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Hormonal Correlates of Bower Decoration and Sexual Display in the Satin Bowerbird (Ptilonorhynchus Violaceus)’

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Hormonal Correlates of Bower Decoration and Sexual Display in the Satin Bowerbird (Ptilonorhynchus Violaceus)’ The Condor93~935-942 0 The CooperOrnithological Society I99 1 HORMONAL CORRELATES OF BOWER DECORATION AND SEXUAL DISPLAY IN THE SATIN BOWERBIRD (PTILONORHYNCHUS VIOLACEUS)’ GERALD BORGIA Department of Zoology, Universityof Maryland, CollegePark, MD 20742 JOHN C. WINGF~ELD Department of Zoology, Universityof Washington,Seattle, WA 98195 Abstract. Male Satin Bowerbirdscourt females with complexdisplays that includestick structuresbuilt adjacentto a decorateddisplay court. Here we reportcirculating levels of testosteroneand corticosteronein a naturalpopulation of SatinBowerbirds and their relation to the developmentof complexmating displays.We found that (1) maleswithout bowers had lowerlevels of testosteronethan bowerholders, (2) testosterone,but not corticosterone, levelswere significantly correlated with male mating success,and (3) levelsof testosterone, but not corticosterone,were consistentlycorrelated with the quality of severalimportant displaycharacters that have been shownto be importantin affectingmale mating success. Theseresults suggest that testosteronelevel is an important componentof male mating success.We considerreasons why variation in testosteronelevels persistsamong mature male bowerbirdsgiven its potentialto affectmale mating success. Key words: Sexual display;Bowerbird; hormone; testosterone; mate choice;Ptilonorhyn- thus violaceus:corticosterone. INTRODUCTION work has shown that female Satin Bowerbirds Male Satin Bowerbirds (Ptilonorhynchusviola- prefer males with well-constructed and well-dec- ceus),like many species in the family Ptilono- orated bowers (Borgia 1985a), and that male rhynchidae, have developed complicated court- competition by destruction of other male’s bow- ship displays oriented around a bower, a stick ers (Borgia 1985b) and theft of decorations (Bor- structure that they build and decorate (Vellenga gia 1985b) affects display characters important 1970; Donaghey 1981; Borgia 1985a, 1986). in female choice. Young males associate with Courtship displays at the bower involve harsh older males at bowers and may learn some ele- calls, vocal mimicry, and jumping or “dancing” ments of display behavior (Vellenga 1970, Borgia postures(Loffredo and Borgia 1986). Naturalists 1986, Loffredo and Borgia 1986). have long been curious about the peculiar nature There is considerable evidence for the role of of these displays, and recently bowerbirds have the steroid hormone testosterone in regulation attracted the attention of evolutionary biologists of male-aggressivenessand courting behavior in becausethey provide a model system for under- birds (Balthazart 1983, Harding 1981, Harding standing the evolution of complex symbolic sex- et al. 1988, Hegner and Wingfield 1987, Wing- ual displays. Investigations of the development field 1984) although this has not been true in all of male display behavior have revealed that there cases(e.g., Rohwer and Wingfield 1981; Wing- is age-relatedimprovement in male displays(Lof- field et al. 1982a, 1987). These studies involved fredo and Borgia 1986; Borgia, in preparation) investigations of bird specieswith relatively sim- and that male dominance may be important in ple courtship displaysand resource-basedmating determining male ability to obtain and hold a systems in which male successin obtaining a display site (Borgia 1985b; Borgia and Gore 1986; mate is primarily dependent on territory quality. Borgia and Loffredo, in preparation). Previous Bowerbirds have a non-resource-based mating system, and female choice appears almost en- tirely dependent on the quality of male display. I Received25 February 1991. Final acceptance30 Investigation of courtship behavior in male Satin May 1991. Bowerbirds is of special interest because the 19351 936 GERALD BORGIA AND JOHN C. WINGFIELD’ unique mating systemallows numerous elements taining matings. We also evaluate the possible of male display to be quantified, and the devel- relationship between hormone levels and male opment of thesedisplay components can be mea- advertising and bower destruction tendencies. sured in relation to hormone levels. Castration of Ptilonorhynchusviolaceus impairs bower METHODS building and displays to females, whereas tes- This study was carried out at Wallaby Creek, in tosterone injections partially restore these be- the Beaury State Forest, 120 km NW of Lismore, haviors (Marshall, 1950), suggestinga major role New South Wales, Australia, and the study site for testosteronein social behavior of this species. has been described elsewhere (Borgia 1985b). Furthermore, becausemale bowerbirds can mate Satin Bowerbirds at Wallaby Creek have been with many different females in a season,there is the subject of an intensive field study from 1980 an opportunity to score the relative quality of to 1987, and general information about the birds male display independently of other factors such was collected over that period. Data for the com- as those found in speciesin which territory qual- parisons reported here were collected from Au- ity affects mating decisions. gust 1984-December 1984. Recent studiesofjuvenile plumage Satin Bow- Birds were captured in baited traps and re- erbirds show that testosteronecan have a critical moved immediately. The birds were color-band- role in determining male successin aggression ed (if not previously captured), scoredfor plum- around feeding sites and in adult male plumage age characters and numbers of ectoparasites, development (Collis and Borgia 1990, 1991). weighed, measured for wing length, and imme- Testosterone-implanted juvenile males showed diately released. Plumage characters and mea- an increased tendency to build bowers, perform surements of wing length allowed sexing and ag- displays and related behaviors, and molt into ing of birds (Vellenga 1970). Individual birds adult plumage, when these birds were compared were banded with unique combinations of col- with a control group. However, the behaviors of ored bands. Our records allowed us to estimate implanted birds were uncoordinated and these the ages of males based on when they obtained birds were not tolerated by bower-holding males. adult plumage. Males exhibit a green female-like These resultssuggest that, although adult-like be- plumage with dark beaks until four years of age, haviors can be induced by testosteroneimplants, at which time the beak begins to gradually turn these birds suffer from an inability to produce completely yellow. At five years, males exhibit fully coordinated behaviors, and, becauseof their a mottled blue and green plumage which, after early attainment of adult plumage, they may be the final molt, becomes entirely blue at seven denied opportunities to observe displays of older years of age. We classified individual males as males. Thus, it is possible that a combination of juveniles if they were less than seven years old. hormonal factors (e.g., testosterone) and hor- Male ranks were establishedby observation of mone independent factors (e.g., experience) may male interactions at trapping sites. We recorded regulate courtship and bower construction in Sat- the identity of birds that were involved in inter- in Bowerbirds. It is also possible that circulating actions, including the band combinations of birds corticosteronelevels may be important since this that initiated and received the attack. Aggres- adrenocortical steroid hormone is known to sup- sivenesswas measured as the ratio of the number pressreproductive behavior either directly or in- of attacks by a bird to the total number of en- directly by inhibiting secretion of reproductive countersby a bird. Encounters are the total num- hormones (e.g., Wingfield and Silverin 1986, ber of times a bird was an attacker plus the num- Wingfield 1988). ber of times it was attacked. This aggressiveness In this study we report levels of testosterone index was used to order males into ranks. Our and corticosterone in individual male birds of aggressivenessindex was highly correlated with different age classesin a wild population. We dominance value (see Collis and Borgia 1990, compare juvenile plumage and adult males and 199 1; Borgia and Loffredo, MS) based on a total assessage-related changes in testosterone and of 2,176 interactions recorded in 1984. corticosterone levels. We test the specific pre- Each permanent bower site was monitored by dictions that circulating concentrations of these a camera controlled by an infrared device that two hormones affect the performance of bower- was triggered when birds entered the bower (see holding males in producing displays and in ob- Borgia 1985a, 1986a). Matings occur in the bow- TESTOSTERONE AND BOWER DECORATION 931 er, and our camera system provided continuous coated charcoal. Standard curves covered the monitoring at bowers through the entire mating range 2-500 pg for testosterone and 7.8-2,000 season. Behavioral observations including be- pg for corticosterone.All sampleswere processed havior around the bower and male advertising within a single assay, and intra-assay variation and display calls were made from hides located was < 10%. For further details of procedure, as- 15 m from the bowers. Male mating success, say validation, and reliability criteria see Wing- number of courtships, and identity of visitors field and Famer (1975); Wingfield et al. (1982b); were determined by review of films (Borgia Ball and Wingfield (1986). 1985a). Females typically mate once per season. STATISTICAL ANALYSIS Rarely females mated twice with a male without moving out of the bower and these were scored
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