The fact that Kerivoula
produced call bandwidths of between 78 and 170 kHz with the first harmonic at repetition rates of up to 200 calls s−1
stands in striking contrast to our present view of echolocation during approach to prey in insectivorous bats. Interestingly, the ‘classical’ pattern was established largely based on temperate species of the same bat family that our seven species belong to—vespertilionids. These likewise emit their buzz calls at maximum repetition rates of 170–200 calls s−1
, but typically only reach bandwidths of 10–30 kHz for the first harmonic (Kalko & Schnitzler 1989
; Schumm et al. 1991
; Kalko 1995
; Surlykke & Moss 2000
; Siemers et al. 2001
). Some bats increase overall bandwidths by shifting energy into higher harmonics (Surlykke et al. 1993
; Siemers & Schnitzler 2000
). The fact that the Kerivoulinae maintain large bandwidths at high repetition rates with only one harmonic might partly be linked to the longer duration of their buzz calls (1.3–1.9 ms versus 0.2–0.8 ms for other species; references as above). This, however, results in even shorter inter-pulse-intervals and higher duty cycles in the Kerivoulinae than in other vespertilionids.
The mean bandwidth of 170 kHz we measured for K. pellucida
buzz calls is the largest bandwidth known for any bat call and, in a more general perspective, for any tonal animal vocalization. The mean starting frequency of these calls was 236 kHz and maximal values reached 250 kHz (cf. b
). These again will be the highest values for a fundamental frequency (= first harmonic) for any known animal vocalization, including all bat echolocation calls. The ability to separate prey from vegetation background is enhanced by broadband calls with high starting frequencies (Siemers & Schnitzler 2004
), and we therefore suggest that the extreme vocal performance of the Kerivoulinae and Murininae evolved as an adaptation to echolocating and tracking arthropods in the dense rainforest understorey. How they overcame the trade-off between repetition rate and bandwidths is unclear as yet. Potentially, a specialization in their vocal apparatus allows these bats to maximize both parameters at the same time. It is interesting to note that the Murina
attained the same good performance in catching tethered prey close to background as Kerivoula
with similar broadband calls, but lower final call rate. The European M. nattereri
sustains bandwidths of more than 100 kHz at about the same low call rate during the initial part of the buzz (buzz I), but then drops bandwidths as repetition rate increases with the onset of buzz II (Siemers & Schnitzler 2000
). The species under study use high bandwidths and high call rate already during search (Kingston et al. 1999
), and approach sequences in our experiments lasted as long or longer than typical approaches in other vespertilionid bats (e.g. Kalko 1995
). Thus these bats can produce broadband calls with short intervals over extended periods of time.
In addition to the lack of bandwidth reduction, a second ‘typical’ feature of vespertilionid buzzes was absent in the Malaysian bats: there was no drop in call frequency. This drop of the first harmonic had been discussed both as the result of vocal limitation (Griffin et al. 1960
; Schumm et al. 1991
) and as an adaptive trait to achieve higher modulation rates with the second harmonic that would then cover the frequency range that is crucial for echo perception (Surlykke et al. 1993
). Clearly, these rainforest bats do not need to drop call frequency for successful prey capture.
The case of the vespertilionids Kerivoula, Phoniscus and Murina highlights the great diversity of echolocation systems and their manifold ecological adaptations—providing surprises even within the putatively well-studied groups of bats. It will be interesting to look for corresponding adaptations in African members of the Kerivoulinae and in the phylogenetically distant, but ecologically similar neotropical rainforest bats.