The Auk 109(3):422-434, 1992

AGE-RELATED EFFECTS OF TESTOSTERONE, PLUMAGE, AND EXPERIENCE ON AGGRESSION AND SOCIAL DOMINANCE IN JUVENILE MALE SATIN (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 (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 ,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 , 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. Controls were given empty implants. Behavioralsampling.--Behavioral interactions at A 12-gaugeneedle was inserted at the nape of the feeding sites were recorded to estimate individual neck, and the implants were pushed under the skin dominance rank before and after treatment. We re- where they remainedthroughout the experiment.This corded the identity of both the initiator and the re- procedure took from 5 to 10 min from capture to cipient of each interaction, and a description of the release. Birds showed few outward signs of distur- initiated behaviorand responseof the recipient. bance as a result of this procedure.They had been There were four categoriesof initiated behaviors frequently handled in previousyears, and mostwere noted: move toward, mild attacks, threats, and severe active around traps soon after release. attacks.Move towardis an aggressiveapproach of the Birdswere recapturedand weighedat varioustimes initiator toward the target with the apparent throughoutthe experiment.Blood samples were taken intent of displacingthe target.Mild attackswere rap- on all testosterone-treated and control birds to deter- id movements(e.g. lunge) that did not involve contact mine their post-treatmentblood-hormone levels. In of the initiator with the target. 1987 the birds were censused and interaction data was Four kinds of threatsaccompanied aggressive dis- taken to determine the dominance status of birds that plays:puff threats,open-beak threats, wing-flip threats, were treated in 1986. To determine the effects of tes- and vocalthreats. Puff threatsinvolved malespuffing tosteroneimplants on various measuresof condition, their feathers while in an erect posture. Open-beak birds were recapturedat this time so that the birds threats involved a bird holding its beak partially couldbe weighed and checkedfor changesin plum- open as it approached another bird without vo- age condition. calizing.Wing-flip threatswere quick movementsof Measuringdominance.--Dominance status was de- the wings away from the body and backagain. Vocal terminedfor eachindividual by calculatingthe num- threatswere harsh-soundingcalls (with the beakheld ber of opponentsdominated using dominancevalues open) directed at a target bird. Severe attackswere (DV): attacksthat involved contact(e.g. hit, strike, maul). There were four typesof responsesby targetbirds DV = arcsin X ø•, (1) to directedaggression: yield to initiator, held ground, mild displacementof the initiator, and severe dis- where X is the proportion of individuals dominated placement of the initiator. Yield to the initiator in- (see Beilharz and Mylrea 1963). Dominance values volves the target bird being displaced.Held ground allow assignmentof rank sothat frequentinteractions means the target bird failed to yield or retaliate in amongparticular dyads are not given undue weight. responseto the directed aggressionof the initiator. Dominance values were calculated before and after Mild and severe displacements involved mild and implantation for all individuals that interacted with severeattacks, respectively, by the target bird in re- five or more nontreated opponents. sponseto the approach,threat, or attack of the ini- Hormoneassays.--Hormone assays were carried out tiator. An all-occurrencesampling method (Altmann with a singleantibody RIA kit (Amersham,Arlington 1974) was used to record behaviors of all birds used Heights, Illinois). We measuredtotal androgen(TA), in the experiment in 1986 and 1987. Interactionsoc- testosterone(T), and dihydrotestosterone(DHT). These curred in or close to the traps, and happened infre- measureswere validated for Satin Bowerbirdplasma quently enoughso that nearly all interactionscould and found to be accurate.Precision of replicateassay be recordedby experiencedobservers. Individuals in from pooledserum was 8.6%(coefficient of variability the flock were censusedfor each observationperiod. within assay).Serial dilution curves for the serum Experimentalprocedure.--Behavioral interactions were parallel to the standardcurve (P < 0.05), which were recorded for two weeks prior to treatment. Ju- insuresno interference from other componentsin the venile-plumagedmales of similarrank (determinedby serum. The sensitivity for T was 2 pg/tube and for number of"interactions won / total interactions"prior DHT was 5 pg/tube (M. A. Ottinger pers.comm.; for to treatment)and plumage condition (age class)were further detailson generalassay procedure, see Ottin- then paired and randomly assignedto a treatment ger and Mahlke 1984). group(see Table 1). This wasdone to assurethat birds Statisticalanalyses.--We determined the effect of of different rank and plumage condition were dis- testosteronetreatment on behavior by comparing tributed equally across all treatment groups. The within-subjectdifferences (before and after implant) treatmentsconsisted of a control group and three tes- among treatments.Comparisons of changeswithin tosterone-treatedgroups. The testosterone-treated individualsamong treatment groups allowed for the birds received either one, two, or three implants of control of: (1) hormonal and behavioral differences testosterone.Crystalline testosterone(Steraloids, Wil- prior to treatment;and (2) time-dependentchanges ton, New Hampshire) was packedinto Dow-Corning in variables. Statisticalcomparisons involved GLM Silastictubing (length 20 mm, inside diameter 1.47 repeated-measuresanalyses (Winer 1971, SAS Insti- July1992] AggressionandDominance inBowerbirds 425

TABLE1. Individual birds in testosterone-treatedand control groupsarranged by treatment,plumage con- dition, and rank (number of interactionswon/total interactions)before implantation in 1986.

Testosterone Control • Bird Implants Plumageb Rank' Bird Implants Plumageb Rankc myd 1 yb 0.306 grp 1 yb 0.409 wwl 1 yb 0.333 ryg 1 yb 0.458 dop 1 yb 0.544 Mean 0.433 lol 1 yb 0.682 Mean 0.466 rgm 2 db 0.070 wmk 2 db 0.071 rrn 2 db 0.092 owo 2 db 0.077 ymr 2 db 0.223 gpk 2 db 0.267 ogn 2 db 0.331 mkr 2 db 0.316 kny 2 db 0.337 rrr 2 db 0.358 ypd 2 db 0.401 mnr 2 db 0.411 dkn 2 db 0.506 nnk 2 db 0.613 dlv 2 db 0.580 rng 2 db 0.690 Mean 0.317 Mean 0.350 rdo 3 db 0.116 dyo 3 db 0.053 nyk 3 db 0.160 pyo 3 db 0.054 dno 3 db 0.200 wnk 3 db 0.270 gnr 3 db 0.311 odp 3 db 0.304 dgw 3 db 0.361 mkm 3 db 0.444 ddp 3 db 0.679 lnp 3 db 0.533 wnp 3 db 0.824 glw 3 db 0.643 rnw 3 db 0.909 ryl 3 yb 0.304 dk! 3 yb 0.374 moy 3 yb 0.377 ppk 3 yb 0.455 rkm 3 bg 0.286 lrp 3 bg 0.327 mkk 3 bg 0.345 pmo 3 bg 0.552 lyr 3 bg 0.409 Mean 0.364 Mean 0.406

Grand mean 0.387 Grand mean 0.365

ßThe different number of empty implants given to control birds did not have an effect on measuredbehaviors. Hence, the different control groups were pooled in determining treatment effects. bdb = dark-billed,green-plumaged male; yb = yellow-billed,green-plumaged male; bg = yellow-billed,mixed blue- and green-plumaged male. • This measurementof rank carriedout sothat birdsof differentranks before implantation were distributedevenly across treatments. tute 1985), Spearman (r•) rank corre!ations, Mann- (TA, t = 0.90, n = 42, P = 0.37; T, t = 0.84, n = Whitney U-tests(Siegel 1988),and Student t-tests(So- 42, P = 0.41; DHT, t = -0.15, n = 42, P = 0.88; ka! and Rohlf 1981). Means are expressedas g + SE. two-tailed P-values).The mean androgen levels One-tailed P-valuesare cited when positive treatment measuredfor the testosterone-treatedgroup (TA, effectswere expected;otherwise, two-tailed P-values are used. œ= 10.7 + 1.3 ng/ml, n = 19; T, œ= 9.5 + 1.3 ng/ml, n = 19; DHT, g = 2.4 + 0.6 ng/ml, n = 19) were significantlyhigher than thoseof the RESULTS control group (TA, œ= 1.8 + 0.6 ng/ml, n = 15; T, œ= 1.7 + 0.6 ng/ml, n = 15; DHT, œ= 0.6 + Hormone implants.--Plasma levels of TA, T, 0.2 ng/ml, n = 15) after implantation (TA, t = and DHT were measured in individuals in con- 5.65, n = 34, P < 0.001; T, t = 4.97, n = 34, P < trol and treatment groups before and after im- 0.001; DHT, t = 2.55, n = 34, P = 0.008; two- plantation. No significant differences were tailed P-values). The testosteroneimplants con- found betweenthe mean androgenlevels of the sistently raised the measuredblood androgens pooledtestosterone-treated groups (TA, œ= 8.0 above basal levels. A significant positive cor- + 2.0 ng/ml, n = 23; T, œ= 7.2 + 2.0 ng/ml, n relation was found between the change in an- = 23; DHT, œ= 3.6 + 1.6 ng/ml, n = 23) and drogen levels and dosesupporting the hypoth- the control group (TA, œ= 5.6 + 1.6 ng/ml, n esis for a dose-responseeffect (TA, r5= 0.53, n = 19; T, g = 5.0 + 1.6 ng/ml, n = 19; DHT, g = = 30, P = 0.002; T, r5= 0.48, n = 30, P = 0.005; 4.0 + 1.6 ng/ml, n = 19) before implantation DHT, r5= 0.39, n = 30, P = 0.02; Fig. 1). The 426 COLLISAND BORGIA [Auk, Vol. 109

20. a repeated-measuresANOVA to determine the effectsof the testosteronetreatment on aggres- 10. sive behavior. Means were compared within O. subjectsfor each class of aggressive behavior (seeabove) over time (before and after implant). -10. Differences between treatments then were

-20. analyzed within the model by the treatment- by-time interaction.We analyzedaggressive en- -30. counters of treatment birds with adult-plum- agedmales (blue plumage) separately from those -40 encounters with females and juvenile-plum- 0 1 2 3 aged males (green plumage) to determine whether or not changesin aggressivebehavior 20. due to treatment are dependent upon the plum-

10. age color (signaledstatus) of the target. Significant differences were observed be- tween the treatment groups in initiated ag- gression(see Table 2). Testosterone-treatedbirds were more aggressivethan controlsand tended

-20. more often to approach,to threaten in the form of wing flips, and to attackseverely green birds -30. (Table 2). Testosterone treatment, on the other hand, had no effect on the tendency of birds to -40 t ! t t 0 1 2 3 initiate aggressivebehaviors toward blue birds (Table 2). The responseof treatment birds when approached, threatened, or attacked by green 10 c birds showed no significant differences be- tween the treatments (Table 3). Testosterone- 0 treated birds held their ground againstthe ini- tiatedaggression of blue birdssignificantly more often than controls (Table 3). We compared the change in hormone level with change in dominance values after implant o -20 for all birds in treatment groups, and found significant relationships in interactions involv-

-30 ing treatedbirds and untreatedgreen birds (TA, o • 2 rs= 0.63, n = 14, P = 0.01; T, rs= 0.59, n = 14, P = O.02;DHT, r•= 0.31, n = 14, P = 0.13). When Numberof Testosteroneimplants blue birds were included in the analysis, rela- Fig. 1. Effectof dose(number of testosteroneim- tionships between change in hormones and plants)on bloodandrogen before and after implan- change in dominance value were marginally tation.Dose of zero (0) signifiesbirds in controlgroup, significant (TA, r• = 0.31, n = 25, P = 0.06; T, r• which were given empty implants.Hormones mea- = 0.29, n = 25, P = 0.08; DHT, r• = 0.02, n = 25, suredwere: (A) total androgens;(B) testosterone;and P = 0.46). Further evidence of a treatment effect (C) dihydrotestosterone(DHT). was found by the significanttreatment-by-time interaction in a repeated-measures ANOVA measured blood androgen levels of testoster- (Table 4). In an analysisof interactionsbetween one-treated birds after implantation, although treatment birds and untreated green birds, tes- above that of nontreated birds (see above), over- tosterone-treated birds had a significant in- lapped with levels measured for adult-plum- crease in dominance over controls after treat- aged males (TA, œ= 4.6 + 0.7 ng/ml, n = 19; ment (Table 4). When blue birds were included T, œ= 4.3 + 0.7 ng/ml, n = 19; DHT, œ= 0.6 + in the analysis,the change was not significantly 0.2 ng/ml, n = 19). different (Table 4). We demonstrated a dose ef- Androgens,aggression, and dominance.--We used fect by a significant correlation between dose July1992] Aggressionand Dominance in Bowerbirds 427

TABLE2. Comparisonbetween pooled testosterone-treatedgroups and control group in the change(after implantationminus before implantation) in meannumbers of initiatedinteractions directed towards females and juvenile-plumagedmales, and adult blue-plumagedmales. Total frequencyof eachbehavior given in parentheses.Probability values (P) are two-tailed.

Treatment- by-time interaction a Control Testosterone

Behavior F P •bef •aft P œ•f œ.•t P Interaction with femalesand juvenile-plumagedmales Direct approach 4.36 0.04 6.12 (120) 5.82 (99) 0.92 8.96 (218) 16.50 (396) 0.002 Threat Puff 2.56 0.12 0.47 (8) 0.00 (0) 0.13 0.37 (9) 0.54 (13) 0.52 Open-beak 0.00 0.99 4.47 (101) 3.88 (66) 0.75 5.83 (142) 5.25 (126) 0.71 Wing-flip 4.88 0.03 0.23 (4) 0.06 (1) 0.28 0.12 (3) 0.42 (10) 0.04 Vocal 0.91 0.35 0.18 (3) 0.29 (5) 0.67 0.17 (4) 0.62 (15) 0.05 Mild attack 0.54 0.47 9.65 (224) 8.71 (148) 0.82 14.50 (357) 17.54 (421) 0.39 Severe attack 6.12 0.02 1.00 (17) 0.06 (1) 0.08 0.25 (6) 1.00 (24) 0.10 Interaction with adult blue-plumaged males Direct approach 0.34 0.56 1.64 (33) 0.94 (16) 0.44 2.96 (71) 2.96 (71) 1.00 Threat Puff 2.36 0.13 0.18 (4) 0.06 (1) 0.52 0.25 (6) 0.50 (12) 0.11 Open-beak 0.46 0.50 3.23 (70) 0.82 (14) 0.17 4.79 (115) 3.92 (94) 0.55 Wing-flip 1.01 0.32 0.23 (4) 0.00 (0) 0.05 0.12 (3) 0.04 (1) 0.40 Vocal 2.72 0.11 0.12 (2) 0.00 (0) 0.30 0.04 (1) 0.17 (4) 0.19 Mild attack 1.27 0.27 4.52 (111) 1.94 (33) 0.53 7.58 (183) 11.08 (266) 0.32 Severe attack 1.80 0.19 0.12 (2) 0.06 (1) 0.82 0.17 (4) 0.58 (14) 0.07 ' F- and P-valuesassociated with treatment-by-timeinteraction in a repeated-measuresANOVA. Time representseffects on aggressionbefore and afterimplant. Means are least-squaredmeans computed by SASProc GLM (SASInstitute 1985). and change in dominance value among birds birds were won by the initiator, 1.7% resulted in green plumage (rs= 0.48, n = 19, P = 0.02; in a tie, and 1.7% were lost by the initiator to Fig. 2A). This relationship was not significant the target. We suggestthat changes in overall when blue birds were included in the analysis dominance of testosterone-treated juvenile- (rs= 0.12, n = 35, P = 0.25; Fig 2B). plumage males might not be expectedin flocks The failure to find significant treatment ef- that consistmostly of adult blue males. fectswhen blue malesare includedmay be due Plumage,aggression, and dominance.--We com- to the high rate of interaction with blue males pared treatment groups in aggressiondirected and the overall dominance of blue males over towards all birds in 1987, the year after im- green males. Adult blue males were dominant plantation. The birds treated with testosterone over juvenile-plumaged males in agonisticin- returned to traps in 1987 in full-adult blue teractions (t = 6.24, n = 123, P < 0.001). Juve- plumage. The birds in the control group re- nile-plumaged males were often the target of mained in juvenile green plumage. In all cases adult blue-male interactions; green males in- the testosterone-treated birds were more ag- frequently initiated, and appearedto avoid, in- gressive than the controls. Significant differ- teractions with adult blue males (Coilis pers. ences were observed in the mean number of observ.). Testosterone treatment had no effect initiated interactions (t = 2.57, n = 26, P = 0.008), on the tendency of the treated birds to initiate direct approaches(t = 2.63, n = 26, P = 0.007), interactions towards adult blue males (Table 4). puff threats (t = 1.93, n = 26, P = 0.03), open- Aggressivedominance was strongly influenced beak threats (t = 2.47, n = 26, P = 0.01), vocal by the number of interactionsinitiated. In 1986, threats (t = 2.17, n = 26, P = 0.02), mild attacks 85.0%of initiated interactionswere won by the (t = 2.60, n = 26, P = 0.008), and mild displace- initiator, 12.2% resulted in a tie, and 2.8% re- ments of the initiator of an interaction by the suitedin a supplantof the initiator by the target. target (t = 2.13, n = 26, P = 0.04). Thesediffer- In 1987, 96.6% of initiated interactionsby all encesmay not be attributableto differencesin 428 COLUSAND BORGIA [Auk, Vol. 109

TABLE3. Comparisonbetween pooled testosterone-treated groups and controlin the change(after implan- tation minusbefore implantation) in numberof aggressiveresponses to initiated interactionsof females and juvenile-plumagedmales, and adult blue-plumagedmales. Total frequencyof eachbehavior given in parentheses.Probability values (P) are two-tailed.

Treatment- by-time interaction' Control Testosterone Behavior F P •bef •aft P œb,f œaf• P Interactions with females and juvenile-plutnaged males Yield to initiator 0.11 0.74 20.88 (426) 15.76 (268) 0.31 27.50 (682) 24.62 (591) 0.50 Recipientheld ground 0.55 0.46 1.56(36) 1.94(33) 0.65 2.46(62) 3.58(86) 0.10 Mild displacement 0.20 0.66 0.12(3) 0.06(1) 0.73 0.46(11) 0.50(12) 0.78 Severe displacement of initiator 0.70 0.41 0.00 (0) 0.00 (0) 1.00 0.00 (0) 0.04 (1) 0.20 Interactions with adult blue-plutnaged males Yield to initiator 0.01 0.94 26.29 (507) 21.53 (366) 0.54 39.87 (963) 34.29 (823) 0.39 Recipientheld ground 5.15 0.03 4.59(87) 2.00(34) 0.41 6.12(147) 12.87(309) 0.01 Mild displacement of initiator 0.84 0.37 0.35 (13) 0.09 (2) 0.67 1.62 (40) 0.50 (41) 0.06 Severe displacement of initiator 0.00 0.96 0.12 (2) 0.00 (0) 0.35 0.17 (4) 0.04 (4) 0.24 aF- andP-values associated with treatment-by-timeinteraction in a repeated-measuresANOVA. Timerepresents effects on aggressionbefore and afterimplantation. Means are least-squared means computed by SASProc GLM (SASInstitute 1985).

hormone levels so much as to differences in tosterone-treatedmales the year after treatment plumagebetween the treatmentgroups in 1987. may be a cost to those males in that an adult We separatedthe directed aggressionof the plumage incites increasedaggression from old- treated birds towards birds in green plumage er males. To test this hypothesis we compared (femalesand juvenile males)and in blue plum- the initiatedaggression of adult-plumagedmales age (adult males)to determine if plumage color toward males in the treatment groups in 1987 of the target affectedthe treated birds' propen- (see Table 6). Testosterone-treated males re- sity to initiate interactions in 1987 (Table 5). ceived more puff threatsand mild attacksthan The testosterone-treated birds tended to be more controlsby adults (Table 6). Testosterone-treat- aggressivetowards both green- and blue-plum- ed males tended to be involved in a greater total aged birds (Table 5). number of interactions. Comparison of attack The plumagechange experienced by the tes- ratios (initiated/total interactions involving

TABLE4. Comparisonbetween pooled testosterone-treated groups and controlin the change(after implan- tationminus before implantation) in dominancevalue among females and juvenile-plumagedmales, and adult blue-plumagedmales. The numberof treatmentbirds included in calculationof dominancevalue givenin parentheses.Probability values (P) are two-tailed.

Treatment- by-time interaction a Control Testosterone

Group F P •b,f •aft P •,f .•,,•t P Dominance among all un- treated birds (including adult males) 0.35 0.56 29.64 (14) 30.71 (14) 0.76 33.38 (21) 37.09 (21) 0.19 Dominance among un- treated juvenile males and females (excluding adult males) 5.16 0.04 44.29 (7) 36.71 (7) 0.04 51.08 (12) 53.33 (12) 0.40

' F- and P-valuesassociated with treatment-by-timeinteraction in a repeated-measuresANOVA. Time representseffects on aggressionbefore and after implantation.Means are least-squaredmeans computed by SASProc GLM (SASInstitute 1985). July1992] AggressionandDominance inBowerbirds 429 adult males) between the treatment groups 40 A ß showedthat testosterone-treated birdshad a • $0 significantlyhigher attackratio than control birds(Mann-Whitney U-test,Z = -2.22,n = • 20 25,P = 0.03).Testosterone-treated birdsmay beara slightlyincreased cost in aggressiondi- • 10 rectedat them by older males. .•-ß 0- ' TO determine the effectsof plumage type on dominance,we calculateddominance values for o• -•o- all testosterone-treated and control birds that . returnedto feedingsites in 1987.The testos- 0 1 2 3 terone-treated birds that developed an adult plumagewere dominantover the controlsstill injuvenile plumage (t = 2.82,n = 19,P = 0.006). 20- Any effect of testosteronetreatment on domi- nancein 1987must be considered indirect. It is unlikelythat the implants were supplying tes- • •0. tosteroneatthe time dominance wasmeasured. Testosteroneimplants typically last two months (Kincl and Rudel 1971), and these measureswere E 0. madeone year after the initial implant. Fur- ._• thermore, the effect of testosterone either di- rectlyor indirectly onplumage had an effect, • -•0. independentofendogenous hormone levels, on dominance relationshipsin 1987. Wetested the prediction from the experience hypothesisthat testosterone-treatedbirds in blue plumage are less dominant than naturally oc- Numberof TestosteroneImplants curring males.In accordancewith this hypoth- Fig. 2. Change in dominancevalue (dominance esis,untreated adult blue-plumagedmales were measuredafter implantation minus dominancemea- more dominant than the testosterone-treated suredbefore implantation)relative to dose(number birds that attained a blue plumage (t = -1.66, of testosteroneimplants). Dose of zero (0) signifies n = 42, P = 0.05). birds of control group, which were given empty im- plants.Plots are: (A) amongbirds in greenplumage; DISCUSSION and (B) amongall birds (includingadult blue-plum- aged males). An interaction of androgen levels, plumage, and experienceappears to determineage-relat- (Baptistaet al. 1987), JapaneseQuail (Selinger ed patternsin overall socialdominance in the and Bermant 1967), Herring Gulls (Larusargen- . Furthermore, we showed in- tatus;Boss 1943), Red-winged Blackbirds (Age- dependenteffects of androgensand plumage laiusphoeniceus; Searcy and Wing field 1980), and on dominancerelationships within an age class. Ringed Turtle-Doves (Streptopeliarisoria; Ben- Testosterone treatment alone increased aggres- nett 1940). Studies that failed to show any re- sion in juvenile-plumaged Satin Bowerbirds, lationship between androgen levels and dom- which led to changes in their dominance po- inance statusdid not control for the confounding sition among untreatedjuvenile malesand fe- effectsof plumage,age, and experienceon dom- males. Testosterone treatment of juvenile- inance, suggestingthe importance of multiple plumaged males did not affect aggressionor factors in overall dominance (Lumia 1972, Roh- dominance reversals among adult-plumaged wer and Wingfield 1981, Ramenofsky 1984, males. Holberton et al. 1989).It hasbeen proposedthat Changesin dominanceas a result of testos- correlations between testosterone and domi- terone treatment among birds of similar age, nance status exist only during heightened pe- plumage condition,and experiencehave been riods of male-male interaction (referred to as demonstrated in White-crowned Sparrows the challenge hypothesis;see Wing field et al. 430 COLLISAND BORGIA [Auk, VoL 109

TABLE5. Comparisonbetween testosterone-treated group and controlgroup in meannumbers of aggressive interactions initiated by treatment birds toward females and juvenile-plumaged males, and adult blue- plumaged males in 1987.

Control a Testosterone b

Behavior œ SE n œ SE n t P

Interactions directed at females and juvenile-plumaged males Total interaction 4.50 1.98 12 11.85 2.84 13 2.09 0.02 Direct approach 2.08 0.85 12 5.69 1.66 13 1.89 0.04 Threat Puff 0.00 0.00 12 0.38 0.21 13 1.73 0.05 Open-beak 0.42 0.23 12 1.38 0.45 13 1.88 0.04 Wing-flip 0.08 0.08 12 0.08 0.08 13 -0.06 0.48 Vocal 0.00 0.00 12 0.61 0.21 13 2.77 0.005 Mild attack 1.83 0.97 12 3.92 0.82 13 1.66 0.06 Severe attack 0.33 0.19 12 0.31 0.21 13 -0.09 0.46 Interactions directed at adult blue-plumaged males Total interactions 2.91 2.14 12 11.08 4.26 13 1.72 0.05 Direct approach 0.82 0.51 12 2.77 1.12 13 1.59 0.06 Threat Puff 0.09 0.08 12 0.54 0.27 13 1.56 0.07 Open-beak 0.09 0.08 12 1.00 0.48 13 1.81 0.04 Wing-flip 0.27 0.25 12 0.08 0.08 13 -0.68 0.25 Vocal 0.00 0.00 12 0.00 0.00 13 -- -- Mild attack 0.91 0.75 12 7.23 2.93 13 2.04 0.03 Severe attack 0.73 0.67 12 0.38 0.31 13 -0.39 0.35

Controlsthat had moltedto a blue plumagein 1987were removedfrom analysis. Only controlshaving a juvenilegreen plumage included. Testosterone-treatedmales that returnedto trapsin 1987had moltedto a full-adult blue plumage.

1987). When levels of aggressionamong males ored dominant adults were dominant over oth- are low, other factors (i.e. individual recogni- er untreated juvenile males in both White- tion, territoriality) may mediate social domi- crowned Sparrows and Harris' Sparrows (Zo- nancein birds (Wing field et al. 1987).Our work notrichiaquerula; Fugle et al. 1984,Rohwer 1985). on Satin Bowerbirds shows that testosterone In similar experiments, where juvenile birds levels can have an overriding effect on domi- dyed to resemble adults interacted with birds nance relationshipsamong juvenile-plumaged of all ages (including adult-plumagedmales), males. However, dominance status acrossage the treatment had no effect on dominance rank classesappears to involve other factors in ad- (Rohwer 1977,Fugle and Rothstein1987, M•ll- dition to androgens. er 1987). Rohwer (1977) argued that the inabil- We showed that plumage had a significant ity of dyed birds to dominate adult birds was effect on dominance status. Testosterone-treat- due to an incongruencebetween the behavior ed juvenile males underwent a premature and signaled status of the treated birds. Al- change into a blue plumage, not observed in though signalinga high status,the treatedbirds control birds, the year following treatment were intrinsically subordinate. Consequently, (Coilis and Borgia unpubl. manuscript).In the the treated birds lost in agonistic encounters year following implant these males were dom- when challengedrepeatedly by adult birds.This inant over the controls who remained in ju- "incongruencehypothesis" (Rohwer 1977) ex- venile plumage, but were subordinate to un- plainshow a statussignaling system can remain treated blue males. Convincing experimental evolutionarily stable and cheating can be pre- evidence exists in support of plumage effects vented by subordinate individuals carrying a on dominancestatus within age and sexclasses plumage signaling a high status.Given the ad- in other species(Fugle et al. 1984, Rohwer 1985, vantages associatedwith carrying an adult Watt 1986). plumage,plumage variation could not be main- Juvenile birds dyed to resemblebrightly col- tained without somecontrol of cheating.To test July1992] Aggressionand Dominance in Bowerbirds 431

T^BLE6. Comparisonbetween testosterone-treated group and controlgroup in meannumbers of aggressive interactionsreceived by femalesand juvenile-plumaged males,and adult blue-plumagedmales in 1987.

Control a Testosterone b

Behavior œ SE n œ SE n t P Interactionsinitiated by females and juvenile-plumagedmales Total interaction 4.50 1.25 12 2.46 0.59 13 - 1.51 0.07 Direct approach 2.08 0.80 12 0.38 0.18 13 -2.14 0.02 Threat Puff 0.00 0.00 12 0.08 0.08 13 0.96 0.17 Open-beak 0.33 0.14 12 0.38 0.18 13 0.22 0.41 Wing-flip 0.00 0.00 12 0.08 0.08 13 0.96 0.17 Vocal 0.00 0.00 12 0.15 0.10 13 1.42 0.08 Mild attack 2.08 0.62 12 1.46 0.39 13 -0.86 0.20 Severe attack 0.00 0.00 12 0.15 0.10 13 1.42 0.08 Interactions initiated by adult blue-plumaged males Total interactions 14.00 3.15 12 28.08 8.21 13 1.55 0.07 Direct approach 3.83 1.10 12 6.85 2.20 13 1.19 0.12 Threat Puff 0.17 0.17 12 1.61 0.63 13 2.16 0.02 Open-beak 1.17 0.65 12 1.00 0.47 13 -0.21 0.42 Wing-flip 0.08 0.08 12 0.23 0.12 13 0.98 0.17 Vocal 1.00 0.43 12 0.69 0.17 13 -0.69 0.25 Mild attack 8.33 1.99 12 18.30 5.31 13 1.70 0.05 Severe attack 0.50 0.29 12 0.77 0.39 13 0.54 0.30

Controlsthat had moltedto a blue plumagein 1987were removedfrom analysis.Only controlshaving a juvenilegreen plumage included. Testosterone-treatedmales that returnedto trapsin 1987had moltedto a full-adult blue plumage.

the incongruencehypothesis, Rohwer and Roh- inance relationships, as well as the timing of wer (1978) dyed subordinate birds to look like when a male attempts to become dominant. dominant adults and, at the sametime, implant- Other evidence supportsthe suggestionthat ed them with testosterone to render them more age-related differences in experience are im- aggressive.These individuals experienceda sig- portant to explain variation in male Satin Bow- nificant increase in overall dominance rank, erbird aggressivebehavior. First, birds treated winning most of their interactions with pre- with testosterone continued to avoid adult blue viously more dominant birds. males after treatment. Controls and testoster- We present a somewhat different pattern. The one-treated birds did not differ in the mean year after treatment, birds given testosterone number of initiated interactions directed at blue implants were behaviorally aggressiveand had birdsafter implant.We know that juvenilegreen an adult plumage. Despite thesechanges, these males typically lose in interactions with adult birds remained subordinate to true adult males. blue males.Second, although testosteronetreat- This implies the existenceof additional factors. ment raised the frequency of aggressiveand These results, coupled with the finding that display behaviors initiated by juvenile-plum- males in different age groups are similar in size agedmales and causedthem to attain adult blue (Borgia and Loftredo in prep.), suggestthat ex- plumage, the form and quality of these behav- perience is important in overall social domi- iors was not affected(Coilis and Borgia unpubl. nance. Satin Bowerbirds are long lived relative manuscript).Testosterone-treated males contin- to Harris' Sparrows.The potential for large dif- ued to give crude displays typical of birds in ferencesbetween individuals in age-relatedbe- their ageclass (Coilis and Borgiaunpubl. manu- havioral experience may be much greater in script). Furthermore, the bowers built by the long-lived versusshort-lived species.Thus, for testosterone-treated males were of a lower over- Satin Bowerbirds, age-related differencesin ex- all qualitycompared to the bowersof older more perience may have a greater role in both dom- experienced males (see Coilis and Borgia un- 432 COLEISAND BORGIA [Auk, Vol. 109

publ. manuscript).The low-quality displaysof providedby the Universityof Maryland and the Uni- testosterone-treatedmales, coupled with age- versity Computer Center. Logisticalsupport was pro- related improvements observedin bower build- vided by the University of Melbourne Zoology De- ing (Borgia1986, in prep.)and display(Loftredo partment and, in particular, M. J. Littlejohn and J. and Borgia 1986) in untreated males, support Hook. Invaluable assistancewas provided in the field by D. Bond,R. Carscadden,M. Coilis,A. Day, C. Dep- the hypothesisthat experience affectsthe de- kin, A. Rosenthall, and S. Webb. We thank the N. and velopment of displaysimportant to females.The J. Hayes,Mulkay, and Bell families,along with the significanceof experiencein the development N.S.W. Forestry Commissionfor their assistanceand of adult male patternsof display has been well for allowing us accessto their property. Hormone documentedin other species.The ontogenyof analysiswas performed by M. A. Ottingerand T. Har- display behaviors such as song (Rice and grove. Statisticaland SAS consultationwas provided Thompson 1968), repertoire size (Nottebohm by S. and L. Douglas,C. Denman,and J. Ott. Valuable and Nottebohm 1978, Derrickson 1987), and criticismsof this manuscriptwere made by L. Chao, motor patternsinvolved in courtship(Marshall E. S. Morton, J. Ott, M. A. Ottinger, M. Shea, and anonymousreviewers. K.C. thanks M. Coilis for her 1954, Kruijt and Hogan 1967, Groothuis 1989) patience and support throughout all stagesof this depends on practice and experience gained by work. young birds (see Shettleworth 1984). It appearsthat experiencehas a critical effect LITERATURE CITED on male dominance and, ultimately, the devel- opment of male display traits.The elevationof ALTMANN,J. 1974. Observationalstudy of behavior: testosteronelevels and plumage changeof ju- Samplingmethods. Behaviour 49:227-267. venile males were insufficient to raise the dom- ARCESE,P., AND J. ]•. M. SMITH. 1985. Phenotypic inance statusof juvenile malesto levels found correlatesand ecologicalconsequences of dom- in older males;thus, advantages associated with inance in Song Sparrows.J. Anita. EcoL 54:817- 830. increased testosterone levels and an adult BAPTISTA,L. F., B. B. DEWOLFE,AND L. AVER¾-BEAUSOLEn,. plumagemay be realizedonly if thesecharac- ters are associated with a sufficient level of ex- 1987. Testosterone,aggression, and dominance in Gambel's White-crowned Sparrows. Wilson perience. Elsewhere,Borgia and Loftredo (un- Bull. 99:86-91. publ. manuscript)have found an age-related BEILHARZ,R. G., AND P. J. MYLREA. 1963. Social po- increasein dominanceboth amongjuvenile and sition and behavior of dairy heifers in yards. adult male Satin Bowerbirds. The failure of Anim. Behav. 11:522-528. youngmales to elevatetheir testosteronelevels BENNETT,M. A. 1940. The socialhierarchy in Ring to render themselvesmore aggressivemay oc- Doves. II. The effects of treatment with testos- cur becausesuch a change fails to provide an terone propiona•e.Ecology 21:148-165. advantagein dominancerelations with older, BORGIA,G. 1985a. Bower quality, number of deco- rations, and mating successof male Satin Bow- more experiencedmales. Elsewhere,we have erbirds(Ptilonorhynchus violaceus): An experimen- shown that premature plumage development tal analysis.Anita. Behav. 33:266-271. causedby testoteroneimplants in Satin Bow- BORGIA, G. 1985b. Bower destruction and sexual erbirds reducesopportunities for malesto learn competitionin the Satin Bowerbird (Ptilonorhyn- important elementsin male display(Coilis and chusviolaceus). Behav. Ecol. Sociobiol. 18:91-100. Borgiaunpubl. manuscript). The combinedef- BORGIA,G. 1986. in bowerbirds. Sci. fectof experienceon male successin aggression Am. 254(6):92-100. and on a male'sability to acquireeffective dis- BORGIA,G., ANDM. A. GORE. 1986. Featherstealing play traits contributesto the prolongeddelay in the Satin Bowerbird (Ptilonorhynchusviolaceus): in attainment of adult plumage and display Male competition and the quality of display. Anita. Behav. 34:727-738. characters in Satin Bowerbirds. BRADBURY,J., AND M. ANDERSSON. 1987. Sexual se- lection:Testing the alternatives.Wiley, New York. Boss, W.R. 1943. Hormonal determination of adult ACKNOWLEDGMENTS charactersand sex behavior in Herring Gulls This researchwas supportedby funds awarded to (Larusargentatus). J. Exp. Zool. 94:181-209. G.B. by the AmericanPhilosophical Society, Harry GLUTTON-BROCK,T., F. E. GUINNESS,AND S. D. ALBON. FrankGuggenheim Foundation, and the NationalSci- 1982. Red deer: Behavior and ecology of two ence Foundation (BNS 81-13477, BNS 83-08154, BNS sexes.Univ. ChicagoPress, Chicago. 85-10483, and BSR89-11411). Additional support was COLLIS,K. 1990. The role of hormones and experi- July 1992] Aggressionand Dominance in Bowerbirds 433

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