ECHOLOCATION IN 2 VERY SMALL BATS FROM THAILAND - CRASEONYCTERIS-THONGLONGYAI AND MYOTIS-SILIGORENSIS

The echolocation and hunting behavior of two very small bats, Craseony cteris thonglongyai (Hill) and Myotis siligorensis (Horsfield), from T hailand, were investigated using multiflash photographs, video, and hi gh-speed tape recordings with a microphone array that allowed determin ation of distance and direction to the bats. C. thonglongyai is the wo rld's smallest mammal and M. siligorensis is only slightly larger. Bot h bats hunted insects in open areas. The search signals of C. thonglon gyai were 3.5 ms long multiharmonic constant frequency (CF) signals wi th a prominent second harmonic at 73 kHz repeated at around 22 Hz. The band width (BW) of the short terminal frequency modulated (FM) sweep increased during the very short approach phase. In the final buzz the CF component disappeared, the duration decreased to 0.2 ms, and the re petition rate increased to 215 Hz (Figs. 2, 3, 4). There was no drop i n frequency in the buzz. The video recordings of C. thonglongyai indic ated that it seizes insects directly with the mouth (Fig. 1). M. silig orensis produced 5.4 ms long CF search signals at 66 kHz. The repetiti on rate was around 13 Hz. In the approach phase an initial broad band FM sweep was added. The buzz consisted of two phases, buzz I and buzz II. Buzz II was characterized by short cry durations (around 0.3 ms), a constant high repetition rate (185 Hz), a distinct drop in frequency , and a prominent second harmonic (Figs. 5, 6, 7). The drop in frequen cy, apparently typical of vespertilionid bats, has been explained by p hysiological limitations in sound production. However, C. thonglongyai produced very short signals at very high repetition rates without any frequency drop. The drop may be of adaptive value since it enables M. siligorensis to produce very short signals with high sweep rates. The drop moves the pronounced second harmonic into the frequency.range of most interest to the bat (Fig. 7D). The sweep rate in this frequency range may now increase to twice the maximum rate that the vocal cords can produce directly. C. thonglongyai and M. siligorensis belong to di fferent superfamilies, Emballonuroidea and Vespertilionoidea, respecti vely. In spite of their phylogenetic distance they produce strikingly similar search signals of narrow BW around 70 kHz with high source lev els (100-115 dB peSPL peak equivalent sound pressure level). We argue that the signal resemblance is due to the similarity in size and hunti ng behavior of the two bats both hunting insects in open areas. High f requencies are heavily attenuated in air, but because of their small s ize the bats are restricted to hunting small insects which only reflec t echoes at high frequencies. Thus, the emitted frequency is probably the lowest possible given the prey size. Hence, the two bats can only maximize the range of their sonar by decreasing the BW and emitting hi gh intensities.