Figure 1. Partial regression plots of the statistically significant predictors of avian infidelity. Axes refer to multivariate residuals after correcting for phylogeny and the effects of other significant parameters in the model. Each data point represents one species and dotted lines depict linear trends in the relationships between variables. doi:10.1371/journal.pone.0032311.g001 variables. Once non-significant terms are removed from this model (final PGLS regression model: R2 = 0.34, l = 0.65; all VIF,1.54, see [55]), we find that the frequency of divorce is higher in species with more pronounced visual ornamentation (b = 0.034, t = 2.348, df = 80, P = 0.021; Fig. 2A) and higher adult mortality (b = 0.089, t = 5.182, df = 80, P,0.001; Fig. 2B). In addition, we find that the Fluctuating Selection and Reproductive Flexibility frequency of divorce is also affected by the interaction between continuity of partnerships and predictability of annual temperature cycles (continuity: b = 0.001, t = 0.021, df = 80, P = 0.984; predict- ability of annual temperature cycles: b = 20.058, t = 22.279, df = 80, P = 0.025; continuity6temperature predictability: b = 0.112, t = 3.604, df = 80, P = 0.001); species that maintain year-long partnerships exhibit higher rates of divorce in more unpredictable environments (Fig. 2C), whereas those that engage only in temporary partnerships exhibit the opposite pattern (Fig. 2D). Discussion Through a comparative analysis of the rates of infidelity and divorce in socially monogamous birds, we found that the process of sexual selection is likely to be influenced not only by intrinsic life history and demographic variables, but also by extrinsic factors such as the variability and predictability of local climates. As noted in earlier studies [25,29], we find positive associations between adult mortality and secondary mate choices in socially monoga- mous birds both within (Fig. 1A) and among breeding seasons (Fig. 2B). These results are in agreement with previous findings that suggest that shorter lifespans lead to a lower tolerance of suboptimal partnerships because the expectation of future breeding opportunities is relatively low (see [26] for review). Similarly, we find that divorce is more common in more ornamented species (Fig. 2A, also noted in [29]), which supports the idea that strong sexual selection can promote the continued search for better reproductive options even after the establishment of breeding pair-bonds [62]. After accounting for these intrinsic factors, we find that the prevalence of infidelity is also positively associated with environmental variability. Although our model of infidelity explains a relatively small amount of the variation compared to that of divorce, the results are consistent with the genetic diversity hypothesis, which posits that more variable environments should favor behaviors that increase the genetic diversity among offspring whenever different environmental conditions favor different genotypes [63–66]. Additionally, our data show that the prevalence of divorce is also influenced by climatic factors, but in this case we find that it is associated with the predictability rather than with the variability of the environment. In other words, while the range of possible environmental conditions appears influence the expression of infidelity, it is the way in which different environmental conditions occur (e.g., the variation among years in the onset, intensity, or duration of environmental cycles) what matters most for divorce. The negative slope in Fig. 2C suggests that the potential for divorce increases when ecological cross-overs are more likely, which is consistent with the hypothesis that species that engage in continuous partnerships use divorce to help prevent, correct, or minimize the negative fitness consequences of suboptimal partnerships [26,32–38]. In contrast, the positive slope in Fig. 2D indicates that temporary partners may experience greater difficulty in detecting or achieving better reproductive options in less predictable environments. In our sample, temporary partners are exposed to much shorter breeding seasons than year-round partners (PGLS: R2 = 0.07, l = 0.17, t = 22.418, df = 84, P = 0.018), meaning that they are likely to experience important fitness losses even from minor delays in egg laying. Because locating and attracting a more suitable partner after divorce can take time (see [26]), this behavior is especially likely to experience strong negative selection in species with very short and unpredictable breeding opportunities. Thus, the positive associa- tion between the predictability of annual climatic cycles and divorce observed in this group suggests a partial release from time PLoS ONE | www.plosone.org 4 February 2012 | Volume 7 | Issue 2 | e32311