Time patterns of sperm whale codas recorded in the Mediterranean Sea 1985-1996

A distinctive vocalization of the sperm whale, Physeter macrocephalus (=P. catodon), is the coda: a short click sequence with a distinctive stereotyped time pattern [Watkins and Schevill, J. Acoust. Soc. Am. 62, 1485-1490 (1977)]. Coda repertoires have been found to vary both geographically and with group affiliation [Weilgart and Whitehead, Behav. Ecol. Sociobiol. 40, 277-285 (1997)]. In this work, the click timings and repetition patterns of sperm whale codas recorded in the Mediterranean Sea are characterized statistically, and the context in which the codas occurred are also taken into consideration. A total of 138 codas were recorded in the central Mediterranean in the years 1985-1996 by several research groups using a number of different detection instruments, including stationary and towed hydrophones, sonobuoys and passive sonars. Nearly all (134) of the recorded codas share the same "3+1" (/// /) click pattern. Coda durations ranged from 456 to 1280 ms, with an average duration of 908 ms and a standard deviation of 176 ms. Most of the codas (a total of 117) belonged to 20 coda series. Each series was produced by an individual, in most cases by a mature male in a small group, and consisted of between 2 and 16 codas, emitted in one or more "bursts" of 1 to 13 codas spaced fairly regularly in time. The mean number of codas in a burst was 3.46, and the standard deviation was 2.65. The time interval ratios within a coda are parameterized by the coda duration and by the first two interclick intervals normalized by coda duration. These three parameters remained highly stable within each coda series, with coefficients of variation within the series averaging less than 5%. The interval ratios varied somewhat across the data sets, but were highly stable over 8 of the 11 data sets, which span 11 years and widely dispersed geographic locations. Somewhat different interval ratios were observed in the other three data sets; in one of these data sets, the variant codas were produced by a young whale. Two sets of presumed sperm whale codas recorded in 1996 had 5- and 6-click patterns; the observation of these new patterns suggests that sperm whale codas in the Mediterranean may have more variations than previously believed.