Loss of the third upper premolar in Suncus murinus and its relation to jaw size

Musk shrew Suncus murinus (Linnaeus, 1766) has a very reduced P3, and it is often missing. Hanamura (1985), based on a sample from Okinawa Island, proposed that the high incidence in P3 loss was a distinguishing characteristic of musk shrew. However, while the Okinawa population lacked P3 in 26 of 95 individuals (27.4%), specimens from Taiwan showed no P3 loss. Thus, the high incidence of P3 loss is not one of distinguishing characteristic of musk shrews. In the Okinawa sample, P4-M3 length vs palatal length in the group with P3 on both sides was significantly greater than that without P3. The relationship between the P4-M3 and palatal lengths showed negative allometry (Okinawa population with P3: y = 0.18 + 0.67a:; Okinawa population without P3: y = 0.21 + 0.67x; Taiwan population: y = 0.15 + 0.68x). Taiwan population had a greater P4-M3 length relative to palatal length than did the Okinawa population because palatal length was greater in the former. These findings suggest that, as in the case of human third molars, a reduction in upper jaw size is responsible for the loss of the third molar in the Okinawa musk shrews.