Character displacement and release in the small Indian mongoose, Herpestesjavanicus

In western parts of its native range, the small Indian mongoose (Herpestes javanicus) is sympatric with one or both of two slightly larger congeners. In the eastern part of its range, these species are absent. The small India n mongoose was introduced, about a century ago, to the West Indies, the Haw aiian islands, Mauritius, the Fijian islands, and Okinawa. All introduction s except possibly that to Mauritius were from the region of Calcutta and Ba ngladesh, where it is sympatric with both congeners. No other mongoose is p resent on any of these islands. In each instance, the introduced population derived from a small propagule. We examined size variation in the maximum diameter of the upper canine tooth (the prey-killing organ) and skull lengt h. In the eastern (allopatric) part of its native range, males of the small Indian mongoose are much larger in both traits than in the western (sympat ric) regions, approaching the size of the smaller of its absent two congene rs, Herpestes edwardsii. Females from the allopatric part of the range are also larger than those of the source region. The species is more sexually d imorphic in the region of allopatry. On all islands to which it has been in troduced, in 100-200 generations the small Indian mongoose has increased in male size and in sexual dimorphism; changes in female size have been sligh t and inconsistent. In general, sizes of island individuals are approximate ly intermediate in size between those in the region of origin (where they a re sympatric and small) and those in the region of allopatry. Sexual dimorp hism is greatest for canine diameter. Thus, H. javanicus shows variation co nsistent with ecological release from competition with its congeners. There is no evidence on whether this variation is genetic, nor on what dietary i tems might be partitioned between species and between sexes. However, morph ological variation is consistently smaller for both traits and both sexes o n the islands of introduction than in any part of the native range, consist ent with idea of a genetic bottleneck imposed by the small propagule size. Neither of the two congeneric mongooses shows morphological variation consi stent with ecological release from competition with H. javanicus in the sou thern part of their ranges, where the latter species is absent.