Individual photographic identification : a key to the social organization of sperm whales

Arnbom, Tom Arne (1987) Individual photographic identification : a key to the social organization of sperm whales. Masters thesis, Memorial University of Newfoundland.

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Abstract

Sperm whales were tracked visually and acoustically in the waters west of the Galapagos between February and April 1985. A method for photographically identifying individual sperm whales is described. Measures of the photograph quality were compared with the certainty with which individuals were identified. A total of 210 females or immatures, 7 large adult males and 6 calves were recognized with certainty and individually identified. A simple model suggested that up to 9% of the females/immatures could not be identified using this method of photographic identification, despite high quality photographs. It was shown that these individuals have a lower number of unique marks on their flukes than the 210 identified females/immatures. The assumption of random sampling when taking photographs of individual sperm whales is discussed. The time and geographical positions of the re-sightings of known individuals suggest that the sperm whales preferred a rich upwelling area. -- The identified females/immatures were clustered into 23 discrete groups. Thirteen of these groups contained more than six associated members. Observations of calves and the high frequency of dorsal fins with a callus suggested that the groups of sperm whales off the Galapagos fell into the category of "mixed groups". Whales recorded as escorting a calf were most probably females. Different females/immatures were observed to escort the same calf, and identified females/immatures were observed with several different calves. -- Large males were observed either as singles, pairs or a set of three. In the observations of identified individuals there was no indication that particular pairs of large males, or large males of a similar size, were preferred companions. No fresh wounds or agonistic behaviour between large males was observed. The lack of sightings of medium-sized males suggest that they do not take part in reproduction in this area. The proportion of large males to mature females suggests that all large males do not migrate to the breeding grounds and do not participate in breeding every year. Identified large males were observed with different mixed groups and, further, different large males were associated with particular mixed groups. There was no indication that some mixed groups associated more with large males, than others. Large males seemed to follow a strategy of searching for mixed groups, instead of holding harems. -- During an attack by killer whales on sperm whales a high degree of coordination of the sperm whales' behaviour was noted. Twenty-one percent of the sperm whale flukes had tooth mark scars of which a majority were probably derived from shark attacks. A difference in the number of unique marks on the flukes between different geographical areas suggests that the method of individual photographic identification relying on uniquely marked flukes may be less successful in other areas.

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