Acoustic mimicry in a predator–prey interaction

To address Müllerian mimicry, we offered a noxious model tiger moth to five E. fuscus and two L. borealis bats for five nights, then substituted a second noxious tiger moth species on night 6 (Fig. 2). Each night, four sound-producing tiger moths were randomly presented along with 12 other palatable, silent control moths: eight pyralids (Galleria mellonella; the moths initially used to train the bats to capture prey) and four noctuid novelty controls size-matched to the experimental tiger moths presented. Three E. fuscus and one L. borealis were presented with Cycnia tenera as the model and Syntomeida epilais as the mimic, two E. fuscus and one L. borealis were presented with the same moths in reverse order. Both groups learned to avoid the noxious model during the course of the first five nights. As the results for both orders of moth presentation were nearly identical, the data were pooled for analysis. C. tenera caterpillars sequester cardiac glycosides from their hostplant, Apocynum cannibinum, and the resulting adults are thoroughly unpalatable to bats (22). S. epilais, which relies on similar cardiac glycoside chemistry for its unpalatability, consumes Echites umbellata as its principal indigenous hostplant and currently relies heavily on Nerium oleander (23). The distributions of all of the moths, bats, and hostplants used in the Müllerian experiments overlap in central and northern Florida. By night 5, all bats had learned to avoid the model completely (Fig. 2; for avoidance behavior, see SI Movie 3; Friedman nonparametric ANOVA, χ² = 43.02; df = 10, p < 0.001; Wilcoxon post hoc test comparing night 1 with 5; Z = −2.388; two-tailed P = 0.017, Q = 0.04). On night 6, when the presumed Müllerian mimics were introduced, only one big brown bat captured a single moth (S. epilais). Müllerian mimicry clearly works in this acoustic system (Wilcoxon test comparing nights 5 and 6, Z = −1.0; two-tailed P = 0.317, Q = 0.423). The E. fuscus showed some catching behavior on night 7, but it quickly decreased and avoidance of the mimic continued for five nights of presentation (Wilcoxon test comparing nights 6 and 10, Z = 0.0; two-tailed P = 1.0, Q = 1.0). To determine whether the bats were generalizing on the basis of the clicks, on night 11 we removed the moths' tymbals and presented the silenced moths to the bats. The percentage of tiger moths caught returned to control levels (Wilcoxon test comparing nights 10 and 11, Z = −2.333; two-tailed P = 0.017, Q = 0.04), but all of these silent noxious moths were subsequently dropped. It is the prey-generated sounds that are driving the mimicry; olfactory cues, wingbeat frequency, and other information from the echolocation stream do not appear to be important.

Batesian mimicry was investigated by again training naïve bats (three E. fuscus and seven L. borealis) for five nights to avoid a model, C. tenera (Fig. 3A; Friedman nonparametric ANOVA, χ² = 66.41; df = 10, p < 0.001; Wilcoxon post hoc test comparing nights 1 and 5, Z = −2.871; two-tailed P = 0.004, Q = 0.01). On night 6, Euchaetes egle was introduced. E. egle larvae feed on milkweeds, including Asclepias tuberosa, but apparently do not sequester the plants' cardiac glycosides in their adult tissues; the moths are totally palatable (22). It is noteworthy that we have often found the hostplants of C. tenera and E. egle in the same fields, and the adult moths resting on each other's hostplants. Fig. 3A shows that while statistically supporting Batesian mimicry (Wilcoxon test comparing nights 5 and 6, Z = −1.414; two-tailed P = 0.157, Q = 0.18), the variance in capture behavior increased markedly when E. egle was introduced. Partitioning the data into bats that were deceived by the Batesian mimic (Fig. 3B) and those that discovered its palatability (Fig. 3C) reveals the cause.

Three E. fuscus and four L. borealis did not capture E. egle when it was introduced on night 6 (Fig. 3B; Friedman nonparametric ANOVA, χ² = 66.41; df = 10, p < 0.001; Wilcoxon post hoc test comparing nights 5 and 6, Z = 0.0; P = 1.0, Q = 1.0). On night 7, all of the L. borealis showed some degree of capture behavior, but as in the Müllerian experiments, the behavior quickly terminated, and avoidance of the mimic was not different between the first and last night of its presentation (Wilcoxon test comparing nights 6 and 10, Z = 0.0; P = 1.0, Q = 1.0). Most of the moths taken during these captures were dropped and not eaten, suggesting that other signals, such as chemical cues or vibration in the bats' catching membranes, induced caution. On night 11, when E. egle with tymbals removed were introduced, all of the silenced moths were caught and eaten (Wilcoxon test comparing nights 10 and 11, Z = −2.646; two-tailed P = 0.008, Q = 0.028). Our results demonstrate that mimetic generalization was driven by the moth sounds.

Three L. borealis discovered the palatability of E. egle (to view a discovered Batesian mimic being captured, see SI Movie 4). Two of them (Fig. 3C, red triangles and gray circles) avoided only the first E. egle presented and subsequently captured the next three tiger moths offered on day 6. Interestingly, most of these captured moths were dropped; it was not until night 7 that these two bats ate all of the E. egle they captured. The other red bat in this group took substantially longer to discover the palatability of E. egle (Fig. 3C, black squares). This animal did not capture any tiger moths on night 6 when they were introduced, but by night 10 it captured and ate 50% of the E. egle offered. On night 11, noxious sound-producing C. tenera were reintroduced. With the exception of one capture, the bats did not touch any of these C. tenera, thus discriminating between the palatable mimic and the unpalatable model. Starting on night 13, silenced C. tenera were presented to these three bats for one, two, and five nights, respectively (only the first night of muted C. tenera presentation is graphed in Fig. 3C). All of these silenced, noxious moths were caught and dropped, showing once again that the prey-generated sounds drive discrimination. Although an omnibus test of planned comparisons for this group revealed statistical differences (Friedman nonparametric ANOVA, χ² = 30.09; df = 12, p < 0.01), no planned pairwise comparisons were significant (Wilcoxon tests comparing nights 1 and 5, 5 and 6, 6 and 10, 10 and 11, and 12 and 13; all two-tailed P = 0.061–0.069, all Q = 0.18).

It is tempting to hypothesize that the close ecological relationship between red bats and moths can explain why only L. borealis discovered the acoustic deception of E. egle. Assaying more E. fuscus might reveal that some big brown bats are capable of discovering the Batesian mimics. Regardless, sound-producing tiger moths, by means of mimicry, enjoy survival benefits from two very different acoustic predators: bats that specialize on moths and bats that are infrequent moth predators. There are >11,000 species of tiger moths worldwide (24) and numerous sympatric species in any single tropical location (25). This diversity and recent discoveries of both tiger beetles (26) and hawkmoths (unpublished work) responding to bats with ultrasonic sounds suggest that acoustic mimicry complexes are likely to be common and rich components of the natural world.

We thank B. French for training and assistance with red bat care; V. Livingston, J. M. Baratta, P. Deal, K. Kelly, S. Knight, C. Hitchcock, T. Pate, and V. Jarvinen for assistance with bat care; S. Garrett and A. Harper for assistance with caterpillar rearing; B. Chadwell and N. Hristov for discussion; and M. Conner and D. Anderson for comments on the manuscript. This work was supported by the Wake Forest University Science Research Fund and National Science Foundation Grant IOB-0615164.