Rapid character displacement of different call parameters in closely related treefrogs ( Hyla cinerea and H. gratiosa )

When the range of similar species begins to overlap, niche separation may develop to maintain species boundaries. Reproductive traits are often subject to these adjustments, particularly when there is selection against hybrids. Shifts in range overlap can result in call interference, increased hybridization, and reduced mating success. Previous research has shown that these conditions can drive reproductive character displacement (RCD). Consistent with RCD, green tree frogs (Hyla cinerea) call at higher frequencies and perch higher in syntopy with their sister species (barking tree frogs, Hyla gratiosa), relative to allotopy. However, the time needed for these changes to occur and the corresponding effects on H. gratiosa remain unclear. We investigated if RCD is detectable in populations of these two species after 8 years or less of syntopy. We found an elevated high-frequency peak in calls of syntopic H. cinerea, while an increase in call duration was detected in syntopic H. gratiosa. Our results suggest that RCD can occur rapidly, in a manner consistent with the plasticity-first model, and that traits improving signal detection may be the first to respond to the pressures of syntopy and selection against hybrids. Related species are expected to change their mating signals when breeding together to avoid mating with the wrong species. Green tree frogs change their mating calls when they breed in the same ponds as barking tree frogs, but it remains unclear how fast these changes can take place, or if there are any changes in barking tree frogs. We analyzed calls and perch locations within areas of recent (<8 years) range overlap in ponds where these species breed together and ponds where only one breeds. We found that when the two frogs co-occur, green tree frogs call at higher frequencies and barking tree frogs increase their calling duration. These changes were in the same direction, but diminished relative to ponds where they have lived together for long periods. Our results demonstrate that behavioral changes to avoid hybridization can occur in as few as 8 years.