Sierra Nevada Aquatic Research Laboratory

05 May, 05 AM - 05 Aug, 05 AM

Title: Sexual Pigmentation, Antioxidant Status, and Reproductive Strategies in Yellow Warblers: Linked via Male Quality or Alternative Allocation Decisions? Purpose of research project: My research explores the information content of sexual pigmentation with respect to phenotypic quality and reproductive strategy, using the yellow warbler as a model species. The purpose of my research is to establish whether sexual coloration is indicative of body condition and antioxidant status, and whether coloration indicates alternative reproductive strategies with respect to the allocation of energy between self-maintenance, mating effort, and paternal effort. During Spring to Summer 2012, I shall further explore whether antioxidant status and body condition mediate alternative reproductive strategies and the information content of sexual coloration using longitudinal sampling and pigment manipulation in a small sub-population of birds (~8-10 males). INTRODUCTION: Alternative reproductive strategies have long proved a theoretical challenge for behavioral and evolutionary biologists, since theory predicts the emergence of a single, optimal strategy through directional selection. Coexistence of multiple reproductive strategies requires more complex mechanisms of selection, such as disruptive selection or tradeoffs between alternative elements of reproductive effort and survival (Gross 1996; Badyaev and Hill 2002; Badyaev and Qvarnstrom 2002; Pryke and Griffith 2009). In sexual species with parental care, tradeoffs between mating effort, parental effort, and self-maintenance may underlie alternative reproductive strategies. Mating effort includes investment towards expensive sexual ornaments, intra-sexual aggression, and inter-sexual display. Parental effort includes all behaviors that enhance offspring quality, and self-maintenance includes investment in immunity, healing, and antioxidant defenses (Trivers 1972; Alonso-Alvarez et al. 2004). Reproductive allocation tradeoffs may be particularly intense in males, which can greatly increase fitness through multiple mating, but may also play critical parental roles (Trivers 1972; McGrath and Komdeur 2003). Antioxidant physiology may be important to mediating reproductive tradeoffs, and constraining sexual ornamentation used in mate attraction. Intense mating effort may generate pro-oxidants that must be combated through up-regulated antioxidant defenses, or otherwise lead to oxidative damage of biomolecules and senescence (Beckman and Ames 1998; Alonso-Alvarez et al. 2008). Further, developing carotenoid and melanin-based pigments, which produce sexual coloration in diverse taxa, may be expensive to oxidative equilibrium. Carotenoids act as antioxidants and immunostimulants, such that allocating carotenoids towards pigmentation may retract from these functions (Alonso-Alvarez et al. 2008; Griffith et al. 2006). Similarly, a tradeoff may occur between developing phaeomelanic pigmentation, and maintaining high levels of the critical antioxidant glutathione (Galvan and Solano 2009). Further, in addition to developmental costs, expressing sexual pigmentation may increase socially-induced aggressive interactions and sexual displays, and thus increase energy expenditure and pro-oxidant production (Qvarnstrom 1999; Safran et al. 2010; Isaksson et al. 2011). My research addresses if tradeoffs between sexual pigmentation, oxidative status, and reproductive behaviors shape alternative reproductive strategies and the information content of sexual signals, using the yellow warbler (Setophaga (formerly Dendroica) petechia) as a model species. Costs to oxidative equilibrium may constrain the high-reward strategy of multiple mating to males in good physiological condition, with males in good condition developing sexual pigmentation and engaging highly in mating competition, while also maintaining oxidative equilibrium and paternal capacity (Safran et al. 2010; Helfenstein et al. 2010). In this case, sexual pigmentation may be an ?honest? indicator of male condition and paternal quality (Hill 1991; Alonso-Alvarez et al. 2008). Alternatively, males displaying high levels of sexual pigmentation may invest in pigmentation and mating competition at the expense of other allocation alternatives, including oxidative equilibrium, overall physiological condition, and paternal care (Candolin 2000). In this scenario, sexual ornaments may negatively predict antioxidant status, physiological condition, and paternal performance, particularly late in the breeding season (Kokko 1998). I will address these hypotheses by enhancing my past research, using longitudinal sampling of antioxidant status across a season, and manipulation of pigmentation in a sub-population. RESEARCH QUESTIONS: 1. Does sexual pigmentation predict oxidative status and body condition over the course of a season? Predictions: Alternative 1: Throughout the breeding season, males with highly developed sexual pigmentation display good body condition and oxidative status. However, males with enhanced pigmentation show a decline in oxidative status and condition across the season, revealing social costs of over-expressing pigmentation. Alternative 2: Both males with highly developed (natural) sexual pigmentation and pigment-enhanced males show poor oxidative status and body condition, particularly late in the season, revealing costs of expressing pigmentation and suggesting that pigmentation reflects alternative allocation ?decisions?. 2. Does sexual pigmentation predict how males behaviorally allocate reproductive effort? Predictions: Alternative 1: High levels of pigmentation predict high mating effort (song, territoriality), and also positively predict paternal effort (incubation feeding, nestling provisioning), indicating high quality of highly pigmented males. However, enhancing pigmentation increases mating effort at the expense of paternal effort. Alternative 2: Both highly pigmented and pigment-enhanced males invest in mating effort over paternal effort. 3. How do differences in sexual pigmentation affect reproductive success? Predictions: Highly pigmented and pigment-enhanced males achieve high reproductive output through extra-pair paternity, but may suffer declines in social nest success. STUDY SPECIES: The yellow warbler (Setophaga petechia) is a good model species for addressing my questions. S. petechia displays biparental care, with males investing paternally by provisioning incubating females and nestlings. Males also invest highly in mating via song, territory defense, and pursuit of extrapair paternity (EPP). Further, male S. petechia display carotenoid-based yellow coloration and phaeomelanin-based ventral streaking (Studd and Robertson 1985; Lowther et al. 1999; Yezerinac and Weatherhead 1997). In some populations, males with extensive phaeomelanic streaking appear to invest highly in territorial aggression, potentially at the expense of paternal care. Further, experimentally increasing ventral streaking on model intruders increases the aggressive responses of territory holders, suggesting social costs of pigment expression (Studd and Robertson 1985; 1988). RESEARCH APPROACH: Body Condition, Oxidative Status, and Sexual Pigmentation: As done in previous seasons, I will capture warblers in mist nets at the beginning of the season, using conspecific playback. To quantify body condition, I will use residuals of body mass on wing chord, and score fat and pectoral muscle. Further, I will assess antioxidant status using ~80μL blood samples. Blood will be frozen pending analysis. At the University of California, Riverside (UCR), I will measure total antioxidant capacity (TAC) of the plasma using the OXY-adsorbent assay (Diacron, International), and glutathione levels in erythrocytes. Additionally, I will assay reactive oxygen metabolites (ROMs) in the plasma, to quantify pro-oxidant levels (Diacron, International). I will use the radio of ROM/TAC x 1000 as a metric of oxidative stress (Costantini et al. 2007). Further, when males are captured at the beginning of the season, I will assess phaeomelanic ventral streaking and carotenoid-based yellow pigmentation. To measure area of ventral streaking, I will place a 1 x 1cm grid adjacent to the breast, and take digital photographs from the front and sides. I will later use the image analysis program ImageJ to determine percentage coverage of ventral streaking. I will average values obtained from multiple photographs to obtain a final value for percent phaeomelanism. Further, I will collect 5 yellow feathers from nonadjacent breast regions. I will later use an Ocean Optics spectrometer to obtain reflectance readings between 400 -700 nm, and use principal components analysis to summarize variance in reflectance spectra. Manipulation of Coloration: Further, when male birds are captured at the beginning of the season, I will increase phaeomelanic streaking to match that of the most phaeomelanic males in the population in a sub-popultion of ~8 males, using non-toxic, colored markers chosen to match natural coloration. This method has been successfully used to enhance phaeomelanic coloration in barn swallows (Hirundo rustica), without detrimental effects on individuals (Safran et al. 2005; 2008). On ~8 control males, I will perform sham application of color. I will select experimental and control males with phaeomelanic pigmentation similar to the population mean (~15 % coverage). I will take photographs of males before and after manipulation, to assess initial pigmentation, and verify the success of manipulation. Behavioral Measures: To measure mating and paternal effort in experimental and non-experimental birds, I will perform 2 hour video-recordings at nests on ~day 2 of incubation and day 3-5 of the nestling stage. I will extract male singing rate, and the rate at which males provisioning incubating females and nestlings. In addition, I will perform a simulated territory intrusion near nests, to assess investment in territorial aggression versus paternal care. Specifically, on day 5-7 of the nestling stage, I will introduce a warbler decoy and song recording ~5 meters from nests. The intrusion will last ~1 hour, during which I will video-record behavior. I will assess territorial response (reflected by song rate and direct attacks on the decoy) and paternal provisioning. Change in Condition: To assess seasonal change in condition, I will recapture males on day 4-7 of the nestling stage by placing nets near nests. I will retake condition-metrics and blood samples for antioxidant analyses. **Note: Shifts in reproductive strategy and costs among pigment-manipulated males are predicted to be facilitated by social interactions with conspecific males and females (Studd and Robertson 1988). Reproductive Success: I will monitor nest success and fledging rates. Further, I will obtain ~5μL blood samples from adults at capture and 4 day old nestlings to obtain DNA samples. I will use DNA paternity analysis to determine the genetic parentage of offspring (Dawson et al. 1997). Statistics: I will perform ANOVA and regression analyses using R 2.9.2 to statistically assess predictions. SIGNIFICANCE: My work will elucidate if tradeoffs between elements of reproductive effort and oxidative balance play a role in maintaining honest sexual signaling of physiological condition, and constraining reproductive strategies. Further, my research will also address the alternative that males invest in sexual pigments at the expense of oxidative status, in which case pigmentation may neither honestly signal body condition, nor paternal potential. Addressing this possibility has significant implications for theories of sexual signaling. Assessing how oxidative status is impacted by pigment enhancement will increase my capacity to detect costs of expressing sexual pigmentation, since individuals that are naturally highly pigmented may be able to sustain high levels of reproductive effort while up-regulating antioxidant defenses and largely avoiding oxidative costs.

Visit #28399 @Sierra Nevada Aquatic Research Laboratory

Approved

Under Project # 21465 | Research

Plasticity in reproductive allocation: Do males adjust mating and paternal effort to individual condition and environment?

graduate_student - University of California, Riverside

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Group of 2 Graduate Student May 5 - Aug 5, 2012 (93 days)
Andrea Grunst May 5 - Aug 5, 2012 (93 days)

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