Abstract:

The startle reflex is little studied in nonmammalian vertebrates, but in mammals it is often characterized by increase in heart rate. Orienting can also occur, characterized by a brief decrease in heart rate. To determine whether these responses occur in a testudinate, the desert tortoise, heart rate and behavior were measured during exposures to simulated jet overflights (94.6--114.2 dB CSEL) and sonic booms (6--10.5 psf). The best sensitivities of the 14 subjects ranged from 23--50 dB SPL (average 34 dB SPL at 250 Hz) measured using ABR. Initial exposure to simulated jet overflights produced a typical reptilian defensive response: freezing. Tortoises became totally immobile for periods of up to 113 min. The average heart rate showed a gradual decline (7--8%), recovering within 2--4 h. Tortoises oriented when exposed to simulated sonic booms and after repeated exposure to subsonic aircraft noise; orienting produced no detectable change in heart rate. These results suggest that (1) high-intensity transients affect desert tortoises by altering activity patterns, but do not have a direct effect on heart rate and (2) tortoises experience a physiological response that produces protracted freezing. [Work supported by USAF, Edwards Air Force Base, and F-22 SPO.]