preference for A over B with or without C. BUT that is not to say that all the bats were going to A 50% of the time and B 50% of the time. This was the case for many of the bats, but in the A/B choice test 2 of the 11 bats strongly preferred A, and one bat strongly preferred B. When C was added in, 3 bats preferred A and one bat preferred B. Only for one bat did preference change with the addition of C. That bat, as predicted, had no preference when there was just A and B and preferred A when C was added. It was the only irrational bat of 11. The other ten had no change in preference for A vs. B when C was added. This individual variation is so typical of animal behavior studies! 

The takeaway

Although many other studies have shown irrationality in animal decision-making, this study did not. There are a number of possible reasons why this could be. The authors propose that the high metabolic demands of powered flight in bats may select for more rational decision-making in foraging choices than in other species. BUT, irrationality has also been shown in hummingbirds! The discordance of these results with other studies could also be an artifact of the system, or stimuli chosen. Regardless, the question is very cool and I am glad to see it tested in this system!