Reduction of emission level in approach signals of greater mouse-eared bats (Myotis myotis): No evidence for a closed loop control system for intensity compensation

Abstract

Bats lower the emission SPL when approaching a target. The SPL reduction has been explained by intensity compensation which implies that bats adjust the emission SPL to perceive the retuning echoes at the same level. For a better understanding of this control mechanism we recorded the echolocation signals of four Myotis myotis with an onboard microphone when foraging in the passive mode for rustling mealworms offered in two feeding dishes with different target strength, and determined the reduction rate for the emission SPL and the increase rate for the SPL of the returning echoes. When approaching the dish with higher target strength bats started the reduction of the emission SPL at a larger reaction distance (1.05 ± 0.21 m) and approached it with a lower reduction rate of 7.2 dB/halving of distance (hd), thus producing a change of echo rate at the ears of + 4 dB/hd. At the weaker target reaction distance was shorter (0.71 ± 0.24 m) and the reduction rate (9.1 dB/hd) was higher, producing a change of echo rate of-1.2 dB/hd. Independent of dish type, bats lowered the emission SPL by about 26 dB on average. In one bat where the echo SPL from both targets could be measured, the reduction of emission SPL was triggered when the echo SPL surpassed a similar threshold value around 41-42 dB. Echo SPL was not adjusted at a constant value indicating that Myotis myotis and most likely all other bats do not use a closed loop system for intensity compensation when approaching a target of interest. We propose that bats lower the emission SPL to adjust the SPL of the perceived pulse-echo-pairs to the optimal auditory range for the processing of range information and hypothesize that bats use flow field information not only to control the reduction of the approach speed to the target but also to control the reduction of emission SPL.

Fig 1. Side (A) and top-view (B) of a typical flight trajectory during an approach from the wall (bat C).

The dots indicate the positions where the bat emitted calls during flight. The black marked area represents the feeding area with the four feeding sites. The echolocation behavior during this passive part of the approach was similar whether they approached the dish directly from search flight or from the wall. SPL of signals emitted during passive approach did not differ (GLMM, F_{(1, 219)} = 0.28, p = 0.598). Therefore, we used pooled data from the two behavioral situations to describe the echolocation behavior during the passive approach. However, we did not pool data of individual bats.

Fig 2. Echolocation parameters for 10 approach flights of bat C (6 approaches from wall, 4 approaches from search flight) when landing on the foam dish (A) and on the reflector dish (B).

Dark gray represents search and passive approach, light gray symbols active approach. The black circles indicate the echo SPL of the reflector dish.

Fig 3. Logarithmic regression analysis of emission SPL reduction (filled circles) and echo SPL increase in relation to target distance (open circles) of all signals emitted during the active approach from 5 landings of each bat on the foam (A) and on the reflector dish (B).

Echoes from the foam dish were only recorded during one flight of bat A.

Fig 4. Individual positions of all bats at the beginning of SPL reduction in 5 active approaches to the foam dish (black symbols) and 5 active approaches to the reflector dish (red symbols).

Mean positions for all bats are marked with filled symbols (mean ± SD). Squares: bat A, diamonds: bat B, triangles: bat C, circles: bat D.

Fig 5. Distance related increase in echo SPL (mean ± SD) of the two types of feeding dishes measured in an ensonification experiment with a signal sweeping in 2 ms from 110–30 kHz with a SPL of 86 dB at 0.4 m.

The dashed reference lines indicate slopes of 12 and 6 dB/hd. Asterisks indicate statistical differences in echo SPL. (Regression equations: reflector 16° y = -37.3log(x) + 41.7; reflector 24° y = -28.6log(x) + 43.1; foam 16° y = -24.6log(x) + 35.5; foam 24° y = -26.0log(x) + 38.2).