Echolocation and body size in insectivorous bats: the case of the giant naked bat Cheiromeles torquatus (Molossidae)

Recent hypotheses propose that the maximum size of aerial insectivorous bats is limited by properties of their echolocation system. According to these hypotheses large bats would (A) have to use low call frequencies which may only allow the detection of large prey. However, large prey may be too scarce to meet a large bat’s energy requirements. Large bats would (B) have low wing beat frequencies and due to energetic reasons also low call repetition rates. These low call repetition rates, however, many not be sufficient for prey detection. Both hypotheses were tested by analysing the echolocation behaviour of Cheiromeles torquatus (150-200g), the largest aerial insectivorous bat.

Cheriomeles searches for prey by alternating calls with the lowest frequencies at 20 and 28 kHz (call duration 30 and 24 ms respectively) at a repetition rate of 6.3Hz, and a duty cycle of 18%. The data are discussed in relation to the foraging behaviour of Cheiromeles.

The call parameters of Cheiromeles are far removed from predictions made in both models When the data of Cheiromeles and some other large bats are included in an allometric version of model (A) the prediction of the model changes distinctly indicating that call frequency might only be a weak constraint on body size, if at all. Call repetition rate seems to be correlated to call frequency and not directly to body size as is assumed in model (B). In nay case, large bats have lower call repetition rates than wing beat rates and the difference increases with body size. Therefore other factors have to be sought to explain the size distribution of aerial insectivorous bats.