KILLER WHALES (ORCINUS ORCA) OF PATAGOAND THEIR BEHAVIOR OF INTENTIONAL STRANDING

WHILE HUNTING NEARSHORE

Juan Carlos López and Diana López

Punta Norte, Península Valdes, Chubut, República Argentina.

Although killer whales are found in all oceans, until recently they have not been studied intensively in any habitat. A review of the literature was supplied by Martinez and Klinghammer (1970), who also listed some of the known prey of killer whales. Prey included many species of fish, squid, pinnipeds, birds and cetaceans. Even remains of a leatherback sea turtle (Deermochelys coriacea) have been found in a killer whale’s stomach (Caldwell and Caldwell, 1969).

Since the early 1970s., more detailed observations on the behavior and social organization of killer whales populations have been made. Most of this work, begun by Bigg (Bigg et al., 1976, Bigg. 1982), and subsequently pursued by Balcomb (Balcomb et al., 1980, 1982), involved animals of the Puget Sound area of northwest Washington State. Bigg was the first researcher to identify individual killer whales by natural markings on the dorsal fin and pigmented "saddle patterns" of the dorsum. In 1975, we initiated a study of killer whales which frequent the near-shore areas of Punta Norte, Peninsula Valdes, in southern Argentina ((42º 05’S, 63º 47’W). This study relied on identification of whales from photographs of their dorsal fins and backs, and on detailed descriptions of behavior as seen through binoculars from shore. Here we provide a brief summary of our findings and report one striking behavioral event, intentional stranding while hunting.

Our observation area at Punta Norte encompasses 4 km of coast. The pebble beaches of this coastline are inhabited by southern elephant seals (Mirounga leonina) and southern sea lions (Otaria flavescens). Most elephant births occur in September and October (Scolaro, 1976), and most sea lions births occurs in January and February (Ximenez, 1976). During parturition and for a few weeks after, when the pups begin to enter the water, killer whales are frequently found close to the coast where they prey on pups and small juvenile pinnipeds, although adult sea lions are taken at times.

We identified 26 killer whales from distinguishing marking on the dorsal fin and dorsum. Six were adult males, as evidenced by their high and erect dorsal fins(Bigg et al., 1976). Five were probably females, because they were either traveling with infants, or engaged in sexual behavior with adult males. Eight were calves or juveniles, and seven were of unknown sex and age. We estimate that the total number of killer whales in this zone is probably not more than 30 adult and juvenile animals.

Thirty-four (3.8%) of 904 sightings of killer whales were of solitary animals. Whales generally fed while in groups of 2-12 animals. Groups were defined as individuals swimming within 100 m of each other. The average number or individuals per group was 3.2 (SD=3.6). Groups of two individuals were sighted most frequently, 410 times (45%).

Killer whales in coastal water near elephant seals and sea lions rookeries often appear to hunt as a cooperating group, with several animals flanking the intended prey to keep it from escaping, while one or more whales chase and capture it (Martinez and Klinghammer, 1970; Condy et al., 1978). During 904 sightings of killer whales near Punta Norte encompassing 936 h, we observed 568 incidences of hinting, and witnessed sea lions or elephant seals caught and killed 181 times (31.9%). Thirty-three (5.8%) hunting incidences were attempted by a particular solitary male. He was successful on 17 attempts. The remaining 535 (94.2%) hunting incidences involved cooperative group attacks, and 164 (30.6%) of these resulted in the capture and death of pinnipeds.

Of the 568 hunting attempts, 203 (35.7%) involved two or more whales in some way corralling pinnipeds in the water. Usually, one or more animals formed a wall in deeper water, and kept the pinniped from escaping in that direction. One killer whale then rushed at the pinniped, which attempted to flee parallel to shore or by beaching itself. Forty-two of the 203 (20.7%) corralling attempts in water were successful. We do not know to what extent food was shared by the whales, since much feeding may have taken place underwater.

Of the 568 hunting attempts, 365 (64.3%) involved one or more killer whales beaching themselves as they rushed toward pinnipeds in the surf zone or in the shore. One hundred twenty-four (34.4%) of these beachings ended in the successful capture of a sea lion or elephant seal. Condy et al. (1978) described a killer whale which unsuccessfully pursued and elephant seal pup onto shore on an island in the Indian Ocean. Because beaching by killer whales are not been described in detail, we do so here.

Typically, the killer whale about to beach itself swam toward shore, actively directing itself toward the prey, and occasionally surfing on an advancing wave. Sometimes other whales milled to the sides of the beaching whales, possibly to keep the prey from escaping in their direction. The whale caught the prey in the tumultuous surf zone or within an advancing wave on the beach, but usually while the prey was still in water. Because the killer whale is much larger than the prey, the killer whales "beached" or grounded while water was still flowing around it, and while the prey was still capable of swimming. Once grounded, the whale arched its body, with the head and tail lifted up, and rocked sideways. This motion usually oriented it parallel to the beach, and a subsequent wave helped to lift it off the bottom. It then swam back into deeper water, carrying the prey in its mouth if it had been successful at capture. None of the whales we observed became permanently stranded, it usually took only two or four large waves to help dislodge grounded animal.

The advantages for the killer whales appear clear; sea lions and elephant seals are clumsy on land and during the transition from swimming to walking in shallow (and specially turbulent) water. Killer whales demonstrate a surprising agility in rotating and flexing their bodies sideways while pursuing the prey. This agility appears to outweigh the disadvantage of being in the turbulent surf zone, and being almost or totally stranded.

A variation of intentional beachings was observed during 27 sessions, with each session including from one to 14 beachings (X=4:SD=4.7). Immature killer whales swam directly toward pinnipeds on the coast, became stranded only centimeters from the pinnipeds, and generally remained next to them for 5-10 seconds. These beachings occurred in areas where an adult killer whale could have been successful in trapping a pinniped. In the 17 of these sessions (62.9%), an adult male floated stationary on the water’s surface within 50-100 m of the shore line, its head pointing in the direction of the site where the juvenile killer whale was beached. In the seven of 17 instances (41.2%), the adult male also charged towards the beach after the juvenile had charged from three to eight times. In these instances, the adult male did not capture its prey, and returned to its previous position. The juvenile then repeated the beaching behavior. Hoese (1971) suggested that young bottlenose dolphins learn to feed during similar types of intentional beachings.

On six occasions, we observed both an adult and a juvenile whale stranding themselves at the same time, separated approximately 4 m from each other, and capturing their prey independently. Each time, the juvenile killer whale captured a sea lion calf, and the adult did. During these hunts, the adult male lifted its prey with a lateral movement of its head, and flung it toward the juvenile killer whale which was positioned on its right side. The juvenile did not catch the flung prey, but held on to the sea lion it had caught itself. Several times, an adult flung a captured live sea lion toward a juvenile which had not caught its own prey, and the juvenile pushed the prey with its head or body, or captured it in its mouth. These observations with adults and juveniles interacting lead us to postulate that the adults may be teaching the young to hunt.

The beaching of killer whales in order to secure food at Punta Norte is an important part of their behavior. Overall, they were more successful at capturing prey by beaching (34.4% success) than by cooperative hunting in water (20.7% success), and since the behavior of beaching requires a high degree of agility, we do not find it surprising to find indications of adults teaching this behavior to their young.

LITERATURE CITED

BALCOMB, K.C., III, J.R. BORAN, R.W. OSBORNE, AND N.J.HAENEL. 1980. Observations of killer whales (Orcinus orca) in greater Puget Sound, State of Washington. NTIS, Springfield, VA. 42 pp.

BALCOMB, K.C., III, J.R. BORAN, AND S.L.HEIMLICH. 1982. Killer whales in greater Puget Sound. Rep.Int.Whal. Comm., 32:681-685.

BIGG, M.A. 1982. An assessment of killer whale (Orcinus orca) stocks off Vancouver Island, British Columbia, Rep.Int.Whal. Comm., 32:655-666.

BIGG, M.A., I.B.MACASKIE, AND G. ELLIS. 1976. Abundance and movements of killer whales off eastern and southern Vancouver Island, with comments on management. Arctic Biol.Sta., Ste. Anne de Bellevue, Quebec. 20 pp.

CALDWELL, D.K., AND M.C. CALDWELL. 1969. The addition of the leatherback sea turtle to the known prey of the killer whales, Orcinus orca. J.Mamm., 50:636.

CONDY, P.R., J.J. VAN AARDE, AND M.N.BESTER. 1978. The seasonal occurrence and behavior of killer whales, Orcinus orca, at Marion Island. J.Zool., 184:449-464.

HOESE, A.D. 1971. Dolphin feeding out of water in a salt marsh. J? Mamm., 52:222-223.

MARTINEZ, D.R., AND E. KLINGHAMMER. 1970. The behavior of the whale Orcinus orca: a review of the literature. Z. Tierpsychol., 27:828-839.

SCOLARO, J. A. 1976. Censo de elefantes marinos (Mirounga leonina, L.) en el territorio continente Argentina. Centro Nac. Patagonico Informe Tecnico 1.4.2., 12 pp.

XIMENEZ, Y. 1976. Dinámica de la población de Otaria flavescens (Shane) en el área de Península Valdés y zonas adyacentes. Centro Nac. Patagónico Informe Técnico 1.4.1., 55 pp.

Submitted 9 January 1984. Accepted 4 July 1984