1The Operational Sex Ratio, the The Operational Sex Ratio, the Potential Reproductive Rate, and Potential Reproductive Rate, and the Opportunity for Sexual the Opportunity for Sexual SelectionSelectionStephen M. ShusterStephen M. ShusterNorthern Arizona University2ObjectivesObjectives1. Definitions of OSR/PRR.2. What do they reallyreallymeasure?3. Do better measures exist?3What Is The Definition of OSR?What Is The Definition of OSR?Originally defined by S.T. Emlen 1976 as: “the ratio of potentially receptive males to receptive females at any time.”A measure of the level of level of competitioncompetitionfor mates, and therefore, of the intensity of intensity of sexual sexual selection.selection.4The Operational The Operational Sex RatioSex RatioEmlen & Oring 1977OSR = Nmature males/NreceptivefemalesA reproductive competition A reproductive competition coefficient.coefficient.OSR>1 = females are rare, competition for mates is intense. OSR<1 = females are abundant, competition for mates is relaxed.5However,However,This definition has not remainednot remainedconsistentconsistent among researchers.3 primary variants of the original concept of OSR:1. Changes in the ratioratio itself.2. Changes in how individuals are are (or are not)(or are not) included in estimates.3. Changes that attempt to incorporate the effects of parental parental investmentinvestment.OSR is widely presumed to predict/induce evolutionary responses by populations6Changes In RatioRatio of potentially receptive males to females at any time (Emlen 1976)Ratio of sexually active males to fertilizable females at any time (Emlen & Oring 1977, and most others). Reciprocal of the sex ratio (RO=1/R=Nmales/Nfemales; Shuster & Wade 2003)Number of males and females ready to mate (Nyman et al. 2006)Ratio of matured females to males (Yamamoto & Edo 2006)Fertilizable females to sexually active males at any given time (Forbes et al. 2005; Prohl 2006).The relative number of members of each sex willing (or able) tomate at any given time (Kemp & Macedonia 2006)The “Operational Sex Ratio” has been expressed in several ways7Changes in Personnel(Clutton-Brock & Vincent 1991; Clutton-Brock & Parker 1992; Ahnesjö et al. 2001; Forsgren et al. 2004)A change in emphasis from allall individuals that are potentiallypotentially receptive,To one that includes only certain receptivecertain receptive individuals at a particularparticular time and in a particularparticular placeAhnesjö, Kvarnemo & Merilaita 2001. Behav. Ecol. 12:397-4018Changes In Emphasis(Clutton-Brock & Vincent 1991; Clutton-Brock & Parker 1992; Ahnesjö et al. 2001; Kokko & Monahagn 2001)The focus on parental care in sexual selection theory has also influenced the changing view of what OSR is.“The costcost of a single breeding attempt, which in iteroparousorganisms can be measured as the probability of death as the probability of death as a consequence of the current a consequence of the current breeding attemptbreeding attempt, has a strong, direct effect on choosiness and well as consistent relationships with both OSR and PRR.”(Kokko & Monahagn 2001)9Evolutionary InterpretationsBiases in OSR are presumed to have significant consequencesVariance in mating success: (Positive effect: Emlen 1976; Balshine-Earn 1996; Kvarnemo et al. 1995; Jann et al. 2000; Jones et al. 2001; Foellmer & Fairbairn2005; Negative effect: Shuster et al. 2001; No effect: Cerchio et al. 2005;Reversal of sex roles: (Emlen & Oring1977; Smith 1984; Berglund et al. 1989; Forsgren et al. 2004; Andersson 2005; Simmons & Kvarnemo 2006)Avoidance of sperm competition(Positive: Møller 1989; Møller & Briskie1995; Hosken 1997; Bateman 1997; Pitnick& Karr 1996; Negative: Pen & Weissing1999; Kemp & Macedonia 2007)Mate selection and choosiness: (Rosenqvist 1993; Berglund 1994; Kokko& Monahagn 2001)Mate guarding/mating duration (McLain 1981; Sillen-Tullberg 1981; Jormalainen 1998; Gao & Kang 2005)Family sex ratio adjustmentMcLain & Marsh 1990; Lopez & Dominguez 2003; Warner & Shine 2007; Aggressive behaviorGrant et al. 2000; Grant & Foam 2002; Changes in oviposition rate Spence & Smith 2005;Female body temperature: (Alsop et al. 2006)Population declines: (Stifetten & Dale 2006)10Summary 1Summary 1The definition of OSR has changedchangedsomewhat since 1976.Working definitions of OSR have attempted to improve the fitto improve the fit between theory and observation.Evolutionary predictions from observed biases in OSR are now commoncommon.11ObjectivesObjectives1. Definitions of OSR/PRR.2. What do they reallyreallymeasure?3. Do better measures exist?12Measuring OSR/PRR (Clutton-Brock & Vincent 1991Clutton-Brock & Parker 1992; Parker & Simmons 1996; Ahnesjö et al. 2001; Forsgren et al. 2004)Considers the effect of certain receptivecertain receptiveindividuals at a particularparticular time and in a particularparticularplace, on the intensity of sexual selectionintensity of sexual selection.13Problems with Leaving Certain Individuals OutThe justification for this is that only certaincertainindividuals reproduce at any time;Including everyone could biascould bias estimates of competition intensity.Specifically, leaving individuals out causes causes errorserrors in estimates of actual selectionactual selection.14The Cause of Sexual SelectionThe Cause of Sexual Selection“If each male secures two or more females, many males would not be able to pair”(Darwin 1871, p. 266).1500.20.40.60.8112345678910Harem Size (H)Proportion of nonmating males (p0)For Harems, For Harems, pp00= = 11--(1/(1/HH))Shuster & Wade 2003; Wade & Shuster 200416Sexual Selection is a Sexual Selection is a Powerful Evolutionary Powerful Evolutionary Force Because: Force Because: For every male who sires young with with k females, there must be k-1 males who fail to reproduce at all. Shuster & Wade 200317When Losers are When Losers are IgnoredIgnoredA significant fraction of the amongamong--group group component of fitness variance goes goes unrecognized.unrecognized.This creates 2 kinds of errors:This creates 2 kinds of errors:1. The average fitnessaverage fitness of the population is overestimatedoverestimated2. The variance in fitnessvariance in fitness for the population is underestimatedunderestimated05101520253035400123456Mates per maleN Malesm = 1.0Vm = 0.9605101520253035400123456Mates per maleN Malesm = 1.56Vm = 0.621800.20.40.60.8112345678910Harem Size (H)Proportion of nonmating males