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.
Life history theory holds that animals that decline with age should invest less energy in staying alive and more in reproduction when they become old, because they have less to lose in doing so. Evidence for such terminal investment, however, is scanty. We measured rates of survival and territorial behavior in a known-age population of common loons and found that males, but not females, suffered a decrease in survival rate and ability to hold a territory after age 14. At the same time, territorial males older than 14 behaved more aggressively and were more apt to give a territorial yodel call towards territorial intruders. Hence, our study appears a rare example of terminal investment in aggressive behavior, whereby old, declining males cling desperately to their territories in hopes of producing a few more offspring before they die.
KeywordsTerminal investment Aggression Territory Loon Gavia immer
We thank three collaborators, Charles Walcott, Jay Mager, and Joel Flory, and many research assistants, especially Andrew Reinke, Nathan Banfield, Keren Tischler, Margaret Klich, Frank Spilker, Lyla Furey, Erin Harrington, and Amy Turcotte. We also thank two anonymous reviewers.
The research was supported by the National Science Foundation (IBN-0316442 and DEB-0717055), the National Geographic Society, the Disney Conservation Fund, and Chapman University.
Compliance with ethical standards
We followed all applicable international, national, and institutional guidelines for the care and use of animals. All protocols for capture, marking and study of common loons were approved by the Chapman University Animal Care and Use Committee.
Conflict of interest
The authors declare that they have no conflict of interest.
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