The Role of Vibration Amplitude in the Escape-Hatching Response of Red-Eyed Treefrog Embryos

Abstract

The function and adaptive significance of defensive behaviors depend on the contexts in which they naturally occur. Amplitude properties of predator cues are widely used by prey to assess predation risk, yet rarely studied in the context of the stimuli relevant to defensive decisions in nature. Red-eyed treefrog embryos, Agalychnis callidryas, hatch precociously in response to attacks on their arboreal egg clutches by snakes and wasps. They use vibrations excited during attacks to detect predators, but wind and rainstorms also excite intense vibrations. Past work has demonstrated that to avoid costly decision errors, A. callidryas nonredundantly combine information from the temporal and frequency properties of clutch vibrations. Here, we demonstrate that embryos also use absolute amplitude and fine-scale amplitude modulation information to refine their hatching decision. We used vibration recordings to characterize the amplitude properties of the most common predator and benign-source disturbances to A. callidryas egg clutches in nature and tested whether embryos at 3 ages across the onset of mechanosensory-cued hatching (4-6 days) respond to amplitude variation during playback of synthetic vibrations to eggs. Older embryos responded to much lower-amplitude vibrations, reflecting a >88-fold decrease in response threshold from 4 to 5 days. To assess how embryos combine amplitude with other vibration properties, we played embryos recorded exemplars of snake attack and rain vibrations of varying amplitudes and patterns of amplitude modulation. The amplitude response curve was steeper for snake recordings than for rain. While amplitude information alone is insufficient to discriminate predator attack from benign-source vibrations, A. callidryas employ an impressively complex strategy combining absolute amplitude, amplitude modulation, temporal, and frequency information for their hatching decision.

Fig. 1.

Amplitude characteristics of common natural vibrational disturbances to Agalychnis callidryas egg clutches reveal that no disturbance amplitude was unambiguously indicative of predator attack. Shown are median, 10th, 25th, 75th, and 90th percentiles for snake attacks (Imantodes inornatus, Leptophis ahaetulla, Leptodeira rhombifera, and Leptodeira ornata), wasp attacks (Polybia rejecta), and vibrations excited by hard rain, strong wind, and routine embryo movements. (A) Peak amplitudes for long periods of each disturbance type. (B) RMS amplitudes for long periods of each disturbance type, including intermittent periods of silence. RMS amplitude for embryo movements was not measured separately for long-period samples. (C) RMS amplitudes for 10-s periods of continuous vibrations sampled from each disturbance, constructed from longer samples by editing out periods of silence. Hard rain is the highest amplitude disturbance. Percentiles are not shown for I. inornatus attacks, and 10th and 90th percentiles are not shown for wind recordings, due to low sample sizes.

Fig. 2.

The hatching response of Agalychnis callidryas embryos to playbacks of vibrational noise at varying amplitudes. (A) Two playback series were conducted using bands of low-frequency noise in a 0.5-s noise/1-s silence temporal pattern. The tine playback apparatus—which presents both motion and tine-contact cues to entire egg clutches—was altered between the two series, both presented to 5-day-old embryos. Escape hatching increases with amplitude until a plateau of maximal hatching is reached. Shown are mean proportion of embryos within each clutch that hatched, ±SE, for each stimulus presented. (B and C) Ontogenetic variation in the amplitude dependence of hatching using bands of low-frequency noise in a 0.5-s noise/1.5-s silence temporal pattern. All stimuli were played through a tray apparatus designed to present vibrations without contact cues to a set of embryos held individually. Escape hatching increases with both amplitude and development. Means not sharing any letter within age groups are significantly different by a post hoc test at the 5% level of significance. Shown are (B) mean proportion of 4-, 5-, and 6-day-old embryos in trays that hatched, ±SE, for each stimulus presented, and (C) the latency of the first embryo in each tray to hatch in response to vibration stimuli of varying amplitudes.

Fig. 3.

The hatching response of Agalychnis callidryas embryos to playbacks of recorded snake attack (Leptophis ahaetulla) and rainstorm exemplars at varying amplitudes. Embryos were presented with two snake attack and three rain exemplars. One snake and one rain exemplar were presented at eight amplitudes, and the remaining exemplars were presented at three amplitudes. Hatching increases more strongly as function of amplitude for snake attacks than it does for rainstorm vibrations. (A) Hatching response as a function of peak amplitude. (B) Hatching response as a function of RMS amplitude. Data are mean proportion hatched ± SE for each stimulus presented.

Fig. 4.

The hatching response of Agalychnis callidryas embryos to playbacks of synthetic vibrational stimuli varying in amplitude envelope shape. Stimuli were matched for frequency (0–100 Hz band limited white noise), temporal pattern (1-s noise/1-s silence), and peak and RMS amplitude, but varied in rise and fall times as shown in the waveform graphics above the data. “Fade in” elicited less hatching than the other two stimuli. n.s. = not significant (P > 0.05). Shown are mean proportion hatched ± SE for each stimulus.