Vigilance and group size in California sea lions Jill Fortuna Abstract A relationship between group size and vigilance (visual scanning of the environment) has been found in many animal species. Decreased individual vigilance is expected to occur in larger groups because animals can rely on their group-mates to detect predators. However, many recent studies do not show a group-size effect, which indicates that more research is needed to determine the functional significance of vigilance and grouping behavior. This study used scan samples and 3-min focal animal samples to determine the effect of group size on individual and group vigilance in groups of California sea lions (Zalophus californicus) hauled out at Point Reyes, California. Increasing group size did not have a significant effect on individual vigilance, although group vigilance did increase significantly with group size. These results indicate that individual vigilance among California sea lions may be influenced by factors other than group size, such as body size and orientation.Introduction Vigilance, or visual scanning of the environment, is a behavior that has been studied extensively in many animal species. Various functions of vigilance have been proposed, including watching for predators, searching for food, and looking for mates (Renouf & Lawson 1986; Quenette 1990). A possible cost associated with vigilance is that time spent being vigilant decreases the time that an animal can spend in activities such as feeding, resting, and grooming (Elgar and Catterall 1981; Mooring and Hart 1995; Treves 2000). A decrease in individual vigilance as group size increases has been reported in many species across different taxa (Da Silva and Terhune 1988; Quenette 1990; Roberts 1996; Treves 2000), including harbour seals (Da Silva and Terhune 1988), Thomsons’ gazelles (Fitzgibbon 1989), Capuchin monkeys (Schaik van et al, 1988), and California ground squirrels (Loughry and McDonough 1988). There are two hypotheses to explain this so-called group-size effect. First, Pulliam (1973) proposed the many-eyes hypothesis, which states that animals in groups can rely on the vigilance of their group-mates to increase the probability of detecting predators and so avoid predation (Quenette 1990; Roberts 1996; Robinette and Ha 2001). Thus animals in groups tend to allocate less time to vigilance behavior and more time to other activities such as foraging (Roberts 1996). Second, the individual risk hypothesis suggests that group size reduces the risk of predation to an individual through dilution and confusion effects. Therefore individual vigilance will decrease with increasing group size if there is less risk of predation to the individual within the group (Roberts 1996; Robinette and Ha 2001). These hypotheses need not be mutually exclusive and may operate simultaneously. Da Silva and Terhune (1988) tested the hypothesis that animals in larger groups have a greater probability of detecting predators. In canoes and on foot, the researchers actively stalked harbour seals (Phoca vitulina) that were hauled out of the water onto beaches. They found that while all groups eventually detected their approaches, harbour seals in larger groups detected the approaches at greater distances. This provides support for the hypothesis that animals in larger groups are less at risk of predation. Despite the frequency with which the group-size effect is reported, recently there have been many studies that have not found evidence of the group-size effect. Some species even show an increase in individual vigilance as group size increases (Renouf & Lawson 1986; Beauchamp 2001; Robinette and Ha 2001). Renouf and Lawson (1986) found that individual vigilanceincreased as group size increased among male harbour seals during breeding season, indicating that the primary function of vigilance was watching for mates, not predators. The practice of scrounging to exploit the food discoveries of group-mates has also been found to increase individual vigilance as group size increases. For example, Robinette and Ha (2001) found that vigilance in northwestern crows was affected not by group size, but by factors such as the tide, which exposed more food sources when low, and the availability of opportunities to steal food from neighbors. Other factors that can contribute to increased vigilance with increased group size may be food density and quality, competition within group, sex, age, social status, distance from cover, time of day, and presence of predators (Quenette 1990; Robinette and Ha 2001). These potentially confounding factors are often not measured or controlled for in vigilance studies (Robinette and Ha 2001). These results, which contradict the predictions of the many-eyes hypothesis and the individual risk hypothesis, suggest that the relationship between group size and vigilance may vary among species or under different ecological circumstances. Some studies have shown that in addition to ecological factors, within-group factors can play a role in vigilance levels. Position within the group has been shown to affect the vigilance of harbour seals, with peripheral members scanning more than central members (Terhune and Brillant 1996). Harbour seals newly arrived at a haul-out site also scan more than harbour seals that have been hauled out for awhile (Terhune and Brillant 1996). These factors may increase or decrease individual vigilance regardless of group size. One species that has not been studied in relation to vigilance is the California sea lion (Zalophus californicus). Groups of California sea lions haul out, or pull out of the water onto rocks and beaches, at many sites around the San Francisco Bay Area, including Point Reyes National Seashore and Pier 39 in San Francisco. The California sea lions that haul out in the San Francisco Bay Area are primarily males and juveniles; females remain at the breeding grounds in the Channel Islands for most of the year (Marine Mammal Center, elec. comm.). Although little is known of the behavior of California sea lions in the wild, it is hypothesized that they haul out in order to rest, as do harbour seals (Krieber and Barrette 1984). Predators of California sea lions include sharks, orcas, and humans (Marine Mammal Center, elec. comm.). Surface predation may have been an important selection pressure during the evolution of the California sea lion, resulting in the need for vigilance. California sea