Large Mindoro Forest Mouse - Evolution and Phylogenetic Relationships

Evolution and Phylogenetic Relationships

The large Mindoro forest mouse belongs to the Chrotomys division, a group within the Murinae that occurs exclusively on the Philippines and in addition to Apomys also includes Rhynchomys, Chrotomys and Archboldomys. Animals in this division share several morphological and genetic features. Within this group, Apomys is by far the biggest and most extensive genus, containing small, inconspicuous wood mice which are common to the whole of the Philippines, while the other, more specialized genera are barely ever found outside Luzon. Apomys itself was divided in two groups by American biologist Guy Musser in 1982, the first being the datae group, containing only the Luzon montane forest mouse (A. datae), and the second the abrae-hylocetes group, containing all other species. Animals in these two groups differ in the way in which the head is supplied with blood from arteries. Ruedas placed the large Mindiro forest mouse in the datae group. This relationship is further supported by other similarities: both species are relatively large for the genus and have a relatively long snout.

In 2003, a phylogenetic study was published which compared DNA sequences from the cytochrome b gene of thirteen species of Apomys. This study confirmed the proposed relationship between the large Mindoro forest mouse (A. gracilirostris) and the Luzon montane forest mouse (A. datae).

The phylogenetic relationships of the large Mindoro forest mouse can be summarized as follows.

This study also found that the large Mindoro forest mouse was the only Apomys species that displayed deep within-species divergences, which were calculated to date back some 400.000 years. The split between the large Mindoro forest mouse and the Luzon montane forest mouse dates back further still at an estimated three million years ago, putting it in the Pliocene. Apomys itself in turn was found to date back more than four million years, while the Chromotys division was estimated to go back another two million years. These dates were all calculated using a molecular clock.

Another, more elaborate phylogenetic study on all of the endemic Filipino genera and species of Murinae, of which the large Mindoro forest mouse is a member, produced different results. According to this study, the Chromotys division originated some ten million years ago (rather than the estimate of six million years ago from the previous study). It was further calculated that the split between the Chromotys division and its closest relatives, a predominantly African group including Mus, Otomys and Mastomys, had happened about sixteen million years ago. However, since the fossil record of Murinae from the Philippines is non-existent, and controversial for Murinae from other areas, these estimates can only be based on the molecular clock, and not on direct paleontological data.

The large Mindoro forest mouse is the third known rodent endemic to Mindoro, after the Mindoro black rat and the Mindoro climbing rat, and not including the Ilin Island cloudrunner, the occurrence of which has not been confirmed on the island. However, in a compilation work on the indigenous mammals of the Philippines, published in 1998, it was announced that there is a second Apomys on Mindoro. This species has yet to be described and has temporarily been given the name of "Apomys sp. E". It may be related to two other undescribed species from Sibuyan Island and Greater Negros-Panay, Apomys sp. A/C and Apomys sp. B. The local population of A. musculus, the only other Apomys known on the island, may also represent a separate species. Several other species occur exclusively on Mindoro (in addition to the undescribed Maxomys species mentioned earlier), including Oliver's warty pig, the tamaraw and the megabat Styloctenium mindorensis. The relatively large number of endemic species on Mindoro can be explained from the fact that, in all probability, Mindoro was never connected to any other landmass. This enabled the animals present on the island to develop in isolation from their relatives.

A remarkable biogeographical aspect of Mindoro is its status as a transition area between Greater Palawan and the rest of the Philippines. On the one hand, species like Oliver's warty pig and the tamaraw are clearly related to animals from Greater Palawan and the rest of South-East Asia, but on the other, Mindoro also harbours animals from genera like Apomys and Chrotomys, which are clearly Filipino. The latter animals reached Mindoro from Luzon, or possibly in some cases Greater Negros-panay. The large Mindoro forest mouse, with its close relationship to the Luzon montane forest mouse, probably falls into the group of animals that reached the island from Luzon.