Group Raids: a Mating Strategy of Male Southern Sea Lions

[Southern sea lions, Otaria byronia, breed in a narrow zone along the high tide mark where males sequester females, defending them against competitors that congregate on the periphery. During four breeding seasons, 15 December to 10 February, 1983-1986, we observed marked animals at Punta Norte, Peninsula Valdes, Argentina, and recorded the form, frequency and consequences of an unusual mating strategy: groups of males raid the breeding area attempting to seize females from resident, territorial males. Raids were often triggered by one male rushing into the breeding area. Females fled from the point of attack and were separated from their pups and were redistributed within the breeding area. Some resident males gained females, others were deposed and lost females, while some raiders gained females and residency in the breeding area. A mean of 144 (sd = 22) raids were observed per breeding season, with 57% of them occurring at peak season, 14-26 January. During this period, raids occurred at a rate of one every two hours of observation. Raids occurred at all hours of the day and at night, the rate, time and site of occurrence being associated with the location and number of females in oestrus, and with tide. The mean size of the raiding party was 10 males (sd = 8, range = 2-40 males); 66 % of the males were subadults. In 36% of 355 raids, at least one female was seized and held for at least a few minutes; in the remainder, all raiders were repelled. There were three outcomes when a raider seized a female: 1) In 53% of the seizures, the male lost the female in less than one hour and was driven out; 2) in 8% of the seizures, the raider could not withstand the attacks of resident males and herded the females a few metres to the periphery of the breeding area, and 3) in 39% of the seizures, the raider remained in the breeding area with one or more females. Subadult raiders were not successful in securing females but in 18% of 484 raids, they seized a pup. Fifty seven percent of the time, pups were abducted from the breeding area and held forcibly likc adult females. At least five of them were killed. Raiders that secured a female during a raid remained in residency a mean of 13 days (sd = 9), a significantly shorter tenure than that of males that set up residence in the breeding area at the start of the season (X = 24 days, sd = 5). Group raids were an effective mating strategy for some participants. The probability of copulating was higher when a male raided the breeding area in a group than when alone. Raiders that seized females and became residents copulated 1.5 times per 100 hrs in the breeding area, achieving a mean of 2.2 copulations per male (variance = 6.0). Founder residents copulated at a similar rate, but were more successful than raider-residents due to longer tenures, achieving a mean of 5.3 copulations per male (variance = 12.3). The incidence and success of raids as a mating strategy was partially dependent on topography. At Puerto Piramide, where males defended territories containing tidepools and access to the breeding area was limited to a few sites at high tide, raids were infrequent and ineffective. Raiders synchronized their rush into the breeding area, keying on the first male to move. However, no alliances or coalitions, suggesting cooperation, were formed. Group raids consist of a collective revolt against the status quo, similar in form to troop takeovers by groups of male hanuman langurs. Outside males can achieve proximity to oestrous females with less risk of injury by raiding in a group rather than by attempting to seize a female by themselves., Southern sea lions, Otaria byronia, breed in a narrow zone along the high tide mark where males sequester females, defending them against competitors that congregate on the periphery. During four breeding seasons, 15 December to 10 February, 1983-1986, we observed marked animals at Punta Norte, Peninsula Valdes, Argentina, and recorded the form, frequency and consequences of an unusual mating strategy: groups of males raid the breeding area attempting to seize females from resident, territorial males. Raids were often triggered by one male rushing into the breeding area. Females fled from the point of attack and were separated from their pups and were redistributed within the breeding area. Some resident males gained females, others were deposed and lost females, while some raiders gained females and residency in the breeding area. A mean of 144 (sd = 22) raids were observed per breeding season, with 57% of them occurring at peak season, 14-26 January. During this period, raids occurred at a rate of one every two hours of observation. Raids occurred at all hours of the day and at night, the rate, time and site of occurrence being associated with the location and number of females in oestrus, and with tide. The mean size of the raiding party was 10 males (sd = 8, range = 2-40 males); 66 % of the males were subadults. In 36% of 355 raids, at least one female was seized and held for at least a few minutes; in the remainder, all raiders were repelled. There were three outcomes when a raider seized a female: 1) In 53% of the seizures, the male lost the female in less than one hour and was driven out; 2) in 8% of the seizures, the raider could not withstand the attacks of resident males and herded the females a few metres to the periphery of the breeding area, and 3) in 39% of the seizures, the raider remained in the breeding area with one or more females. Subadult raiders were not successful in securing females but in 18% of 484 raids, they seized a pup. Fifty seven percent of the time, pups were abducted from the breeding area and held forcibly likc adult females. At least five of them were killed. Raiders that secured a female during a raid remained in residency a mean of 13 days (sd = 9), a significantly shorter tenure than that of males that set up residence in the breeding area at the start of the season (X = 24 days, sd = 5). Group raids were an effective mating strategy for some participants. The probability of copulating was higher when a male raided the breeding area in a group than when alone. Raiders that seized females and became residents copulated 1.5 times per 100 hrs in the breeding area, achieving a mean of 2.2 copulations per male (variance = 6.0). Founder residents copulated at a similar rate, but were more successful than raider-residents due to longer tenures, achieving a mean of 5.3 copulations per male (variance = 12.3). The incidence and success of raids as a mating strategy was partially dependent on topography. At Puerto Piramide, where males defended territories containing tidepools and access to the breeding area was limited to a few sites at high tide, raids were infrequent and ineffective. Raiders synchronized their rush into the breeding area, keying on the first male to move. However, no alliances or coalitions, suggesting cooperation, were formed. Group raids consist of a collective revolt against the status quo, similar in form to troop takeovers by groups of male hanuman langurs. Outside males can achieve proximity to oestrous females with less risk of injury by raiding in a group rather than by attempting to seize a female by themselves.]