ANÁLISIS Y DISCUSIÓN

Playback and monitoring equipment were as in experiment 1. Threshold measurements were carried out after experiment 1 was finished. The main problem was that due to the strong aversiveness of the stimuli in the previous experiment it proved very difficult to attract animals to the feeding station. In animals that had sensitised in the previous experiment more than a day’s break and substantial food rewards were required to make the animal approach the feeding tube again (and in some cases the animal would still only approach for a short time). After an animal got accustomed again to wait in front of the feeding station experiments were carried out. The edge of the cup was lowered as a cue for the seal to position itself in front of the feeding station. If the animal was closer than 40cm either lying on the bottom of the pool or drifting motionless in front of the feeding station the playback started. This involved presentation of 1 kHz and 200ms long pure-tone pulses with rise-time of 5ms. These pulses ranged in intensity from 140-180 dB re 1µPa in increments of 5 dB. Since subtle responses like neck twitches could only be analysed appropriately by watching the videos from the underwater camera on a big screen, no classical staircase design was applied which would have required a decrease or increase of stimulus intensity depending on the observed response. Therefore, all nine intensity steps were presented in a pseudo-randomised order. Four playback blocks each containing all nine intensities in a different pseudorandomised order were carried out for each of the eight animals. A 1min interval separated each intensity step within a block. The time interval between the end of one block and the start of the following block was at least 1h. No more than 2 blocks were presented on any one day. If two blocks were measured on the same day a 12h recovery period was introduced before the third block was presented on the following day. The startle threshold was defined as the 75% response threshold. This means that an animal had to exhibit a neck twitch or a whole body startle in at least 3 out the 4 blocks at a defined received level and all higher received levels. Behaviour was coded from the videos as “neck twitch” or “whole body startle” if there was a clear shift (“jump” or “curving”) of the whole body in the video frame. The highest intensity stimulus was only presented twice in the two sessions furthest apart. This was done because the maximum sound pressure level was potentially getting close to the threshold for onset of temporary threshold shift (TTS) which has been determined to occur at sound exposure levels

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(SEL) of 183 dB re 1μPa-s dB in a harbour seal (Kastak et al. 2005). Given that the 180 dB re 1μPa stimulus tested in my study had a sound exposure level of only 173 1μPa2_s-1 _{(due to its short duration) and the hearing thresholds at the frequencies}

tested by Kastak et al 2005 were lower, my stimulus can be expected to have had no impact on hearing. However, higher intensities were not tested in this study to account for the uncertainty in the limited data available on onset of minor TTS in pinnipeds. In addition, the loudspeaker started to distort the signals at levels higher

than 180 dB re 1μPa.

The 200ms long 1 kHz pure tone signals were synthesised in Cool Edit pro in 5 dB amplitude steps. The correct received levels for the threshold measurements were obtained through a series of sound field measurements in the pool. First, the power amplifier was roughly adjusted to produce the expected output. Then in a series of calibration trials prior to the experiment the output of the power amplifier was adjusted so that the 9 intensities would coincide with received level ranging from 140- 180 dB re 1µPa. At lower amplitudes this also required an adjustment of the signal in the digital domain. Received levels were then measured at 8 different positions where seals typically positioned their head when stationing in the experiment. Each position was measured several times on different days and values were then averaged over all eight positions. The variation of received levels between the measured 8 positions was on average +/- 3 dB. If the pool had to be emptied and equipment had to be removed between tests of different individuals a new calibration was carried out. Slight changes in orientation of the transducer had no apparent systematic effect on measured received levels. No changes in the output of the transducer were found over the course of the experiment from 2006 to 2007.

Results

The startle thresholds are given in table 2. Startle responses occurred at received levels down to 155 dB re 1µPa in two animals (see table 2). Startle thresholds defined as 75% response threshold to 1 kHz pulse were found to be between 160- 165 dB re 1 µPa for all animals that sensitized in the previous experiment. Taking the hearing threshold from the composite audiogram (see appendix 1) into account, this would reflect a sensation level of 80-85 dB. However, no startle threshold could be determined for the three animals that habituated in experiment 1. All three animals showed no apparent skeletal muscle contraction, eyelid closure or any other signs that would be indicative for the startle reflex. This includes the two exposures to the loudest stimulus which had a received level of 180 dB re 1µPa.

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Table 2: Startle thresholds and life history information for the 8 seals tested in experiment 1 and 2. No startle threshold could be determined for the three animals that habituated in experiment 1 since they never showed any sign of startle.

Experiment 3: Field trials investigating larger scale