Syllable acoustics, temporal patterns, and call composition vary with behavioral context in Mexican free-tailed bats

Abstract

Recent research has shown that some bat species have rich vocal repertoires with diverse syllable acoustics. Few studies, however, have compared vocalizations across different behavioral contexts or examined the temporal emission patterns of vocalizations. In this paper, a comprehensive examination of the vocal repertoire of Mexican free-tailed bats, T. brasiliensis, is presented. Syllable acoustics and temporal emission patterns for 16 types of vocalizations including courtship song revealed three main findings. First, although in some cases syllables are unique to specific calls, other syllables are shared among different calls. Second, entire calls associated with one behavior can be embedded into more complex vocalizations used in entirely different behavioral contexts. Third, when different calls are composed of similar syllables, distinctive temporal emission patterns may facilitate call recognition. These results indicate that syllable acoustics alone do not likely provide enough information for call recognition; rather, the acoustic context and temporal emission patterns of vocalizations may affect meaning.

Figure 1

Temporal emission patterns of Tadarida vocalizations. (a) and (b) are multisyllabic calls with clumped syllable distributions and syllables are never emitted singly. (c) Monosyllabic calls with a uniform syllable distribution and (d) monosyllabic calls with a random distribution. (a) Maternal directives, (b) irritation calls, (c) mounting calls, and (d) marking calls.

Figure 2

Spectrograms of calls emitted by T. brasiliensis. (a) Three herding calls, (b) four marking calls [also see Fig. 1d], (c) four mounting calls [also see Fig. 1c], (d) three isolation calls, (e) one directive call [also see Fig. 1a], (f) one irritation call [also see Fig. 1b], (g) two protest calls (each call is from a different bat), (h) two warning calls (each call is from a different bat), (i) face-rubbing call, (j) nose-rubbing call, (k) investigation clicks, (l) food click, (m) food solicitation call, (n) two alarm calls (each call is from a different bat), and (o) four echolocation calls.

Figure 3

Territorial∕courtship song. (a) One complete song showing the three types of phrases: chirp, buzz, and trill. (b) Expanded section of a chirp, which has two types of syllables: types A and B. (c) Expanded section of a trill. (d) Expanded section of a buzz.

Figure 4

Spectrograms of syllable types. (a) Clicks, NR=nose rubbing, IN=investigation, and FO=food click. (b) Brief downward frequency modulated syllables (down FMs), IR=irritation, SB=song buzz, ST=song trill, HE=herding, FS=food solicitation, EC=echolocation, and SCA=song chirp type A. [(c) and (d)] Long syllables, WN=warning, AL=alarm, MT=mounting, PT=protest, MK=marking, IS=isolation, FC=face rubbing, DR=directive, and SCB=song chirp type B.

Figure 5

Cannonical functions from discriminant function analyses. (a) All syllables fell into three distinct types. Function 1 was correlated with duration (r=0.99) and function 2 was correlated with peak frequency (r=0.99). The lines depict 80% normal bivariate density ellipses. (b) Down FM syllables. Function 1 was correlated with all variables. Function 2 was correlated with end and peak frequencies. We recognized two syllable subgroups: The first subgroup consisted of echolocation (EC), escape (ES), food solicitation (FS), and song chirp type A (SCA) syllables (open symbols) and the second subgroup consisted of irritation (IR), song buzz (SB), herding (HE), and song trill (ST) syllables (filled symbols). [(c) and (d)] Long syllables. Function 1 was correlated with frequency features, function 2 was most correlated with duration (c), and function 3 was most correlated with maximum frequency (d). AL=alarm, DR=maternal directive, FC=face rubbing, IS=isolation, MK=marking, MT=mounting, PT=protest, and SCB=song chirp type B. The lines depict 80% normal bivariate density ellipses.