Ultrasonic Differentiation Between Two Species of Chinese Pygmy Dormice (Genus Typhlomys) With Support for the Size-Signal Allometry Hypothesis

The genus Typhlomys comprises six species that all exhibit exceptional climbing agility in arboreal habitats, of which five have been established to use ultrasonic echolocation in the 80-120-kHz frequency range to navigate among tree branches. Here, we investigated the ultrasonic vocalizations of the remaining and recently recognized species, T. fengjiensis, and compared its ultrasonic and morphological traits with its sibling species T. daloushanensis. Both species produced frequency-modulated (FM) ultrasonic calls that lacked harmonic structure, consistent with echolocating calls established for other members of this genus Typhlomys. This FM echolocation call structure is well-adapted to navigating along branches in dense foliage conditions in the forest understory. Importantly, however, the specific call structures of T. fengjiensis and T. daloushanensis exhibited significantly different ultrasonic characteristics, with different numbers of pulse groups, in support of phonic speciation. T. fengjiensis was on average larger than T. daloushanensis and vocalized at a lower frequency and for a longer duration, in support of the signal-size allometry hypothesis. Furthermore, T. fengjiensis has the lowest ultrasonic call frequency among Typhlomys spp., corresponding with it being the largest member of this genus. Bergmann's law does not provide a compelling explanation of the body mass differences between T. fengjiensis and T. daloushanensis, due to the likely overlap in their elevational distribution. Further research is needed to establish if differences in habitat selection and diet, or differences in social and reproductive behavior, might best explain this local species divergence based on phonic traits.