The Auk 109(3):422-434, 1992 AGE-RELATED EFFECTS OF TESTOSTERONE, PLUMAGE, AND EXPERIENCE ON AGGRESSION AND SOCIAL DOMINANCE IN JUVENILE MALE SATIN BOWERBIRDS (PTILONORHYNCHUS VIOLACEUS) KEN COLLIS AND GERALD BORGIA Departmentof Zoology,University of Maryland,College Park, Maryland 20742, USA ASSTRACT.--Weinvestigated the relationshipbetween age and dominancein the Satin Bowerbird(Ptilonorhynchus violaceus) to understandwhy youngmales delay maturation. Tes- tosteroneimplants were used to experimentallyadvance the expression of adultcharacteristics in juvenile-plumagedmales. Testosterone implants increased both aggression and dominance amongjuvenile-plumaged males. In theyear following implantation, treated males molted intofull adultplumage and maintained their dominance over controls at feedingsites even thoughtheir implants were probably empty. Despite the changesin circulatingandrogens, aggression,and plumagebrought on by the testosteronetreatment of juvenile-plumaged males,age-related differences remained in aggressionand socialdominance between the treatedand untreatedbirds. These age effects are consistentwith the hypothesisthat male experiencein maleaggressive encounters plays an important role in determiningdominance relationshipsand, ultimately, affects the developmentof maleadult-plumaged characters. Received12 September1990, accepted 13 January1992. THEEVOLUTION of exaggeratedmale display still in juvenile plumage(Marshall 1954),they traitscommon in manypolygynous species has do not acquirea full-adult blue plumageuntil attracted much attention and discussion (see their seventhyear (Marshall 1954, see also Vel- Bradburyand Andersson 1987). It is knownfor lenga 1980). Prior to that time, young males severalof thesespecies that exaggeratedmale maintaina greenplumage and are behaviorally displaytraits are fully expressedonly after a subordinatewhile feeding in flocksand when relativelylong developmentaldelay (e.g. Mar- visiting the bowersof older males(Vellenga shall 1954, Gilliard 1969, Selander 1972, Vel- 1970, Coilis 1990). Young males spend a great lenga 1980, Studd and Robertson1985, Foster dealof time observingolder malesat their bow- 1987).For birds,the ultimate causesof this delay ers, and practicebower building and display are not well understood. For most birds, unlike behaviors when the owner is absent from the mammals(e.g. Clutton Brock et al. 1982),phys- bower site (Vellenga1970, Borgia in prep.). As ical characters(e.g. size) that might directlyaf- youngmales grow older,they becomemore ag- fectmale success in aggression(and, hence, their gressiveand involved in the behaviorsper- ability to influencedisplays of othermales) do formedby adult males,constructing temporary not changewith ageafter earlyjuvenile devel- bowers at sites removed from the bowers of opment.There is, however, much evidence that adult males (Vellenga 1970). Both male domi- experiencecan affect display traits (Shettle- nanceand ageare importantfactors in a male's worth 1984),and delaysassociated with gaining ability to hold a bower site,and to becomesuc- thisexperience may be importantin the devel- cessfulin attracting females by maintaining a opmentand deployment of elaborateddisplays. bower of high relative quality (Borgia1985b, SatinBowerbirds (Ptilonorhynchos violaceus) are 1986,in prep., Borgiaand Gore 1986).Among especiallysuited for the studyof delayedde- adult males,mating successis skewedin favor velopmentof male displaycharacters. Adult of a small minority of maleswho are able to males build elaborate structures called bowers, build high-quality bowerswith many decora- where males court females (Vellenga 1970, tions on their display courts(Borgia 1985a). Donaghey1981, Borgia 1985a). Young males do Two hypothesesprovide plausibleexplana- not hold bowers and undergo a long delay in tions for why there is delayedmaturation of bothplumage and behavioral development. Al- display charactersin male Satin Bowerbirds. thoughyoung males produce viable sperm while First, previous experiencein aggressiveinter- 422 July1992] Aggressionand Dominance in Bowerbirds 423 actionsmay be important in determining male or to reproduce(Rohwer et al. 1980, Lyon and aggressivedominance. Aggressive dominance, Montgomerie 1986, Foster 1987). These alter- in turn, may be necessaryto allow malesaccess natives need not be mutually exclusive. to display sites or other prerequisitesfor at- Here we investigate the first of the above tracting mates. Young males may delay matu- hypotheses.We used testosteroneimplants to ration until they have gained the experiencein induce premature maturation of adult charac- aggressionnecessary to compete successfully ters in juvenile-plumaged males. After treat- with older males for mates. Previous studies of ment, we compared levels of aggression and aggressionin birds suggestthat experiencemay dominance position of the treated birds with be important in socialdominance. In domestic the levels found in controls and untreated adult- fowl (Gallusgallus), Smith and Hale (1959) and plumage males. We tested the prediction that Ratner (1961) have shown that conditioning,by aggressive experience is important in overall way of learned dominanceor submission,had socialdominance: that young birds treatedwith a significant effect on an individual's domi- testosteroneare expectedto remain subordinate nance position. Lumia (1972) found that the to older, experienced males. We investigated combination of testosterone treatment and ag- both the immediate effectsof implants and the gressiveconditioning led to the greatesteffect effectsof implants one year later on males that in increasing aggressionand dominance posi- had prematurely obtained adult plumage. tion of male pigeons. Testosterone treatment alone had no effect on aggression,and condi- METHODS tioning alone led to only small changesin ag- gression and dominance of the treated birds. This study was conductedfrom August through Prior experiencein aggressiveencounters also December (1986 and 1987) as part of an investigation had an effect on social dominance in Red Grouse of Satin Bowerbirdsthat began in 1980 (see Borgia (Lagopuslagopus; Moss et al. 1979),Japanese Quail 1986). The field site is located 150 km southwest of (Coturnixcoturnix; Ramenofsky 1984), and Song Brisbanein the BeauryState Forest, New SouthWales, and White-crowned sparrows(Melospiza melodia Australia, and has been described elsewhere (Borgia and Zonotrichialeucophrys; Arcese and Smith 1985a).This work was carried out in a valley formed by WallabyCreek, adjacent to a systemof ridgeswhere 1985, Baptistaet al. 1987). bower sites have been located over eight previous Second,young malesmay fail to developadult years(Donaghey 1981, Borgia 1986). From Augustto display charactersbecause they have insuffi- October, Satin Bowerbirdsare commonly found in cient experience in using these charactersto the morning and late afternoons in open paddocks produce effective displays.Experience can be in predominantly single-sexflocks (5-50 birds). In critical in producing effectivedisplays and, un- paddocks,the birds generally feed on grassshoots. til it is gained, costly investment by young in Satin Bowerbirds may move singly or in flocks to traits needed for aggressivedominance and in forage along the creek in flowering trees (e.g. GraveIlia the display traits is avoided. Under this hy- sp.), fruiting trees,and shrubs(e.g. Rubussp.). In late pothesis,delayed maturation occurs because of Octoberand early November,the flocksbegin to dis- perse, and males spend more time visiting and tend- the limited advantagesof dominanceat a young ing bowers. The breeding seasonbegins in mid-No- age, and not becauseof the inability of young vember and continuesthrough mid-December. malesto becomeaggressively dominant. Plum- Three feeding siteswere establishedin mid-August age development may be delayed when male and maintained through October in paddockswhere reproductive successdepends strongly upon birds had fed previously.Interactions were recorded age-relatedfactors, such as learned components daily at two of the three sitesbetween 0600 and 1200 of display, or when precocial development of (seebelow). Siteswere occasionallymonitored in the adult charactersbrings significant costs, or both. afternoon. Birds were attracted to traps set in these Costs leading to delayed maturation may in- locationsthat were baited with bread. The traps were clude: (1) increasedrisk to young males ex- operated by an observer in a blind 20 m away. We measuredwing length, tarsuslength, bill length, and pressingdisplay traits (Lack1954, Selander 1965, weight of capturedbirds. Also, they were scoredfor 1972, Marler and Moore 1988); or (2) reduced plumagecharacters. Birds were banded with individ- toleranceof young males by older, more ex- ually distinct color-band combinations.Birds were periencedmales, that may impair young males' aged and sexedbased on plumage and wing length ability to learn display (termed the facilitated- (see Vellenga 1980). Blood was drawn from the wing learning hypothesisby Coilis and Borgia1990) vein of all experimentalbirds prior to treatment (see 424 COLLISAND BORGIA [Auk, Vol. 109 below). Blood sampleswere collectedin two to four mm, external diameter 1.96 mm), and the open ends heparinized capillary tubes (75 mm x 1.4 mm i.d.), were sealedwith silicone(silastic adhesive). Implants and were centrifuged soon after collection. Plasma were soakedin methanol to insure they were ade- was frozen for hormoneanalysis in the laboratory. quatelysealed.