Fruit resources shape sexual selection processes in a lek mating system

The degree to which within-population variation in sexual trait expression relates to resource heterogeneity remains poorly explored. This is particularly true in lek-mating species, where genetic explanations for male phenotypic variance and mating success are dominant. Here, we demonstrate a link between fine-scale fruit resource availability and indices of male mating success in the white-bearded manakin (Manacus manacus), a lek-mating frugivorous bird that produces energetically costly courtship displays. We used motion-activated camera traps to monitor male display behavior and female visitation at male courts while concurrently conducting twice-monthly fruit surveys around courts. We observed significant variability in ripe fruit biomass among display courts and leks, and mean fruit biomass at courts significantly predicted male display rates. In turn, male display rate was the strongest predictor of female visitation to courts. Causal modeling supported the hypothesis that hyper-local fruit availability indirectly affects female visitation via its direct effects on male display rate. The demonstration that resource availability at fine spatial scales predicts display rate in a lekking organism—for which resource-related variables are typically not considered to play important roles in shaping male reproductive variance—has implications for the expression, honesty, and maintenance of sexually selected traits under fluctuating ecological conditions.

Methods
Fruit biomass data was collected via visual fruit surveys. Survey plots were circular with a 10-meter radius and centered around male white-bearded manakin display courts. Both ripe and unripe fruits were counted during surveys, although only ripe fruits were used in analyses. Fruits were identified into morphospecies, and representative samples of fruits were obtained and weighed to obtain an average wet weight per berry for each morphospecies.

Video data was collected using motion-activated camera traps that were placed 1-2 meters from male display courts. Videos were then transferred from SD cards to hard drives and subsequently annotated for behaviors of interest by trained observers. Videos collected at courts that were monitored for < 3 weeks and/or using a different brand of camera were excluded from analyses. Displays by juvenile males were also excluded from analyses. 

UTM coordinates for each display court were gathered using a handheld GPS, and points were then uploaded into QGIS for calculation of centrality and nearest neighbor distances. Canopy cover at display courts was estimated using Canopeo, a mobile application that rapidly assesses the proportion of green cover from photographs.