Coevolution of auditory sensitivity and temporal resolution with acoustic signal space in three songbirds

In this article, the question, “Does coevolution between senders and receivers produce a close match between signal design and sensory biology?” was posed. Prior to their experiment, the researchers hypothesized that coevolution between senders and receivers would produce a close match between signal design and sensory biology. They planned to test this hypothesis by conducting experiments on wild-caught tufted titmice, house sparrows, and white-breasted nuthatches. They were looking to compare aspects of acoustic signal space with both the frequency range of auditory sensitivity and temporal resolution.

First, the researchers conducted an auditory brain-stem response (ABRs) test to determine the frequency range of auditory sensitivity. The ABR test records a minute change in voltage from the scalp surface that reflects the response to auditory nerve and brain-stem nuclei. This test is evoked by tone bursts and is widely used in other vertebrates, including bird species, to assess the frequency range of auditory sensitivity. An envelope-following response (EFRs) test was conducted to test the correlation between acoustic signal space and temporal resolution. The EFR determines the number of waveform peaks that coincide with the maximum amplitude in the stimulus after accounting for a delay. This test had been done in the past on mammals but never birds. The success with mammals made the researchers believe that it could also be an effective technique in this experiment.

The ABR stimuli were 5-ms tone bursts presented at a rate of 31.1 stimuli per second and at eight differing intensity levels. The ABR was therefore considered the average response to 1000 stimulus repetitions. Then using this data, ABR values for each species were cross-correlated using computer software. The EFR test used AM tones and was considered the average response to 1000 stimulus repetitions. Then using the same computer program previously mentioned, the EFR values were correlated.

The researchers found that ABR’s showed a maximum sensitivity at 2.2-3.2 kHz, but a greater sensitivity in titmice at the maximum frequency tested. Overall, however, the ABRs were similar to the other species involved in this study as well as those involved in previous studies using other bird species. EFRs showed a slightly greater temporal resolution in titmice and sparrows than nuthatches, but were overall very similar. Subsequent statistical analysis using ANOVA’s helped the researchers see the significant effects of frequency on ABR thresholds as well as the effects of FM on EFR amplitude.

From this data they concluded that high-frequency sensitivity varied with the maximum frequency of species-specific vocalizations and temporal resolution varied with the maximum number of waveform peaks in response to amplitude. They therefore could accept their hypothesis, and conclude that coevolution between senders and receivers does indeed produce a close match between signal design and sensory biology. The additionally added that the titmouse could use high-frequency communication as a sort of alarm call to conspecifics.

This article provides us with an example of coevolution that builds on what we learned in lecture. As we talked about, coevolution of signals occurs between senders and receivers and in this article, we noticed a correlation between the frequency sensitivity of a species and its response to differing vocalized frequencies. this means that as vocalizations changed, the individual’s ability to interpret them also changed. This study sets precedence in determining whether other non-avian species also coevolve signal sending with reception. If this correlation has been found to occur in birds, maybe we can experimentally determine if it happens via the same mechanism in other species as well.

Henry, K. & Lucas, J. (2008) Coevolution of auditory sensitivity and temporal resolution with acoustic signal space in three songbirds. Animal Behaviour, 76, 1659-1671.