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The face that sank the Essex: potential function of the spermaceti organ in aggression

David R. Carrier^*, Stephen M. Deban and Jason Otterstrom

Department of Biology, 201 South Biology Building, University of Utah, Salt Lake City, UT 84112, USA

View larger version (39K): [in a new window]   Fig. 1. Diagram of the anatomy of the head of the sperm whale Physeter macrocephalus. The relative size and position of the two oil-containing compartments of the spermaceti organ, the spermaceti and junk, are shown in relation to the skull. Modified from Clarke (1979).

View larger version (23K): [in a new window]   Fig. 2. Phylogenetic hypothesis of the taxa used in this study compiled from Messenger and McGuire (1998) and Irwin and Arnason (1994). Species in which head-butting has been observed during male—male aggression are marked with a filled circle. See text for references. The numbers at the terminal branches represent the percentage sexual dimorphism in body length (top) and the percentage of the lateral projected body area made up by the melon (bottom). Numbers at nodes represent the contrast values for the node. The contrast value for a given node of the tree was calculated by taking the average of the descendant nodes along one bifurcation and subtracting it from the average along the other bifurcation and dividing this difference by a measure of the variance weighted by the lengths of the subsequent branches (Felsenstein, 1985).

View larger version (7K): [in a new window]   Fig. 3. Diagram of measurements taken to determine relative melon area. The shaded area is the lateral projected area of the melon. The index of body area is the head depth times the body length, measured from the eye to the tip of the flukes.

View larger version (13K): [in a new window]   Fig. 4. Independent contrast analysis of evolutionary change in relative melon area and evolutionary change in sexual dimorphism in body length for 21 species of cetacean. Positive values on the y-axis represent the evolution of an increase in the lateral projected area of the melon relative to the lateral projected area of the rest of the body. Negative values on the y-axis represent the evolution of a decrease in the lateral projected relative area of the melon. Positive values on the x-axis represent the evolution of longer males relative to females and negative values represent the evolution of shorter males relative to females. This result shows that species that have evolved relatively larger males tend also to have evolved relatively larger melons.

View larger version (21K): [in a new window]   Fig. 5. Graph of the modeled peak accelerations of the target and attack whales during a collision at 3 m s-1 for different ratios of damping constants. Each line extending from the origin illustrates the accelerations that the model produced for a given ratio of damping constants for the tissue and spermaceti dampers. The number beside each line is the ratio of the tissue damping constant of the target whale to the spermaceti damping constant of the attacking whale. The magnitude of damping increases as the lines extend from the origin. As the ratio becomes larger, the accelerations experienced by the target whale become larger relative to the accelerations of the attacking whale. Twice the acceleration due to gravity (2g=19.6 m s-2) is the estimated acceleration above which fatal injury is likely to occur for a vertebrate the size of the modeled whales (Farlow et al., 2000). Hence, the area above the horizontal line represents accelerations that would probably injure the target whale and but not the attacking whale. The filled circle on the line for a damper constant ratio of 16 represents the damping constants used for the sample acceleration traces shown in Fig. 6.

View larger version (12K): [in a new window]   Fig. 6. Sample acceleration traces from the model of a collision of two 39 000 kg sperm whales. The acceleration experienced by the target whale is shown with the solid line and the acceleration experienced by the attacking whale is shown by the dashed line. In this case, the ratio of the damping constants was 16 (2 040 480 N s m-1 for the tissue damper and 127 530 N s m-1 for the spermaceti damper).

View larger version (146K): [in a new window]   Fig. 7. Frontal view of a stranded bull sperm whale showing the location of scars on the anterior aspect of the spermaceti organ. The bull is lying on its left side. The relative size and shape of the two major compartments of the organ can be distinguished: (A) junk and (B) spermaceti. Note the `battering-ram'-like appearance of the junk compartment and that the scars are largely restricted to the junk. The scars probably result from tooth-raking during male—male aggression as has been suggested by Best (1979) and Kato (1984). The scars are usually concentrated on the anterior end of the junk, suggesting that tooth-raking occurs during head-butting. If the scars were the result of jaw-to-jaw wrestling matches, which have been observed to occur between male sperm whales (Berzin, 1972), we would have expected them to be concentrated at the angle of the mouth. Reprinted with permission from Møhl (2001).