The potential role of reproductive interference in the decline of native green treefrogs following Cuban treefrog invasions

Abstract

Reproductive interference is increasingly implicated in the decline of native taxa following alien species invasions. We examined reproductive interference in the context of acoustic modulation of hormone levels and declines in native green treefrogs (Hyla cinerea) following invasions of Cuban treefrogs (Osteopilus septentrionalis) in the southeastern United States. The motivation for our study stemmed from similarities in spectro-temporal properties of Cuban treefrog courtship calls and green treefrog aggressive calls. The aggressive calls of green treefrogs stimulate glucocorticoid production in rival conspecific males, which suppresses androgen production and reproductive behavior. We thus hypothesized that native male green treefrogs exposed to advertisement calls of this invasive species exhibit increased levels of circulating glucocorticoids. We found that broadcast aggressive calls of green treefrogs stimulated glucocorticoid production in conspecific males but that Cuban treefrog vocalizations did not increase glucocorticoid levels in green treefrogs. In natural choruses, the density of calling male green treefrogs and the proportion of non-calling males were positively correlated with glucocorticoid levels and negatively correlated with androgens, whereas body size was positively correlated with androgens. After controlling for these variables, there was no evidence that hormone levels or behavior differed in choruses of green treefrogs in the presence and absence of Cuban treefrogs. Together, our results suggest that interactions among male green treefrogs modulate hormone levels but that the presence of Cuban treefrogs, calling or not, is not altering levels of glucocorticoids or androgens in breeding male green treefrogs.