Echolocation behavior and source level reduction in the echolocating bats Myotis myotis and Eptesicus fuscus when catching airborne targets of different strength

Abstract

The narrow space passive gleaning forager Myotis myotis and the edge space aerial forager Eptesicus fuscus were trained to catch mealworms or reflectors, small point-like targets of differing target strength, in the aerial-hawking mode and their motor and echolocation behavior was recorded and described. It was assumed that the comparison of the approach behavior of the two ecologically different species confirms the hypothesis that the sound pressure level (SPL) reduction during the approach, a behavior that is reported for all bats, has the function to adjust the relation of the perceived emission and echo SPL in a range where the ranging performance of bats is optimal. The motor behavior during prey capture was rather similar in both species, and both species formed a pouch with wing and tail membrane when catching prey. According to changes in echolocation behavior the behavioral phases search flight, initial approach, and terminal approach with buzz 1 and buzz 2 were discriminated to describe a capture event. Eptesicus fuscus used search signals with a source level (SL) of about 96 dB re 1 m which were much louder than those of Myotis myotis which had a search SL of about 80 dB re 1 m. These values correspond to an emission SPL at the ears of about 106 dB and 90 dB. When approaching the target both species reacted in a similar way with a reduction of sound duration, pulse interval, and SPL. The start of the approach or the reaction distance which was determined as the distance where these reactions started depended in both species on the target strength (TS) and decreased with lower TS. With M. myotis, the approach began immediately after the calculated target detection at a target distance of 103 cm (for the mealworm target) and a distance of 125 cm (for the reflector target) - each with an echo SPL of approx. 20 dB. In E. fuscus, which emit search signals with a SL 16 dB higher than that of M. myotis, the calculated detection distances were distinctly further out than the begin of the approach reaction as defined above. The reaction distance also differed according to the different target strength of the prey items between 132 – 173 cm. The start of the approach was triggered when the echo level of the three target types surpassed 35.3 dB, 38.5 dB and 42.3 dB SPL (referring to the emission SPL at the microphone). With the begin of the approach both bats started with the reduction of the emission SPL. M. myotis reduced the SPL from about 90 dB to about 75 – 70 dB at the end of buzz 1 at a distance to the target of 17 – 19 cm thus producing echo SPLs of 37 – 35 dB whereas E. fuscus where buzz 1 ended rather early at distances around 50 cm reduced the SPL from 106 dB to between 95 – 88 dB emission SPL thus producing echo SPLs between 46 – 35 dB. In M. myotis with its low search SL the reduction rate for the emitted signals was with 3.3 and 3.2 dB/hd (hd = half distance) rather low which resulted in a distinct echo increase during the approach to about 40 dB. In the louder E. fuscus the reduction rate was rather high with 11.3, 10.5, and 6.5 dB/hd at the three targets which kept the echo level constant near about 40 dB throughout the whole approach. These results support the working hypothesis of this thesis that the adjustment behavior of bats approaching prey is adapted for a precise ranging performance and depends in each species on the echolocation conditions of their ecological niche. Bats with high search SL have high reduction rates during the approach and keep the echo SPL rather constant near 40 dB whereas bats with low search SL have lower reduction rates of the emitted signals so that the echo SPL is raised during the approach to about 40 dB. The measured data do not allow making exact estimations about the perceived SPL ratios at the cochlea since it is not known how the middle ear reflex effects the auditory input during the approach.