6.3 Impacts on Signals
6.3.1 Halting the Evolution of Increasing Signal Magnitude
applications in their own research. One significant challenge in elucidating when JNDs impact behavior is the occurrence ofjust-meaningful differences or JMDs (Nelson and Marler 1990). A JMD is a measure of what difference in signal magnitude is perceived by receivers as behaviorally significant (see also Chaps.7 and8). JMDs can depend upon an animal’s motivational state, which can vary with hunger, reproductive cycle, age, or health condition. If a receiver does not respond to a difference in signal magnitude, it becomes necessary to consider whether the lack of response is due to not detecting the difference (due to its JND) or not caring about the difference (due to its JMD). Overcoming the difficulty of distinguishing between JNDs and JMDs is one of the challenges to tackle in proportional processing research, and we discuss potential solutions in Sect.6.7.
In some cases, female preference patterns clearly reflect Weber’s law. In these cases, the female preference strength depends on the proportional difference between two call values. This specific situation has been demonstrated in tu´ngara frogs,Physalaemus pustulosus(Akre et al.2011). In tu´ngara frogs, females prefer to mate with males that produce an extra element appended to the vocalization—a chuck (Chap.4). Males can add 1–7 chucks but usually produce only a few chucks at natural choruses (Bernal et al.2009). Males add chucks in response to female elicitation behaviors (Akre and Ryan2011), but females do not consistently prefer more chucks with statistical significance (Bernal et al.2009). Instead, they demon- strate a preference for more chucks that is constrained by their ability to discrim- inate chuck number due to their dependence on proportional processing (Fig.6.3;
Akre et al.2011). Depending on the costs involved, this reduced benefit of adding chucks is likely to cause the evolution of increasing signal magnitude to cease.
The cost of signal production determines whether proportional processing will limit the evolution of increasing signal magnitude. When the cost of increasing signal magnitude is linear or exponential, such that each increment added requires an equal or increasing effort, the cost to benefit ratio at each additional increment becomes increasingly dominated by cost, because the benefit shrinks as females are less likely to notice the additional increment. When this occurs, proportional processing by females could be the reason that evolution of increasing signal magnitude ceases. The energetic requirement of producing additional signal Fig. 6.3 Proportional processing of male tu´ngara frog (Physalaemus pustulosus) calls by potential mates and predators. Both the preferences of female tu´ngara frogs for a mate (black circlesand line) and predatory attacks by fringe-lipped bats (Trachops cirrhosus;gray circlesandline) target males that produce more chucks, significantly fitting a model based on proportional comparison of chuck number. Subjects were tested in nine different two-stimulus choice tests in which the paired call alternatives differed in absolute and relative chuck number. For female frogs, the ratios were 0:1, 0:3, 1:2, 1:3, 1:4, 1:5, 2:3, 2:4, and 3:4. The ratios for bats were 0:1, 0:2, 0:3, 1:2, 1:3, 1:4, 1:5, 1:6, 1:10, 2:3, and 2:4. (From Akre et al.2011, permission granted by AAAS)
increments is one type of cost that might increase in this way. But not all costs will increase with magnitude in this fashion. The fundamental frequency of some animals’ vocalizations, for example, will increase in magnitude as body size decreases, such that signalers experience reduced cost with increasing magnitude.
This would reduce the likelihood of limiting signal elaboration, even when the additional benefits for each increment are minimal.
Interestingly, predation pressure as a cost of signal production might not increase in a linear or exponential manner with additional signal increments. This is because changes in predation pressure, like the preferences of female tu´ngara frogs, depend on perception of increasing physical stimulus magnitude. Predators might also experience difficulty in discriminating higher-magnitude signals from potential prey. When this is the case, predation pressure is not expected to increase dramatically with signal elaboration. In fact, this appears to be the case in tu´ngara frog call evolution. Their major predator, the fringe-lipped bat (Trachops cirrhosus), discriminates between potential prey vocalizations by attending to chuck number, with a preference to attack individuals that produce more chucks (Chap. 11). The bats, like the female frogs, are constrained by proportional processing in their ability to actually attack the male making more chucks (Akre et al.2011; see Fig.6.3).
Generally, it is expected that predators drawn to conspicuous elaborate signals will limit the evolution of increasing signal elaboration (Zuk and Kolluru1998).
Predators might preferentially attack individuals that produce more elaborate sig- nals due to factors such as ease of localization (Page and Ryan2008) or quality of meal (Bernal et al.2007). However, the impact of predation depends in part on their ability to discriminate signal magnitude. If predators face the same proportional processing constraints as conspecifics do, the impact of predation on signal evolu- tion should actually decrease at higher magnitudes, as occurs with the predatory bats that target tu´ngara frogs (Akre et al. 2011). In cases where eavesdropping predators havesmallerJNDs than conspecific receivers, however, predators will be better than conspecific receivers at finding the higher-magnitude signals. This might be a common situation, because predators are usually bigger than prey and thus have bigger sense organs, which can lead to lower JNDs, as does the size of the eye in vision (Cronin et al.2014). In this situation, the selective pressure posed by predation would increase, and predation would quickly halt the evolution of increasing signal elaboration. If predators instead havelargerJNDs than conspe- cific receivers, predators will be relatively less capable of distinguishing between high-magnitude signals. In this situation, the selective pressure posed by predators would be negligible, and the evolution of increasing signal elaboration would depend completely on conspecific response (Fig.6.4).