reproductive success (e.g., number of grand-offspring), thereby favoring reproduction with multiple partners. Given that suboptimal social partners can be highly detrimental to fitness, we hypothesized a link between environmental uncertainty and the prevalence of secondary mate choices among socially monogamous birds. To test this idea, we focus here on the rates of extra-pair mating (hereafter infidelity, [25]), and divorce [26–28], two types of secondary mate choices that have been sampled in a consistent way across a taxonomically diverse array of avian species (see Methods). Environmental uncertainty could influence the prevalence of these behaviors among socially monogamous birds in two alternative ways. One possibility is that environmental uncertainty increases the prevalence of infidelity and/or divorce relative to what would otherwise be expected from each species’ life history and demography (for reviews on the effect of these variables on secondary mate choice see [25,29,30]). Although we acknowledge that divorce and infidelity may not be adaptive in every species (or even in both sexes, see [31]), this hypothesis assumes that, on average, these behaviors can prevent, correct, or minimize the negative consequenc- es of suboptimal partnerships (see [25,26,30]). For example, extra-pair offspring tend to have higher survival rates [32], fledge in better condition [33], be more immunocompetent [34], and have higher lifetime reproductive success [35] than their within-pair half-sibs. Similarly, reproductive success and partner quality typically increase as a consequence of re-pairing after divorce [36–38]. An increase in the prevalence of infidelity and divorce is particularly likely when the frequency of environmental change is relatively low because under those conditions, individuals that have already experienced an ecological crossover may now acquire reasonably accurate informa- tion about the relative quality of available options in the new ecological setting. Alternatively, environmental uncertainty could lead to lower rates of infidelity and divorce by interfering with an individual’s ability to determine if there are better reproductive options available. In many species, previous experience and familiarity with a breeding partner can lead to a measurable increase in reproductive success (see [26]). Because divorce and infidelity can compromise such benefits by straining or ending social partnerships, it may only be profitable for individuals to engage in these behaviors when it is highly likely that they will benefit from them. A decrease in the prevalence of infidelity and divorce is therefore likely when experience and familiarity with a partner are more important than mate quality for reproductive success, or when ecological cross-overs are frequent enough that conditions are likely to change again before a new partnership can be established. Thus, environmental uncertainty could potentially lead to either a decrease or an increase in infidelity or divorce, depending on the natural history of the species involved. Here we explore the effects of environmental uncertainty on the evolution of reproductive behavior among socially monogamous birds. To estimate the extent to which populations are exposed to ecological cross-overs, we measure the variability and predictability of local temperature and precipitation patterns. After accounting for the potential effects of life history (i.e., adult mortality, ornamen- tation, dichromatism) and demography (i.e., continuity of partner- ships, coloniality) [see 25,29,30], we demonstrate significant associations between environmental parameters and the incidence of avian infidelity and divorce. These findings illustrate how sexual selection may be susceptible to the globally increasing environmen- tal uncertainty that is resulting from climate change (see [39,40,41]). Methods 1. Behavioral and Life History Variables Infidelity is defined as cases in which at least some offspring in a brood are fathered by individuals other than the social partner Fluctuating Selection and Reproductive Flexibility [25]. We therefore measured this behavior as the proportion of nests in a population containing extra-pair young. Divorce is defined as cases in which individuals establish a breeding partnerships with a new mate even though their former partner is still alive and present in the population [26]. A comprehensive search of the literature published through 2010 yielded data on infidelity for 277 species and on divorce for 163 species. Sources were located via Web of Knowledge (http:/www.webofknowledge. com), Google Scholar (http://scholar.google.com/), and by direct inspection of the reference lists of major reviews on these topics. All secondary records were checked against primary sources for accuracy and to determine study locations. We geo-referenced all study sites using BioGeomancer WorkBench version 1.2.3 (http:// www.biogeomancer.org) and confirmed every locality through direct inspection in Google Earth 5 (http://earth.google.com). We then computed climatic variables (see below for more detail) from the nearest weather station to each study site from the Global Historical Climatology Network (http://www.ncdc.noaa.gov) us- ing a maximum cutoff distance of 100 km (great-circle distance between weather stations and study populations: Mod- e = 12.22 km; Mean = 31.15 km). In addition, we used molecular data from GenBank to build independent phylogenetic hypotheses for our comparative analyses (see below for more detail). Because climatic or molecular data were not available for every species, our final sample includes 122 species for infidelity and 86 species for divorce (Table S1). We analyze these two datasets separately because infidelity and divorce have been previously linked to different causal factors(see [25,26]), and because in our sample, there is no evidence of a correlation between these behaviors (Pearson’s product-moment correlation of phylogenetically inde- pendent contrast: t = 0.6122, df = 22, P = 0.547) (but see [42]). In exploring the effects of climatic uncertainty on reproductive flexibility, we also consider the roles of other potential covariates such as life history and demography. Several variables are thought to be important for the evolution of infidelity and divorce and have received different levels of support in analyses of single species or closely-related groups. To avoid overparameterization of our models, we consider only those variables that explain broad-scale, interspecific differences in previously published studies. Earlier comparative analyses have failed to find any strongly supported predictors in the case of avian infidelity, but breeding density, parental care, and adult mortality appear to have at least some explanatory power (see [25,30]). Specifically, higher frequencies of extra-pair mating are expected under higher breeding densities because physical proximity may increase the availability of alternative reproductive options and the opportunity for extra- pair copulation [43]. Similarly, a higher incidence of extra-pair mating is expected when females are able to rear offspring on their own because they have little to loose if their cuckolded partners retaliate with a reduction in parental care [44]. In addition, extra- pair mating is expected to be higher in species with shorter lifespans because when future reproductive opportunities are unlikely, abandoning a clutch or withholding parental care in response to infidelity is expected to be highly maladaptive [45]. Our models therefore include coloniality (as a proxy for breeding density) and adult mortality (measured as the per-year probability of dying for both sexes combined) as potential predictors of avian infidelity, and control for the mode of parental care by including only species that are socially monogamous, show bi-parental care, and do not breed in social or cooperatively breeding groups. Note that the metric of adult mortality in this model (as well as in the model of divorce; see below) captures the range of fast/slow life history strategies that is known to influence reproductive decisions [46]. PLoS ONE | www.plosone.org 2 February 2012 | Volume 7 | Issue 2 | e32311