Bats of this monotypic genus occur on Bougainville, Choiseul, Santa Ysabel, and Guadalca.n.a.l (see Fig. 4). These four islands probably were contiguous during the maximum lowering of sea level in the Pleistocene (see Durham, 1963:362-363). Bats of the genus _Pteralopex_ are the only kind in the Solomons having a distribution that can be correlated with former land connections between islands.
The distributions of 16 species of megachiropterans known from the Solomons are summarized in Table 5 and in Figure 16. The larger islands (in terms of surface area and elevation) in general have the highest number of species (Guadalca.n.a.l 10, Choiseul 9, and Bougainville 8). But Fauro, one of the smallest islands for which data are available, has six species of megachiropterans whereas San Cristobal and Malaita, two of the larger islands, have only three and four species, respectively.
Possibly this difference signals the need for additional collecting.
Bougainville and Choiseul, about 60 miles apart, have seven species of megachiropterans in common (Table 5). Fauro, 25 miles southeast of Bougainville and 35 miles west of Choiseul, shares five species with each of these islands (Fig. 16). _Pteralopex atrata_ and _Pteropus rayneri_ occur on Choiseul and on Bougainville, but not on Fauro.
Individuals of these species are the largest fruit bats in the Solomons, and their absence on Fauro suggests, therefore, that this small island is ecologically unsuitable, at least in some months, for the support of populations of bats that require relatively large amounts of food. The small size of the island is consistent with this hypothesis, but several other islands as small as Fauro do support populations of the large kinds of _Pteropus_, at least in some months.
TABLE 5. A Summary of Distribution of All Species of Megachiropteran Bats Known from the Solomons. Only Islands Well Known Faunistically Are Listed.
Column headings:
A: Bougainville I: Vella Lavella B: Choiseul J: Kolombangara C: Santa Ysabel K: Russell D: Ndai L: Guadalca.n.a.l E: Malaita M: San Cristobal F: Florida N: Ugi G: Fauro O: Rennell H: Shortland P: Ontong Java
===================+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+== SPECIESABCDEFGHIJKLMNOP -------------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-- R. amplexicaudatusXXXXXXXP. atrataXXXXPt. hypomela.n.u.sXPt. admiralitatumXXXXXPt. tonga.n.u.sXPt. howensisX Pt. rayneriXXXXXXXXXXPt. woodfordiXXXXPt. mahaga.n.u.sXXD. inermisXXXXXXXXXXXXM. lagochilusXXXXXXXM. woodfordiXXXXXXM. aurantiusXXN. albiventerXXXXXXN. majorXXXXN. malaitensisX+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-- Totals89614464354103231 -------------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--
Santa Ysabel has six species of megachiropterans and 10 occur on Guadalca.n.a.l (Table 5). These two islands, separated by about 100 miles of water, share five species (_Rousettus amplexicaudatus_, _Pteralopex atrata_, _Pteropus rayneri_, _Dobsonia inermis_, and _Nyctimene albiventer_). The Nggela Group, in which Florida is the largest island and the only one from which bats have been collected, is 50 miles southeast of Santa Ysabel and 30 miles north of Guadalca.n.a.l (Fig. 16).
Four species of megachiropterans are known from Florida (_Dobsonia inermis_, _Macroglossus lagochilus_, _Melonycteris aurantius_, and _Nyctimene major_). Three of these are known from Guadalca.n.a.l and one occurs on Santa Ysabel. This situation resembles the one involving Fauro, Bougainville, and Choiseul because none of the large bats (_Pteropus_ and _Pteralopex_) is known from Florida, even though two species of large bats that occur on Santa Ysabel to the northwest occur also on Guadalca.n.a.l to the south. Possibly Florida and the smaller islands that comprise the Nggela Group are ecologically unsuitable for large bats, or perhaps these small islands can support only limited numbers of individuals during part of a year.
[Ill.u.s.tration: FIG. 16. The number of megachiropteran species known from individual islands (number within a circle) is compared with the number of species common to two different islands (number without a circle). For names of islands see Fig. 2.]
Some of the small islands in the Solomons have populations of large fruit bats. For example, _Pteropus admiralitatum_ and _P. hypomela.n.u.s_ have been reported from the small islands in the Russell Group (Table 5). Possibly these species do not live concurrently in the Russells; specimens of the two were obtained in different years. Two small megachiropterans, _P. woodfordi_ and _Melonycteris woodfordi_, also inhabit the Russells. Shortland, a small island about 15 miles south of Bougainville, supports one large bat, _P. admiralitatum_, as well as smaller megachiropterans.
Kolombangara and Vella Lavella are about the same size and are separated by about 15 miles of water. _Rousettus amplexicaudatus_, _Pteropus rayneri_, _P. woodfordi_, _Macroglossus lagochilus_, and _Nyctimene albiventer_ have been collected on Kolombangara but only _P.
admiralitatum_, _P. rayneri_, and _Dobsonia inermis_ have been found on Vella Lavella. The difference in the known megachiropteran faunas is more striking when one compares each island with adjacent islands. Two species on Vella Lavella occur also on Choiseul, which is about 35 miles northeastward, and two species occur also on Shortland, which is 120 miles northwestward (Fig. 16). Four of the five megachiropterans on Kolombangara also have been found on Choiseul, about 50 miles northward (Table 5). _Pteropus rayneri_ is the only megachiropteran known from both Kolombangara and Vella Lavella, even though the islands are separated by only a few miles of water. Inadequate data possibly account for the differences in the megachiropteran fauna, but I suspect that some other factors are involved. Although Vella Lavella and Kolombangara do have one species (_P. rayneri_) in common, a different subspecies occurs on each island--_rubia.n.u.s_ on Kolombangara and _lavella.n.u.s_ on Vella Lavella (Fig. 17 and Table 6). This indicates that some factor or factors are operating to keep megachiropterans from moving frequently or easily from one island to the other.
Each of several subspecies of species in the genus _Pteropus_ are known from one or two small islands separated by only a few miles from other islands on which different subspecies occur (see Fig. 6). Judging from this kind of distribution, these bats do not move frequently from island to island. Possibly this is because they cannot easily cross water barriers, or are not inclined to do so because food is abundantly available throughout the year on their home island. Because "flying foxes" frequently are seen in flight over water several hundred yards from sh.o.r.e, the first factor probably is unimportant--at least where short distances are involved. It seems most likely that when abundant food is available these bats have no reason to move even moderate distances.
[Ill.u.s.tration: FIG. 17. The number of subspecies of megachiropterans known from individual islands (number within a circle) is compared with the number of subspecies common to different islands (number without a circle). For names of islands see Fig. 2.]
Distributions of subspecies of polytypic species are summarized in Table 6 and Figure 17. Generally, more subspecies are known from the larger islands than from the smaller islands (Guadalca.n.a.l with 5, Bougainville, Choiseul, and Santa Ysabel with 4, Fauro with 2.) The distributions of some subspecies can be used to judge the differential effectiveness of water gaps between islands. The distribution of _Pteropus rayneri lavella.n.u.s_ and _P. rayneri rubia.n.u.s_ is an example.
Choiseul and Santa Ysabel are separated by about 50 miles of water (see Fig. 17) but have three subspecies in common (_Pteropus rayneri grandis_, _Dobsonia inermis minimus_, and _Nyctimene albiventer minor_.) Choiseul is about 50 miles from Kolombangara and about 35 miles from Vella Lavella, but shares no subspecies with these smaller islands although some species are shared (Tables 5 and 6). From these data one can conclude that exchange of genes between populations on Choiseul and populations on Santa Ysabel is frequent but for some reason exchange of genes between populations on Vella Lavella and Choiseul and Kolombangara and Choiseul is infrequent. A series of small islands (Rob Roy, Wagina, and the Arnavon Islands, not named on the maps) connect Choiseul and Santa Ysabel in stepping-stone fashion (see Fig. 17). Possibly these small islands enhance movement of megachiropterans between Choiseul and Santa Ysabel.
TABLE 6. A Summary of Distribution of Polytypic Species of Megachiropteran Bats in the Solomon Islands. Only Islands Well Known Faunistically Are Listed.
Column headings:
A: Bougainville I: Vella Lavella B: Choiseul J: Kolombangara C: Santa Ysabel K: Russell D: Ndai L: Guadalca.n.a.l E: Malaita M: San Cristobal F: Florida N: Ugi G: Fauro O: Rennell H: Shortland P: Ontong Java
===================+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+== SUBSPECIESABCDEFGHIJKLMNOP -------------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-- P. atrata atrataXXP. atrata ancepsXXPt. a. solomonisXXXPt. a. colonusXPt. a. grandisXPt. r. rayneriXXPt. r. grandisXXXPt. r. rubia.n.u.sXPt. r. lavella.n.u.sXPt. r. monoensisX Pt. r. cognatusXXPt. r. rennelliXD. i. inermisXXXXXXXXXXD. i. minimusXXN. a. bougainvilleXXXN. a. minorXXX+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-- Totals4441212232152221 -------------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--
Florida, of the Nggela Group, is approximately halfway between Santa Ysabel and Guadalca.n.a.l. _Pteralopex atrata anceps_ occurs on Santa Ysabel and on Guadalca.n.a.l but is unknown from Florida. Fauro lies between Bougainville and Choiseul. _Pteralopex atrata atrata_ and _Pteropus rayneri grandis_ occur on Choiseul and on Bougainville but are unknown from Fauro. As suggested earlier, small islands like Fauro and Florida possibly cannot support large fruit bats, although they probably would utilize these small islands when in transit between larger islands.
Fauro apparently is important to the distribution of the two subspecies of _Dobsonia inermis_ and _Nyctimene albiventer_ in the Solomons (see Figs. 9 and 13). In both species, one subspecies is found in the eastern chain of islands and one subspecies is found in the western chain.
Specimens of _Dobsonia inermis_ from Fauro and Bougainville can be identified as the subspecies _inermis_ whereas those from Choiseul are a.s.signable to the subspecies _minimus_. _Nyctimene albiventer bougainville_ occurs on Bougainville but specimens of _N. albiventer_ from Fauro and Choiseul can be identified as the subspecies _minor_.
Although interchange of genes occurs between populations on Bougainville and Fauro in the case of _D. inermis_, the population of _N. albiventer_ on Fauro is at least partially isolated from the population on Bougainville.
Rennell and Ontong Java are relatively isolated from other islands in the Solomons (see Fig. 17). Only one kind of bat (_Pteropus howensis_) is known from Ontong Java and apparently is endemic to that atoll.
_Pteropus tonga.n.u.s geddiei_, one of the megachiropterans that occurs on Rennell (Table 5), also is found in the New Hebrides and on New Caledonia (Table 4). This makes _P. t. geddiei_ the only megachiropteran bat in the Solomons that is more closely related to bats on islands to the southeast of the Solomons than to bats on other islands of the Solomons, the Bismarcks, or New Guinea, to the north and west. The other species of megachiropterans (_Dobsonia inermis_ and _Pteropus rayneri_) on Rennell are found also on other islands in the Solomons. It is to be noted that Mayr (1931) regarded the avifauna of Rennell as most nearly like that of the New Hebrides and New Caledonia. He suggested that the prevailing winds from the southeast have been important for birds that have reached Rennell. The New Hebrides and New Caledonia are four and a half times farther from Rennell than are San Cristobal and Guadalca.n.a.l.
On first consideration a person might doubt that the winds would be favorable enough to compensate for the great distance between Rennell and the New Hebrides and New Caledonia. Darlington (1938) has used the formula X n/m to obtain a comparison of barriers of different widths.
[X = the probability of an individual crossing a barrier of width m; the probability of an individual crossing a similar barrier of width n is the ratio n/m.] If this formula is applied here, one finds that winds from the southeast (that is, from the New Hebrides and New Caledonia) would have to be more than 100 times more favorable than winds from the northeast (from Guadalca.n.a.l and San Cristobal) in order to compensate for the distance of Rennell from the New Hebrides and New Caledonia.
Even so, tropical storms with unusually strong winds, frequent during some parts of the year, possibly account for the present distributional pattern of bats and birds that live on Rennell.
Whatever the means by which bats of the species _P. tonga.n.u.s_ reached Rennell, the fact remains that specimens from Rennell cannot be distinguished from specimens of _P. tonga.n.u.s geddiei_ from the New Hebrides and New Caledonia, more than 500 miles to the southeast.
NOTE: An important and interesting paper on zoogeography of bats, which was published too late to be included here, is: Krzanowski, A., 1967, The magnitude of islands and the size of bats (Chiroptera), Acta Zool.
Cracoviensia, 12:281-348.
LITERATURE CITED
ANONYMOUS.
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