Sentinel Behavior: A Review and Prospectus

Sentinel behavior can be defined as coordinated vigilance, usually from exposed positions. The literature contains anecdotes of sentinel behavior in many species, yet most of these anecdotes simply label particularly vigilant individuals as sentinels. Coordination has been documented quantitatively for only two species and verbally described for a few other species. Pending quantitative tests, we may regard sentinel behavior as repeatedly hypothesized but rarely documented. To further quantitative tests, a method of comparing the observed gaps without sentinels to those expected by chance is described and illustrated using data available from the literature.Sentinel behavior varies with ecological and social conditions. Natural rates of feeding correlate broadly with rates of sentinel behavior, and experimental food supplements have increased sentinel behavior dramatically. Sentinels generally are far more likely to detect predators and give alarm calls than are foragers. Although all individuals may engage in sentinel behavior, it tends to be more common in adults and males. Individual rates of sentinel behavior sometimes decrease with group size, though this effect seems to vary across species and even seasons. Sentinels often give calls besides alarm calls, yet these do not function simply as termination or continuity signals. Foragers are sometimes closer to cover than sentinels and sometimes farther away. Overall, the evidence is consistent with sentinels being safer than foragers. Evidence now exists that sentinel behavior functions, at least in part, to protect vulnerable juveniles. Therefore sentinels may be safe but should not be described as selfish. © 2015 Elsevier Inc.