Adaptation

Adaptation describes the usefulness of a given trait in terms of an organism’s fitness.  In Tinbergen’s four areas, it is commonly referred to as adaptive value and describes why and organism “is the way it is” (Suzy Renn’s B342 :Animal Behavior  course website).  It represents an ultimate cause or process and can be conflated with phylogeny, which describes the “sequential changes in a species across time” (Suzy Renn’s B342 :Animal Behavior course website).

Just How Is Clicking Adaptive?

While most researchers agree that moth clicking is defensive in nature, there are many hypotheses as to how the behavior actually deters bats.  One hypothesis, the startle hypothesis, claims that moth clicks are timed in such a way as to throw bats off course at the last possible moment, thereby "startling" it before it has made contact.  Another hypothesis, the jamming hypothesis, claims that the timing and frequency of moth clicks are intended to confuse the bat by somehow disrupting its perception.  The final hypothesis, the aposematic hypothesis, claims that clicks are intended to warn bats of chemical defenses that render moths unpalatable to them.

Experimental Desgin

 

Hristov and Conner (2005) created an experimental design that de-coupled chemical unpalatability from sound production to determine how naive bats would learn to respond to moths that differed naturally in their sound production and unpalatability (Figure 1).  Each of the three hypotheses predicts a different pattern of learning behavior.  The jamming hypothesis predicts that bats will have difficulty capturing clicking moths throughout the week-long study, regardless of whether the moths are unpalatable or not (Figure 2).  The startle hypothesis predicts that bats will initially have difficulty capturing clicking moths regardless of palatability, but will get progressively better at catching over time. The aposematic hypothesis predicts that bats will never have any difficulty capturing clicking moths, but will learn to avoid them if they are also unpalatable.

Figure 1. Arctiid moth species naturally possesing different combinations of chemical defense and sound production.  Moths that possess chemical defense are labeled C+, while moths that do not are labeled C-.  Similarly moths that possess that ability to produce sound are labeled S+ and those that are not are labeled S-.  (Hristov and Conner, 2005)

 

The results support the aposematic hypothesis in that all bats tested had the ability to capture sound producing moths, however when sound production was paired with unpalatability, bats quickly learned to avoid clicking moths.  Conversely, when sound production was paired with palatability, bats captured clicking moths with the same frequency as control moths.  In other words, the bats only learn to avoid moth clicks if they are paired with chemical defense.  This kind of acoustic warning system could have been selected for over evolutionary time under an individual or kin selection model.  In the former, a clicking moth benefits from higher survival rates and therefore has a statistically higher chance of leaving behind offspring than a silent moth.  In the latter, a clicking moth that is killed in the process of 'teaching' the bat may still increase the survival rates of his clicking brethren (Hristov and Conner, 2005).  The experiment by Hristov and Conner does not label the jamming and startle hypotheses as patently incorrect.  It merely provides evidence for aposematism as the dominant mechanism by which moths repel bats.  Jamming has been shown to operate in some species, although even here it is likely an embellishment to an existent aposematic signal (Fullard, 1979).  The jamming hypothesis requires an incredible degree of specificity, both in terms of timing and frequency and might therefore be expected to evolve more slowly, or secondarily (Hristov and Conner, 2005).  The startle hypothesis is likely to generate an effect that is too ephemeral to become an effective bat deterrent, especially in environments where bat-moth encounter rates are high (Hristov and Conner, 2005).

Figure 2.  Predictions for experimental moth (dotted red) and control moth (dotted blue) interactions with big brown bats under the acoustic aposematism, jamming, and startle hypotheses.  Solid lines show results for experimental moths (red) and control moths (blue) (Image from Hristov and Conner, 2005)

 

Benefit for Bats

True aposematic signaling is beneficial to bats as it warns against unfavorable prey.  By avoiding unpalatable moths, bats do not waste time and energy attempting to catch them.  Without the warning of sound, unpalatable and palatable moths would be hunted in the same way.  Unpalatable moths would be caught and handled, but would eventually be dropped and left uneaten due to their chemical defenses.  This scenario is not ideal for either party as the moth survival rate is often low after handling, and the bat has expended energy for no gain. 

On the other hand there is no benefit to the bats in avoiding a mimic.  In fact, in this case, the bat has been duped out of a perfectly delicious meal.  Bats would therefore most benefit from an ability to determine which moths were mimics and which were not.  While bats do have the ability to learn to decouple moth clicking from unpalatability, it is often difficult for them to do so in natural settings as mimics are almost always sympatric with their models (Ihalainen, E et al., 2008).