Male advertisement calling tactics in northern and southern populations of spring peepers (pseudacris crucifer)

Abstract

The evolution of reproductive isolation is the foundation of species formation but identifying the factors that underlie this process can be challenging. Reproductive isolation can arise because of ecological divergence or direct selection on traits associated with the mating system; however, these drivers are not mutually exclusive, and one may influence the other. Often populations of a species that are geographically isolated also display divergence in behavioural characteristics, as is evident in my focal species, the spring peeper (Pseudacris crucifer), a broadly distributed North American treefrog. Male spring peepers in the northern portion of the species’ range exclusively call from ground-level within breeding aggregations, whereas individuals in populations at the southern portion of the species’ range tend to perch arboreally when calling. I test whether mating call perch site selection in spring peepers might relate to seasonal changes in vegetation structure and thus influence acoustic transmission success. Results of playback transmission experiments conducted in Florida and Ontario suggest that southern populations might have higher acoustic transmission success when calling from arboreal perches while transmission of spring peeper mating calls in northern habitats appear more effective at ground-level over short distances or exhibit no differences between the two perch heights over longer distances. Although relative humidity, air temperature, and obstacles such as vegetation can affect acoustic transmission, my samples sizes and thus statistical power were insufficient to compare directly variation of transmission success between Florida and Ontario; however, I did document trends that implied differences between the two regions. My results suggest that behavioural differences in male calling between Florida and Ontario are caused, at least in part, by pressures to optimize acoustic transmission of mating calls in their respective habitats. If true, these divergent selective pressures might ultimately contribute to the evolution of reproductive isolation. Future research should focus on determining why acoustic transmission of mating calls differs between northern and southern environments, and whether differences in calling behaviour would affect mating success in these two regions.