Multimodal cues improve prey localization under complex environmental conditions

Abstract

Predators often eavesdrop on sexual displays of their prey. These displays can provide multimodal cues that aid predators, but the benefits in attending to them should depend on the environmental sensory conditions under which they forage. We assessed whether bats hunting for frogs use multimodal cues to locate their prey and whether their use varies with ambient conditions. We used a robotic set-up mimicking the sexual display of a male túngara frog (Physalaemus pustulosus) to test prey assessment by fringe-lipped bats (Trachops cirrhosus). These predatory bats primarily use sound of the frog's call to find their prey, but the bats also use echolocation cues returning from the frog's dynamically moving vocal sac. In the first experiment, we show that multimodal cues affect attack behaviour: bats made narrower flank attack angles on multimodal trials compared with unimodal trials during which they could only rely on the sound of the frog. In the second experiment, we explored the bat's use of prey cues in an acoustically more complex environment. Túngara frogs often form mixed-species choruses with other frogs, including the hourglass frog (Dendropsophus ebraccatus). Using a multi-speaker set-up, we tested bat approaches and attacks on the robofrog under three different levels of acoustic complexity: no calling D. ebraccatus males, two calling D. ebraccatus males and five D. ebraccatus males. We found that bats are more directional in their approach to the robofrog when more D. ebraccatus males were calling. Thus, bats seemed to benefit more from multimodal cues when confronted with increased levels of acoustic complexity in their foraging environments. Our data have important consequences for our understanding of the evolution of multimodal sexual displays as they reveal how environmental conditions can alter the natural selection pressures acting on them.

Keywords: eavesdropping; environmental complexity; localization; mixed-species chorus; multimodal communication.

Figure 1.

Robofrog presentation decreased flank attack angle of bats. (a) Robofrog set-up and zone of analyses. Shown are the frog model, a circular zone 60 cm from the model that we used to score approach angle, with 0° being the direction of the perch, and a circular zone at 30 cm from the perch that we used to score attack angle. Flank attack angle was defined as the differences in degrees between zone of approach and zone of attack, which is 45° (90 minus 45) in this example. (b) Median flank angles made by bats during experiment 1 and 2. Boxplots depict model estimates of fixed effect of the robofrog and show that bats make narrower flank angles during their attacks when the robofrog was present (red) compared to control conditions (blue). Stimulus presentation was switched off immediately after the bat left the perch (experiment 1) or after the bat entered the zone of attack (experiment 2). (Online version in colour.)

Figure 2.

Multimodal cues increase approach directionality of bats under increased acoustic complexity. (a) Individual examples of the angles of approach made during the different experimental treatments. Each dot represents the compartment where a bat approached the frog model during a single trial. We show examples for trials with and without robofrog presentation as well as during trials in which either 0, 2 or 5 speaker platforms were playing D. ebraccatus male calls. For each bat, we assessed approach directionality by calculating the MRV length of all six trials from a treatment category. An MRV length of 1 indicates that a bat always approached from the same direction, whereas an MRV length of 0 indicates large variability in approach direction. (b) Approach directionality of all bats for all six treatments. Boxplots depict model estimates of fixed effects and lines indicate individual measures of approach directionality. (c) Post hoc model to illustrate the difference in directionality due to the robofrog under the three different D. ebraccatus treatments. The multimodal cue from the robofrog only altered approach directionality when five speakers broadcast D. ebbracattus calls. (Online version in colour.)

Figure 3.

Increased flight latencies under increased acoustic complexity. Bats took longer to leave their perch and approach their prey when additional speakers broadcast D. ebraccatus calls. Boxplots are derived from model estimates for different D. ebraccatus treatments as well as robofrog treatment (robofrog in red, control in blue). The y-axis depicts natural log-transformed flight latencies. (Online version in colour.)