As we former predicted, A. olivacea showed higher levels of morphological variation in comparison with O. longicaudatus, along the southern Patagonia of Chile. Moreover, such variation was associated to cranial size differences. Our morphometric analyses showed that in A. olivacea, individuals from Isla Wellington are of bigger cranial size if compared to their conspecifics of any other sampled locality along Patagonia. Additionally, we also observed a slight tendency to a bigger cranial size for specimens from Isla Riesco. In fact, the probability of inhabiting an island increased with higher values of cranial size in A. olivacea. Instead, the latter pattern was not observed in Oligoryzomys longicaudatus. Actually, this species did not exhibit significant differences in cranial size between insular and mainland individuals, although a very subtle variation of specimens from Wellington, Capitán Aracena and Navarino islands was observed. Thus, our data suggest that geographic isolation could be a relevant feature behind the cranial morphological differentiation of A. olivacea in the Fuegian archipelago. Moreover, such differences seem to vary according to classical evolutionary trends described for mammals (i.e., the Island Rule [4, 11]). However, further studies should be carried out to test exhaustively the prediction of the Island Rule in this sigmodontine rodent.
How can we explain the differences in the pattern of morphological variation for the two rodents studied in Patagonian localities? We suggest that a synergistic effect of the entangled geomorphologic scenario of Patagonia, the historical glacial processes that affected the region, and the idiosyncratic dispersal capability (vagility) of each species, might be among the major reasons. Successive events of connection and isolation between Tierra del Fuego island, the southern tip of continental South America, and several islands surrounding these land masses (e.g. Harrison and Navarino) were produced during Pleistocene period . Product of ice sheets and sea level dropping (up to 100 m), potential geographic scenarios for the passage or dispersal of biota through glacial bridges between mainland and islands could have occurred in the Magallanic region. Thus, small mammal populations may have been in geographic contact through insular and mainland territories in Patagonia, with low possibilities of local differentiation due to isolation, especially for O. longicaudatus due to its greater dispersal capacity compared to A. olivacea [24, 44].
On the other hand, Lomolino  exhibited a marked positive curvilinear correlation between area of the island and body size, and a negative correlation between body size and distance from the island (the longer the distance, the smaller the body size). In our results, we found a subtle increase in cranial size for specimens of A. olivacea from Isla Riesco, which is very close to the mainland, separated from the continent by narrow channels and sines (i.e., the Otway Sinus). Specimens from Tierra del Fuego island, however, did not show an evident pattern of cranial size differentiation with respect to the mainland. As Lomolino  suggested, body size of individuals inhabiting large islands, seems to vary as if they were at the continent. This would be the case for populations from Tierra del Fuego. In addition, the latter island is very close to the mainland. In fact, the narrowest portion between Tierra del Fuego and the mainland (Primera Angostura) is about 3.7 kms, and with a surface of 48,000 km2 represent the biggest island of the Fuegian archipelago. Meanwhile, the Wellington and Riesco islands, with their 5500 and 5000 km2 respectively, constitute the second and the third largest islands sampled in this study, being almost 10 times smaller than Tierra del Fuego. Hence, the geographic proximity of Tierra del Fuego to the mainland, as well as its large geographic extension, could resemble a small continental area. Therefore, we should not expect major differences in cranial size for the small mammals inhabiting this area. In fact, specimens from Tierra del Fuego resulted about the same size with respect to those of the continent in our study. Regarding Oligoryzomys from Navarino Island, we just found a subtle increase in cranial size. This island located below Tierra del Fuego, is separated from the latter through the Beagle Channel by less than 50 kms, and it constitutes the smallest island (2473 km2) sampled in this study. Furthermore, we just analyzed only three specimens of Oligoryzomys from Navarino, so probably we were not obtaining the real variation present on this island.
Our data supported the tendency of Abrothrix olivacea to increase its cranial size on islands. This fact was particularly recognizable for individuals from Isla Wellington. Some studies evaluating changes in size and morphology in different orders of insular mammals, have concluded that these evolve at a faster rate than their mainland congeners, and by a greater rate when they occur in smaller islands  (although see ). The latter scenario could explain the increment of cranial size in A. olivacea from Wellington island, since as it was mentioned above, that insular area is almost 10 times smaller than Tierra del Fuego. In addition, this phenomenon could be also related to the strong selection pressure faced for the island populations of A. olivacea in Wellington, due to the longer history of isolation across the last two glacial periods (~ 0.2 Mya ). This is besides Isla Wellington is very close to the continent and surrounded by other islands. Furthermore, Isla Wellington is located next to the Southern Icefields (a remnant of the LGM), an area that was severely glaciated during the LGM of the Pleistocene. However, the presence of glacial refuges in Isla Wellington according to palynology and fossil record of insects, was later corroborated for Abrothrix olivacea and the deer Hippocamelus bisulcus [23, 48, 49]. On the other hand, no species of carnivores have been recorded on this island ). The only known predators of this system are some raptors occasionally sighted by local people (Park Rangers of Bernardo O’Higgins National Park, personal communication to ER-S). Thereby, the strong cranial size differentiation of A. olivacea (as noticed in our linear and geometric morphometric analyses) may have its origins in the absence of predators, and long periods of isolation to monopolize local resources. In fact, in the light of most recent molecular evidence, the geographic forms of A. olivacea from Isla Wellington, are recognized as the subspecies Abrothrix olivacea markhami , proposal that we would be supporting with our morphometric data.
The slight increase of cranial size that we observed in O. longicaudatus from Isla Wellington, agreed with previous studies that have shown a subtle mass increment for this mouse on Patagonian islands , but such small variation has not been associated with the occurrence of an insular race of O. longicaudatus. However, in the Harrison and Capitan Aracena islands of Patagonian fjords (54° S) a new species of Oligoryzomys has been recently described, Oligoryzomys yatesi ) based on morphology, chromosomes and molecular sequence data. The relevant point here is that some male specimens of O. yatesi where about double the size if compared to its sister species O. longicaudatus from mainland Magallanes region. Two of us (REP & ER-S) captured nine specimens from Harrison Island (the type locality for this new species), an island that is half the size of the smallest islands sampled in this study.
Interestingly, our morphological data did not recognize the occurrence of the other two subspecies of A. olivacea in Patagonia, A. o. canescens and A. o. xanthorhina, as it was proposed by Rodríguez-Serrano . The first subspecies would be mostly restricted to the Brunswick Peninsula and mainland Magallanes: Torres del Paine, Magallanes National Reserve, Fuerte Bulnes and other nearby localities sampled in this study, whereas, A. o. xanthorhina would be restricted to islands such as Tierra del Fuego and Riesco . Nevertheless, ongoing phylogeographic studies for Patagonian populations of A. olivacea are recovering two major clades in the area: one restricted to islands and the other for the mainland, which would correspond to both subspecies canescens and xanthorhina, respectively (unpublished data). These results agreed with the finding of two allopatric clades between Tierra del Fuego and Patagonian mainland populations evaluated with molecular data by Lessa et al.  and Abud . The incongruence between morphological and molecular variation could be explained because the former usually responds to similar selective pressures along distributional gradients. This might not be reflecting the intrinsic genetic variation due to, for example, the biogeographical processes that affected the Patagonia ecoregion . By contrast, Oligoryzomys longicaudatus that also characterizes for having a wide distributional range (27–54° S), it does not show a strong structured population pattern along its geographic distribution, and in Patagonia a single subspecies is recognized, O. l. magellanicus . Furthermore, underway phylogeographic studies in the area are not showing any structure for the latter taxon either in the mainland or the islands of Fuegian archipelago.
Overall, our study highlights that the morphological changes experienced by small rodent taxa in the Patagonia could be explained partially in terms of classical biogeographic trends (i.e., island size and distance or ecological pressure release, the Island Rule). However, factors such as the glacial history of Patagonia and the distinct dispersal abilities of A. olivacea and O. longicaudatus could be considered important subjacent causes. Therefore, it is possible that the higher levels of morphological variation observed in the crania of A. olivacea, compared to O. longicaudatus, could be a tendency of the former species to rapidly adapt to local conditions. Moreover, A. olivacea displays strong population structure by its lower vagility, scenario that could be reinforced under insular conditions (i.e., A. o markhami from Wellington island). We hope to stimulate future researches that extend our geographic sampling along mainland and islands of the Fuegian archipelago to test the findings of this study. In this line, our linear and 3D-geometric morphometric data are valuable for a better comprehension of evolutionary tendencies of small mammals in the remote Patagonian region, in terms of the taxonomy of both species, and for eventual management programs.