Geographical variation in squirrel mating calls and their recognition limits in the widely distributed species complex

Acoustic signals are one of the reproductive isolation mechanisms that can be a driving force for speciation. Evidence that geographic variation in mating calls can cause reproductive isolation is abundant for birds and frogs, but not for mammals. In the Callosciurus genus of tree squirrels distributed in Southeast Asia, males emit mating calls to attract females and the sound properties are known to be distinctly different among coexisting congeneric species. We investigated whether mating calls emitted by males differ between ten populations of the widely distributed C. erythraeus / C. finlaysonii complex. Based on the acoustic properties, the populations were divided into four groups (South Vietnam, Taiwan, eastern China, and western areas including Thailand and western China), which resulted in a high discrimination rate of 94.7% by a discriminant function analysis. A playback experiment was also conducted to determine whether the mating calls from different localities were as effective in attracting conspecific individuals as those in the same locality. In Kanagawa, Japan, where squirrels were introduced from Taiwan in the 1950s, squirrels responded more frequently to the playback sounds recorded in Taiwan compared to those recorded in Thailand. In addition, the response of squirrels decreased with an increase in the difference in acoustic characteristics compared to that from the original habitat. These results suggest that local differences in acoustic characteristics could be one of the mechanisms that promote reproductive isolation of geographically distant populations of Callosciurus . Significance statement Acoustic signals are one of the reproductive isolation mechanisms that can be a driving force for speciation. The tree squirrel genus Callosciurus (Sciuridae) is widely distributed in Southeast Asia and is one of the few suitable mammals for testing the premating isolation mechanism using acoustic signals. The mating calls of the C. erythraeus / C. finlaysonii complex were divided into four geographical groups (Taiwan, Vietnam, eastern China, and Thailand) by a discriminant function analysis. Playback experiments revealed that the squirrels responded more frequently to the sounds of their own group compared to those of other groups. In addition, the response of the squirrels decreased with an increase in the difference in acoustic characteristics compared to that from the original habitat. Local differences in acoustic characteristics could be one of the mechanisms that promote reproductive isolation of geographically distant populations of Callosciurus .