RESEARCH PAPERSInfluence of Age and Prior Experience on Territorial Behaviorand the Costs of Defense in Male Collared LizardsAndrew M. Schwartz, Troy A. Baird & Dusti K. TimanusDepartment of Biology, University of Biology, University of Central Oklahoma, University Drive, Edmond, OK, USAIntroductionTerritory defense evolves when the benefits of main-taining exclusive access to areas containing one ormore resources essential for survival and reproduc-tion exceed the costs of excluding competitors(Brown 1964; Brown & Orians 1970; Emlen & Oring1977; Davies & Houston 1984; Stamps 1994). Def-ense costs may include the time and energy expen-ded in aggressive contests and displays (Marler &Moore 1988, 1989; Vehrencamp et al. 1989; Marleret al. 1995; Hack 1997) especially when asymmetriesin the physical attributes of competitors are small(Parker 1974; Leimar & Enquist 1984). Territorialdefense may also increase mortality as a conse-quence of high energetic costs (Marler & Moore1988; Dufty 1989) or increased predation risk (Tuttle& Ryan 1981; Ryan et al. 1982; Bradbury et al.1989; Whitehouse 1997a; Lange & Leimar 2001).Lastly, defense of territories may interfere withopportunities to court and mate with females (Clut-ton-Brock et al. 1979: Baird et al. 2001).Because territorial defense is costly, it is reasonableto hypothesize that natural selection may act on thebehavior of defenders in ways that reduce costs andhence increase the net benefit of spatial defense.Territory owners sometimes reduce defense costs bydecreasing aggression towards familiar occupants ofCorrespondenceTroy A. Baird, University of Central Oklahoma,100 N. University Drive, Edmond, OK 73034,USA. E-mail: [email protected]: February 10, 2006Initial acceptance: April 21, 2006Final acceptance: June 16, 2006(S. Forbes)doi: 10.1111/j.1439-0310.2006.01297.xAbstractIn polygynous species where males maintain strong interseasonal phil-opatry to the same breeding territories, older individuals have priorexperience defending their areas, whereas younger individuals aredefending a territory for the first time. Theoretical and empirical studiespredict that under such conditions the cumulative costs of defense maybe lower for older experienced males as a consequence of familiaritywith environmental and/or social conditions in their particular localhabitats. We used quantitative data from focal observational studies andintroduction experiments to test the hypothesis that older collared lizardmales (Crotaphytus collaris ) with prior territorial experience (3 yr+ males)acquire larger territories allowing them to court more, different females,with greater frequency without incurring higher defense costs. Consis-tent with this hypothesis, 3 yr+ males controlled significantly larger ter-ritories and courted more females, without having significantly higherrates of territory patrol, frequencies of advertisement display, or aggres-sive interactions with same-sex competitors. Moreover, the intensity ofresponses to size- and age/experience-matched tethered intruders by3 yr+ males was lower than that by 2 yr males in their first season ofterritory defense. Our results support the hypothesis that age/prior occu-pancy of territories lowers defense costs allowing males to defend largerareas and increase opportunities to court females, perhaps increasingmating opportunities. By contrast, 2 yr males may need to respond moreaggressively to intruders because their ownership of territories is tenu-ous as a consequence of shorter territory occupancy.EthologyEthology 113 (2007) 9–17 ª 2007 The AuthorsJournal compilation ª 2007 Blackwell Verlag, Berlin9neighboring territories (dear-enemy phenomenon;Jaeger 1981; Krebs 1982; Ydenberg et al. 1988; Fox& Baird 1992; Temeles 1994; Husak & Fox 2003),and by utilizing displays that are ‘honest signals’ ofresource holding power but are less costly than overtaggression (Whiting et al. 2003; Husak 2004). Malesdefending breeding territories may maximize adver-tisement to same-sex competitors when these behav-iors detract the least from courtship of females(Baird et al. 2001). Familiarity with specific territoryattributes (e.g. local terrain; Baugh & Forester 1994)acquired through prior occupancy may promoteeffective detection and defense against intruders(Alcock & Bailey 1997; Eason et al. 1999; Sloan &Baird 1999). Experienced males may also be moresuccessful in agonistic contests with same sex com-petitors and have greater access to females becausethey have prior knowledge of the locations and phe-notypic qualities of both (Waser & Wiley 1980; Aus-tad 1983; Holberton et al. 1990; Kim & Zuk 2000).Economic approaches have proven useful for mod-eling the conditions under which defense of space isadaptive (Brown 1964; Davies & Houston 1984).Because reproductive success of males is often lim-ited by access to females (Trivers 1972), males arepredicted to maximize the size of defended areas toexclude same-sex competitors from the home rangesof numerous females (Davies 1991; Zamudio &Sinervo 2003). These models generally predict thatboth the cumulative benefits and costs of territorydefense increase as some function (linear or curvi-linear) of increasing territory area (Davies & Houston1984; Hixon 1987). Territories of a given size areexpected to be economically defensible at axialcoordinates where the elevation of the cumulativebenefit curve exceeds the curve describing thecumulative defense costs (Fig. 1). If there is an upperlimit on the capacity of individuals to exploit thedefended resources (e.g. food), then the benefitcurve should asymptote (Brown 1964; Davies &Houston 1984). However, assuming that the numberof females that males can mate with is not limited,the benefit curve for breeding territories should con-tinue to increase to the extent that males are able toexpand to overlap additional female home ranges.The shapes of cumulative defense cost curves willbe influenced by any factor(s) that alter the costs ofexcluding competitors vying for the defendedresource(s). Under the hypothesis that prior experi-ence defending the same territory reduces one ormore costs, the elevation and/or slope of the curvefor older experienced males is expected to be lowerthan that for younger males lacking prior experience(Fig. 1). Older males, therefore are predicted toacquire larger territories without increasing thedefensive behavior patterns that are costly (aggres-sion and display). Conversely, some studies