Pacific Science (1973), Vol. 27, No.1, p. 92-98 Printed in Great Britain

The Ecology of in the Tonga Islands I

JOHN TWIBELL2

THE INFLUENCE on crop damage of nor­ shipping came numerous introduced plants and vegicus, Rattus rattus, and the native Polynesian . The arrival dates for the common , rat, Rattus exulans, was studied during the es­ Rattus norvegicus, and the "European" roof rat, tablishment of a rat control program for the Rattus rattus, are not known, but are believed Tongan Department of Agriculture in 1969. to be more recent, probably since the increase This was the first long-term study of Tongan of regular shipping trade and the construction rodents. Previous scientific literature on Ton­ of wharves. gan is very sparse. The Kingdom of Presently rodents account for approximately Tonga, or Friendly Islands, consists ofapproxi­ 20 percent of the agricultural losses and mately 150 small islands with a combined area $50,000 worth ofeconomic loss each year (Twi­ of about 256 square miles at lat 21 0 S. The bell, unpublished). This is a conservative esti­ majority ofthese islands are composed ofraised mate based on damage counts and observation. coral limestone; however, there is a row of six In some areas destroy or damage up to volcanic islands on Tonga's western border. 50 percent of the coconuts, which represent the Tongatapu, the location of the government main economic crop in Tonga. center, is the largest and most important island. The following are acknowledged for their The Ha'apai island group lies 80 miles north of greatly appreciated assistance in making this Tongatapu, and 150 miles north is the Vava'u article possible: E. H. Bryan, Jr., manager of group. is 420 nautical miles east and Samoa the Pacific Scientific Information Center, Ber­ is 480 miles north. The climate is tropical and nice P. Bishop Museum, Honolulu; John is influenced seasonally by trade winds. Pittman, Agriculture Department, Kingdom of Concerning the people, T. F. Kennedy Tonga; James Wilson, South Pacific Commis­ (1958) says: "Polynesians, ranging southwards sion; Timothy Lawler and Dan Brant, Hum­ from Samoa, first settled in Tonga in about boldt State College. 500 A.D. They found a widely-scattered group of high volcanic and low raised-coral islands. METHODS They established themselves, not only on the main islands, but also on many of the smaller From January to December, 1969, rodents islands, which, although now uninhabited, were obtained by snap traps and livetraps set show evidence of past settlement, probably on adjacent to and in agriculturalareas todetermine a family-unit basis." Some students of Pacific densities, habitat preference, food habits, and prehistory put the date ofarrival ofPolynesians related crop damage. Various means of in Tonga much earlier, and suggest that they control were also tested; anticoagulant poisons came to Tonga from Fiji. These details are not were the principal control methods em­ important to this paper; Polynesians have been ployed. traversing the central Pacific for many cen­ External measurements were taken according turies, and during this period doubtless trans­ to the British system of measuring tip of nose ported the to many islands in to anus for body length and from anus to tip of their voyaging canoes. (See Tate, 1935.) tail integument for tail length. These combined Since Captain Cook's first visit in 1773, measurements were used in Table 1. Western civilization has brought trade, mis­ Some rats were maintained alive in captivity sionaries, and perhaps rats to Tonga. With this for tests of food preference, behavior, and general observation. r Manuscript received 23 March 1972. Five islands were trapped to give an indica. 2 Route no. 1, Box 63, Trinidad, California 95570. tion of the distribution present (Table 2 92

_ ¥ ...ill Ecology of Rodents in Tonga-TWIBELL 93

TABLE 1

TOTAL LENGTH OF ADULT Rattus exu/ans FROM MID-PACIFIC ISLANDS

X BODY LENGTH LOCATION (mm) 2 SE N

Kure Atoll, 292 80 (W. O. Wirtz, 1972) Ponape, 282 16.1 ±6.82 22 (D. H. Johnson, 1962) Upolu, Western Samoa 259 15.8 ±4.84 42 (R. R. Marples, 1955) Tongatapu, Tonga 241.2 18.3 ±6.9 28 (Present study, 1969)

TABLE 2

RELATIVE ABUNDANCE OF SPECIES TRAPPED ON FIVE TONGAN ISLANDS

Rattus Rattus Rattus norvegicus rattus exu/ans LOCATION (%) (%) (%) NUMBER

Tongatapu 56 5.5 38.5 55 'Eua 29 71 14 Vava'u 21 11 68 28 Niuatoputapu 100 29 Hunga 12.5 50 37.5 8 Total 134 and Fig. 1). No individuals of Mus musculus vanced in the kingdom. Some fertilizers and were caught, observed, or reported to have limited farm machinery are used. been caught, contrary to other reports (Carter, All three species of Rattus are present. R. Hill, and Tate, 1945). The five islands trapped norvegicus damages approximately 30 percent of differ in relation to geological makeup, size, such ground crops as sweet potatoes, peanuts, trade contact, agricultural methods employed, watermelon, and corn (Fig. 2). R. rattus is the and composition of the Rattus population. sole cause ofcoconut damage, which amounted to a 20-percent loss during the study. R. exulans also contributed to ground crop damage to RESULTS tapioca, sweet potatoes, peanuts, and corn. On Tongatapu Tongatapu the Norway rat is present near agri­ Tongatapu was the most intensively studied cultural areas whereas the Polynesian rat is island because it is the largest and most pro­ more frequently found in native tropical bush. ductive agriculturally and has the best facilities Jackson and Strecker (1962) found a similar for transportation. Government offices are in pattern of distribution in Micronesia. the capital city, Nuku'alofa. Tongatapu is a Trapping of the same area in 1965 yielded flat, uplifted, coral island, with a maximum 65 percent R. exulans and 25 percent R. norvegi­ height of 270 feet. The topsoil is composed of cus (Bianchi and Smythe, 1965). In contrast, my 2 or 3 feet of volcanic clay. Approximately 60 trapping showed Norway rats to be more abun­ percent of the 100-square-mile land area is de­ dant (Table 2), suggesting that the Polynesian voted to raising coconuts and bananas for ex­ rat is possibly being forced out of the agricul­ port. The agricultural methods are the most ad- tural habitat into the native bush areas. Modern 94 PACIFIC SCIENCE, Volume 27, January 1973

~'NinJ1'O'oU 7{eiUfLL~fava'u ~j· ";( (71nCcmI) 4. J{unqa I r::> 0 • 1.3 } 'V~~" 'Niuataputapu o' .·V. ) " ("". ••. ~ I Late 5r ••) I.CQi - ( . ) VA VA 'u G R. 0 U P ~ / '-~ / I ) \. ) ( ) l ) "-J

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Fonurifo'ou 1. o

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FIG. 1. Map of Tonga. Numbers indicate sampling areas, Ecology of Rodents in Tonga-TWIBELL 95

FIG. 2. Corn damaged by Norway rats (Rattus norvegicus). agricultural practices seem to favor R. nor­ mately 20 percent here also. Apparently R. vegicus. exulans thrives well on guava fruit, which is On two occasions I observed that, when abundant among the coconut groves. No members of the same sex of R. norvegicus and R. specific food studies were made to demonstrate exulans were put together in an observation this; however observation has verified it. cage, the Polynesian rats were killed by the larger and more aggressive Norway rats within Vava'u 30 seconds. These data suggest that R. nortJegicus is the more aggressive species, which charac­ Vava'u (the name of the main island is 'Uta teristic may result in an increase in its numbers Vava'u) is the second largest island in the king­ on the island. dom, 34.6 square miles. It is about 165 nautical or 187 statute miles north of Tongatapu, and like Tongatapu it is composed of raised coral 'Eua limestone, with a rich topsoil, and a maximum 'Eua is a relatively pristine and mountainous height of 670 feet on its western coast. Vava'u island. Its maximum elevation is 1,078 feet and is the second-best copra-producing area in the its area is 33.7 square miles. Approximately Tonga Islands. Approximately 30 percent of 20 percent of the land area is used for agricul­ the tillable land is used at one time. The other ture. The absence of Norway rats on 'Eua good soil is allowed to lie fallow for 4 to 6 years (Table 2) may be due to the limited trade (a daily to regenerate itself. launch) and lack of agricultural areas. Coconut All three species of Rattus were present here, damage by individuals of R. rat/us is approxi- but R. exulans dominates in relative abundance 96 PACIFIC SCIENCE, Volume 27, January 1973

FIG. 3. Coconuts damaged by roof rats (Ratttls ratttls).

(see Table 2), which is unlike the situation in population seems to be most abundant in this Tongatapu. This could be due to several crop. Coconut loss due to rats is assessed at factors: (1) the Norway rat has not been in between 50 percent and 70 percent (see Fig. 3). Vava'u as long as it has been in Tongatapu, This is similar to Smith's observations (1968) in (2) the environment is not well suited to R. the Gilbert Islands. norvegicus because it is less developed agricultu­ rally, or (3) R. tJorvegicus exhibits different be­ Niuatoputapu havior on this island. Niuatoputapu (also called Keppel's Island) is one ofthe most isolated islands in the kingdom. Hunga Except for neighboring Tafahi, which is a Hunga is an outlying island near 'Uta rugged volcanic peak 2,000 feet high with an Vava'u, in the Vava'u group, with an area of area of 1.3 square miles, it is 50 miles from any about 1.85 square miles and a maximum eleva­ other island. It is visited by a government ship tion of 245 feet. It is visited only by local about once a month, but has no wharf other launches. Primitive agriculture is practiced here. than one which will accommodate launches. It There are no motor-driven vehicles or ad­ is a volcanic island with an area of 5.7 square vanced farming techniques on the island. Copra miles, fertile lowlands surrounding a 350-foot sold for export is the only source of livelihood peak. for the people but, unfortunately, the R. raltus Only R. exulans was found on Niuatoputapu,

tMA; ¥!!b Ecology of Rodents in Tonga-TwIBELl 97 but it was found in large numbers. Trapping areas is not available, it will move to places results in August 1969 indicated about 40 rats adjacent to fields and subsist on native fruits, per acre. No coconut damage was observed and seeds, roots, and insects. no rats were caught in coconut palms. This was 3. R. exulans appears best suited to habitats of quite different from conditions found in the native plants. The sensitive plant (Mimosa Tokelau Islands, 300 nautical or 340 statute pudica) produces an abundant amount of miles north of Samoa (Wodzicki, 1968). On seeds that are palatable to them. The Poly­ Niuatoputapu only slight agricultural damage nesian rat is a ground dweller in Tonga; it was evident on some root crops such as the frequently builds subterranean nests near or giant taro and sweet potato. One-hundred­ under Ischaemum grass. Therefore, R. exulans percent damage is common for corn crops. and R. norvegicus may be in competition for the same habitat.

DISCUSSION SUMMARY Compiled data provide evidence to compare The three species of Rattus are apparently the body size of R. exulans (see Table 1). Its distributed according to time of arrival in the mean body length of 241.2 millimeters from islands, habitat preference, agricultural condi­ Tongatapu is significantly smaller than on tions, and possibly competitive relationships. other central Pacific islands. Tate (1935) sug­ Islands with more trade and advanced agricul­ gested that the Tongan R. exulans is derived tural techniques are likely to have roof rats and from more northerly populations. Although Norway rats. Both ofthese rats cause economic Tate referred to the homogeneity ofR. exulans losses and are potential disease carriers. The from different islands, the evidence presented more widely spread indigenous species, R. here demonstrates that identifiable strains have exulans, remains relatively unimportant eco­ evolved. nomically and is found in native habitat and Predators are rare on the islands, except for root crops where R. norvegicus is not present. one species ofowls and feral cats (Baker, 1946). R. norvegicus dominates grassland and agricul­ The major determinants of rat population size tural areas whereas R. rattus is usually found seem to be available habitat and food. near its preferred coconut trees for climbing A general habitat preference was observed and nesting. for each species:

1. R. rat/us occasionally lives on the jungle LITERATURE CITED £loo1-, but it is usually trapped on the trunks or in the crowns of coconut palms. This rat BAKER, R. H. 1946. A study of rodent popula­ typically gnaws a hole in green coconuts and tion on Guam, Mariana Islands. Ecol. proceeds to drink the milk and eat the meaty Monogr. 16: 393-408. material adhering to the edges of the shell BIANCHI, FRED, and R. SMYTHE. 1965. Report (Fall, Medina, and Jackson, 1971). Nests with of rodent control clinic given in Tonga 1965. young frequently are found inside large Sponsored by East-West Center, University coconut shells. Of all three species of rats of Hawaii. Mimeographed. Copy on file, released alive on the surface of coconut tree Department of Agriculture, Nuku'alofa, trunks, only R. rattus attempted and was Tonga. successful in climbing to the tops of the CARTER, T. D., J. E. HILL, and G. H. H. TATE. crown. This is a further indication that roof 1945. Mammals of the Pacific world. Amer. tats are most suited for living in treetops. Mus. Nat. Hist., New York. 263 p. 2. R. norvegicus lives in burrows associated with FALL, M. W., A. B. MEDINA, and W. B. JACK­ agricultural crops, among tree roots, and SON. 1971. Feeding patterns of Rattus rattus around tall grasses such as Ischaemum. If and Rattus exulans on Eniwetok Atoll, Mar­ suitable habitat for nesting in agricultural shall Islands. J. . 52: 69-76. 98 PACIFIC SCIENCE, Volume 27, January 1973

JACKSON, W. B., and R. L. STRECKER. 1962. TATE, G. H. H. 1935. Rodents of the genera Ecological distribution and relative num­ Rattus and Mus from Pacific islands, collected bers, p.45-63. In: T. I. Storer [ed.] Pacific by the Whitney South Sea Expedition, with island rat ecology. Bull. Bishop Mus., discussion of the origin and races of the Honolulu 225. 274 p. p,adfic rat. Bull. Amer. Mus. Nat. Hist. 68: JOHNSON, D. H. 1962. Rodents and other 145-178. Micronesian mammals collected, p. 21-44. WIRTZ, WILLIAM 0., II. 1972. Population In: T. I. Storer [ed.] Pacific island rat ecology. ecology of the Polynesian rat, Rattus exulans, Bull. Bishop Mus., Honolulu 225.274 p. on Kure Atoll, Hawaii. Pacif. Sci. 26 (4): KENNEDY, T. F. 1958. Village settlement in 433-465. Tonga. N.Z. Geogr. 14 (3): 161-172. WODZICI