An unusual case of care-giving behavior in wild long-beaked common dolphins (Delphinus capensis) in the East Sea
Supportive behavior in cetaceans refers to care or help provided by a healthy individual to a disabled or sick individual. Cremer et al. (2006) divided the care-giving behaviors of dolphins into two types. The most frequent type involves care by a mother dolphin for her calf in difficulty, whereas the second type involves the support of an adult dolphin to maintain a dead dolphin afloat. Care-giving behavior of a mother dolphin to a calf, known as nurturance behavior, has been reported previously. For example, Caldwell and Caldwell (1966) discussed the report of McBride and Kritzler (1951), which described a bottlenose dolphin, Tursiops truncatus, in captivity attempting to lift her stillborn calf to the surface with her back. Moura et al. (2009) reported a case of care-giving behavior in rough-toothed dolphins (Steno bredanensis) in Brazilian waters. Most descriptions of nurturance behavior are of adult bottlenose dolphins supporting a dead or stillborn calf at the surface (Tayler and Saayman 1972, Cockcroft and Sauer 1990, Wells 1991).
All previously reported observations of care-giving behavior appear to have involved only one or two adult animals, which exhibited rescue activities for calves by pushing the animal to the surface or stimulating it by biting. Here we describe the behavior of several common dolphins (Delphinus capensis) toward a dying conspecific, including attempts to support the stricken individual.
Our group has been conducting sighting surveys to accumulate information on the distribution and abundance of small cetaceans every other month in the coastal area of the East Sea since 2004. The survey was in the region of the observation site in Figure 1. Using the R/V Tamgu No. 12 (70 GT), we cruised track lines while observing from the upper bridge at 4 m above sea level. A DSLR camera (Nikon D2Xs with Nikkor 80 –200 mm and 17–55 mm lenses) and a camcorder (Sony HDR-FX7) were used to identify the observed cetaceans and record their behaviors. Identification of individuals and behavior analysis were accomplished by using both photographs and video tape in the lab.
On 27 June, during a survey conducted from 26 June to 1 July 2008, we encountered two groups of long-beaked common dolphins, Delphinus capensis. The second group, sited 14 km from the coast of Korea (35º42’20”N, 129º38’42”E), contained approximately 400 animals. More than 500 streaked shearwaters, Calonectris leucomelas, were following the common dolphins, which were foraging and moving together towards the southeast. The vessel approached the school several times for observations, during which several dolphins would occasionally approach and swim alongside the bow of the boat. At least four pairs swimming at the rear of the group appeared to be mothers with a calf, based on the size of individuals in each pair.
When we had traveled approximately 3 km southeast from the first sighting point, we noticed that several dolphins had gathered close together and were splashing in the water on the right side of the vessel. This group comprised at least 12 animals that were swimming very slowly (at 1143 local time). Among them, one dolphin was wriggling about, with its body inclining to one side or the other and its abdomen showing at the surface. A group of 5–10 animals was swimming slowly around the perimeter of the dense cluster of dolphins.
Based on distinguishing characteristics (wounds, scars, white patches, and fin shape) we assigned several dolphins in this group identification codes for inclusion in our photo-ID database. The wriggling dolphin was identified and cataloged as EC08001 in the photo-ID database. Nine other dolphins in the group were identified by their wounds, scars, white patches, and shape of their fins. Two of them were very notable; one had a severely damaged dorsal fin (EC08002) and the other had a scar around its dorsal fin (EC08003). We did not detect any external wounds or scars on EC08001 during the observations or from the photos and videos that were examined in the laboratory after the survey. We were able to confirm from video that EC08001 was a female of approximately 1.7 m body length, by comparing with the interval between the handrails on the starboard side of the boat. Based on the location of the gonads EC08003 was presumed to be a male. When the vessel approached within approximately 200 m of EC08001, the dolphin showed its belly and we noted that the white parts of its abdomen were slightly more reddish than those of the other dolphins in the group. When EC08001 rolled its body and pounded the surface water by waving its fluke, the pectoral flippers did not move and appeared to be paralyzed.
Approximately 12 dolphins closely crowded EC08001, while between 5 and 10 other dolphins circled the cluster, alternately joining and departing from the central group. The dolphins crowded in the center appeared to help EC08001 maintain its balance by pushing it from the side and supported its body from below using their bodies (see supplemental video).
When the dolphins in the outer group left the size of the central group decreased to 11 animals, including EC08003. EC08002 was no longer seen. The remaining dolphins began to arrange their bodies horizontally, assuming a raft-like formation, which supported EC08001 from below (1217, Fig. 2). This formation was maintained while EC08001 moved onto the raft, and one dolphin, which positioned itself below EC08001 turned upside-down. The individual in this position was alternated among several dolphins, including EC08003. The group remained in this organized formation for 3 min, and then began to disperse one by one. When only five animals remained in the central group, one tried to hold EC08001’s head up with its beak while EC08003 and another one turned their bellies to EC08001’s (1220; Fig. 3). The dolphins came within 1 m of the research vessel, but did not appear to be bothered by the vessel or the observers. There was no apparent injury or bruise on the flippers, fluke, or body of EC08001 that would indicate an attack or a ship strike. EC08003 turned its belly to EC08001 while it kept swimming along the left side of EC08001.
At 1221, EC08001’s body was vertical in the water with its head above the surface, and the dolphin did not appear to be breathing, at which point we concluded that EC08001 was dead. Five dolphins, including EC08003, continued to interact with EC08001 by rubbing and touching its body, even though ECO08001 appeared to show rigor mortis (Fig. 4). At 1224, two or three dolphins moved under the carcass and released bubbles. At this point, we attempted to remove the body from the water, but it sank and disappeared at 1227. Although we could no longer see the body, five dolphins remained at the site where EC08001 sank. We left to resume our survey at 1232. EC08003, identified clearly by its notable scar on the dorsal fin, which had been a part of the rescue group from the beginning, was one of the remaining dolphins. EC08009, identified from the raft-formation stage, appeared frequently and left with EC08003.
While we conclude that the behaviors directed toward the stricken individual included attempts to support it, contact between the ventrum of some of the surrounding dolphins and EC08001 suggest a possible sexual component to some of the interactions. We identified one of these dolphins (EC08003) as a male. However, we did not detect any erections in our video and photo review.
There are few studies about social structure of D. capensis. Viricel et al. (2008) reported that the social organization of D. delphis does not appear to be matrilineal or based on kinship. Even though the sex of the dolphins surrounding EC08001 was not confirmed, their behaviors and the role in the raft-formation were very similar to those of EC08003. In the early stages, around 20 animals of the 400 in the large group participated in care-giving behavior. No calves were involved. When EC08001 died and five dolphins were left, they separated into two groups, EC08003 with EC08009 and the other three dolphins.
Most previous reports of care-giving behavior comprise observations of instinctual parental behavior by one or two adult dolphins, mostly bottlenose dolphins, to support a dead calf at the surface. For example, Félix (1994) reported the case of a bottlenose dolphin that carried a dead calf. In that case, however, no other dolphins assisted, even though others were foraging very close by. Although Félix (1994) was not able to determine the relationship and/or the sex of the animals involved in the care-giving behavior, he presumed that the adult animal supporting the dead calf was its mother.
Previous reports of dolphins carrying dead group members involved either bottlenose dolphins or other toothed dolphins in Brazil. In addition, the calves or young dolphins that have been the subjects of care-giving behavior have exhibited fresh parallel scratches on their bodies from the adults’ teeth (Félix 1994, Cremer et al. 2006, Moura et al. 2009). No tooth scratches were visible on the flippers or fluke of EC08001 and no helpers were observed biting or scratching the body when it was floating on the surface.
Additional research on common dolphin societies in the wild as well as the competition of dolphins is needed to expand our understanding of this type of apparent altruism.