This Case Study was prepared for the Case Studies session of the 1997 Annual Meeting of the Statistical Society of Canada in Fredericton, New Brunswick. Some of the analyses presented there will appear in a forthcoming issue of The Canadian Journal of Statistics, including an analysis co-authored by Joe Farruggia, Peter Macdonald, and Roman Viveros-Aguilera.
Seals possess varied repertoires of underwater vocalisations. Geographic variation in call types have been reported for Weddell and bearded seal species, and the variations have been attributed to the isolation of breeding populations within these species.
The present study is concerned with harp seals (Phoca groenlandica), and in particular the herds from Jan Mayen Island, Gulf of St. Lawrence and Front. The data comes to us courtesy of Dr Jack (John) M. Terhune, Department of Biology, University of New Brunswick, St. John.
Tagging and morphometric studies suggest little exchange between the Jan Mayen Island, Gulf of St. Lawrence and Front harp seal herds. Vocalisation differences among these three herds would provide evidence that the groups are reproductively isolated.
A difficulty is that researchers do not know what constitutes a single vocalisation (or call) type, nor do they know how many call types seals have. Previously vocalisations were classified by a researcher listening to tape recordings and subjectively categorising the calls. A problem with this is that calls occur along a continuum and different researchers have varying criteria for what they think constitutes a different call type. As regards using calls to identify discrete seal populations, if the definitions of calls are very specific (resulting in a large number of call types) then between-herd differences may be exaggerated, while if call types are very broadly defined then actual interherd differences may be missed. A related issue is that the repertoire often has to be defined before other research on the potential behavioural effects of the vocalisations can be addressed. An objective classification method using only a few easily measured variables would be useful.
Several pictures of the spectrographic pattern of underwater vocalisations can be found in Terhune (1994). In the data set described below each picture is represented by several summary measures.
The data were obtained from underwater recordings of harp seals in three herds. One thousand calls from each of the three herds (Jan Mayen, Gulf of St. Lawrence and Front) were recorded, and several features of each recording were noted. They are:
v1 ELEMDUR - this is the duration of a single element of a harp seal underwater vocalisation. It is measured in milliseconds.
v2 INTERDUR - this is the time between elements in multiple element calls. It is measured in milliseconds. Note that not all calls have multiple elements so this variable is absent in single element calls. Where absent, a value of NA is recorded in the data.
v3 NO_ELEM - this is the number of elements of the call. In harp seals all of the elements within a single call are similar and the spacing between them is constant.
v4 STARTFREQ - this is the pitch at the start of the call or the highest pitch if the call has an extremely short duration (call shape 0 below). This variable is measured in Hertz (Hz) but it probably should be converted to octaves using the following formula:
octave = log2(Hz)
where the logarithm is base 2. This equates 16 Hz as octave 4, 32 Hz as octave 2, 4096 Hz as octave 9 etc. The purpose for converting from Hz to octaves is that mammals hear in terms of octaves. One octave is a doubling of the pitch. Thus the difference between 16 and 32 Hz is the same as that perceived between 1 and 2 kHz.
v5 ENDFRE - this is the pitch at the end of the call or the lowest pitch if the call has an extremely short duration (call shape 0). It also should probably be converted to octave band measures.
v6 WAVEFORM - this codes a series of waveform shapes (a plot of amplitude vs time) which lie more or less along a continuum. The waveform shapes are:
frequency modulated sinusoidal 9 slight frequency modulated and complex 8 sinusoidal (pure tone) 7 complex (irregular waveform) 5 amplitude pulses 4 burst pulses 3 knock (short burst pulse) 2 click (very short duration) 1
v7 CALLSHAP - this codes a series of call shapes as they would appear in a sonogram spectral analysis (a plot of frequency vs time). The shapes lie along a continuum. Some of the shapes and their codes are (approximately):
/ 9 _/ 8 _ / 7 /\ 6 _ or . 5 \/ 4 \_ 3 _ \ 2 \ 1 | 0
v8 HERD - this is the herd from which the recordings were obtained. The codes are as follows:
Jan Mayen Island 1 Gulf of St. Lawrence 2 Front 3
Terhune, J.M. (1994) Geographical variation of harp seal underwater vocalisations, Can. J. Zoology 72(5) 892-897.
Bruce Smith
Departmentt of Mathematics & Statistics
Dalhousie University
Halifax, NS B3H 3J5
Phone: (902) 494-3339
FAX: (902) 494-5130