Aging Male Loons Make a Terminal Investment in Territory Defense

Animals that senesce experience a decline in residual reproductive value (RRV), such that old individuals can expect reduced breeding success compared to young ones. According to life history theory, animals with low RRV, which thus have less to lose, should shift resources away from self-maintenance and towards reproduction, an expectation called terminal investment. In a population of common loons whose survival and territorial behavior were measured throughout life, males 14 years and older exhibited clear senescence, as they showed lower survival, reduced body mass, and far greater susceptibility to territorial eviction than younger males. While older males invested no more effort than young males in feeding or protecting their chicks, they increased territorial yodeling by 35%, showed more aggression towards territorial intruders, and, following eviction from original territories, resettled with great frequency on vacant, unproductive territories nearby. Our findings thus provide support for terminal investment in territorial behavior. Hyper-aggressive behavior by old, declining males might explain the unusual occurrence of lethal combat for territories in this species