Relationships between vocalization characteristics and sperm traits in a neotropical treefrog

Sexual selection can drive the evolution of any trait that enhances mating success and, when females mate with multiple males, competitive fertilization success. Sperm competition theory predicts a fundamental trade-off between traits under pre- and postmating sexual selection when resources are limited. However, variation in male condition and so resource availability for all fitness traits could lead to a positive association between the same traits across males. Such a link could be used by females to infer male fertility based on the expression of male traits under premating sexual selection (phenotype-linked fertility hypothesis). Most studies examining these types of covariation have focused on ornaments and armaments, but another widespread and costly trait category is acoustic signaling. In frogs, studies have examined sexual selection on either vocalization or sperm characteristics, but the link between them has been largely overlooked. Here, we examined the phenotypic covariation between multiple acoustic characteristics and testicular or sperm traits in the Brazilian treefrog Scinax crospedospilus . In multivariate analyses, we found the size of sperm heads and tails to covary positively or negatively with vocal traits, whilst sperm number increased with the call rate. By exploring beyond physical traits under premating or testis size under postmating sexual selection, our results revealed how males invest in different multivariate traits across episodes of selection to maximize their fitness. Including further traits is important considering the widespread use of other modes of costly sexual signaling and that multiple ejaculate traits may affect competitive fertilization success.