Advertisement displays often seem extravagant and expensive, and are thought to depend on the body condition of a signaller. Nevertheless, we know little about how signallers adjust effort based on condition, and few studies find a strong relationship between natural variation in condition and display. To examine the relationship between body condition and signal elaboration more fully, we characterized physiological condition and acoustic displays in a wild rodent with elaborate vocalizations, Alston's singing mouse, We found two major axes of variation in condition-one defined by short-term fluctuations in caloric nutrients, and a second by longer-term variation in adiposity. Among acoustic parameters, song effort was characterized by high rates of display and longer songs. Song effort was highly correlated with measures of adiposity. We found that leptin was a particularly strong predictor of display effort. Leptin is known to influence investment in other costly traits, such as immune function and reproduction. Plasma hormone levels convey somatic state to a variety of tissues, and may govern trait investment across vertebrates. Such measures offer new insights into how animals translate body condition into behavioural and life-history decisions.

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These elaborate behaviors often depend on the modification of, and are ultimately limited by, existing morphology. In Alston's singing mouse (Scotinomys teguina), males attract mates by producing long and loud songs at lower frequencies than most muriod rodents

Courtship displays are dramatic examples of complex behaviors that vary within and among species. Evolutionary explanations for this diversity rely upon genetic variation, yet the heritability of complex phenotypes is seldom investigated in the field. Here, we estimate genomic heritability of advertisement song and body condition in a wild population of singing mice. The heritability of song exhibits a systematic pattern, with high heritability for spectral characteristics linked to vocal morphology, intermediate heritability for rhythmic patterns, and lower but significant heritability for measures of motivation, like song length and rate. Physiological measures of condition, like hormonal markers of adiposity, exhibited intermediate heritability. Among singing mice, song rate and body condition have a strong phenotypic correlation; our estimate suggests a comparable genetic correlation that merits further study. Our results illustrate how advances in genomics and quantitative genetics can be integrated in free-living species to address longstanding challenges in behavior and evolution.