Effect of anthropogenic noise on the echolocation pulses of the bats Molossus sinaloae and Mormoops megalophylla

Anthropogenic noise interferes with the acoustic signals of various wildlife species.  For bats that use echolocation, noise can mask the information received in the echo.  The effect of anthropogenic noise on the time and frequency components of echolocation pulses emitted by the aerial insectivorous bats Molossus sinaloae and Mormoops megalophylla in urban and natural habitats were evaluated.  We hypothesized that the frequency components of pulses would increase in response to masking, while time components would not change significantly.  To this end, acoustic recordings of both species were made in the two types of habitats using ultrasonic detectors; simultaneously, the intensity of the ambient noise was measured.  Frequency (kHz) and time (ms) were analyzed for each echolocation pulse using the BatSound 4.2 software.  Consistent with our hypothesis, the results showed that under background noise of 75 dB in an urban environment, M. sinaloae increased the low and high frequencies of its echolocation pulses by 5.8 kHz on average.  For M. megalophylla, no increase in pulse frequencies was observed.  Contrary to our expectation, the time components of pulses for M. sinaloae were modified, being of shorter duration in urban sites.  Increasing the maximum amplitude-frequency by M. sinaloae may be a response to the Lombard effect, i. e., the increase in vocal amplitude in response to increased background noise.  It is important to carry out studies focused on understanding the modification of echolocation pulses, mainly for species living in urban environments.