Population Dynamics of Varanus indicus Daudin in a Limestone Forest of Guam

Population Dynamics of Varanus indicus (Daudin) in a Limestone Forest of Guam

by
David R. Gardner

A proposal for a thesis submitted in partial fulfillment
of the requirements for the degree of Master of Science in Biology

Introduction

The Pacific monitor lizard Varanus indicus (Daudin) has a major predatory role in Micronesian forests--it feeds on reptile and bird eggs, land snails, rodents, insects, crabs, small lizards, and carrion (Dryden 1965). Evidence in the research library of the University of Guam Marine Laboratory also indicates that the monitor lizards also feed on fish--fish specimens preserved in the research library had been regurgitated by a cornered monitor in Palau. However, little ecological work with this animal has been done. Existing studies of Varanus indicus involve aspects of the monitor's feeding habits and food preferences (Dryden 1965) and the lizard's utility as a rat control agent (Uchida 1967). Other authors have mentioned the lizard during the course of their own studies (Crampton 1921, Fulbeck 1947, Kaneshiro 1950, Gressitt 1952, Fosberg 1956, Bates and Abbott 1958), but other than commenting about the introduction of monitors by humans and the resulting lizard-human interactions, they have done little more than to document the wide distribution of Varanus indicus.

Although lizards in general have received the greatest attention among reptiles with regard to population dynamics (Porter 1972), most of these studies have been limited to specific life-history aspects of a few species within the families Iguanidae and Agamidae (Auffenberg 1978). These studies have dealt with mostly temperate environments; quantitative studies in the tropics have not been undertaken (Porter 1972). Varanids are tropical and subtropical old-world lizards (Schmidt and Inger 1957). Because they attain large sizes, are difficult to maintain in controlled experimental environments, and tend to have low population densities in the wild, the monitors have not been the subject of extensive ecological studies (Auffenberg 1978).

Varanids (and other families of lizards) have been assumed to have life-history and population characteristics similar to those of the iguanids and agamids (Auffenberg 1978). Auffenberg (1978) has shown in comprehensive studies, however, that the social and feeding behaviors of Varanus komodoensis Ouwens differ significantly from those of iguanids and agamids. Noting that there is only one genus, Varanus, in the varanid family, Schmidt and Inger (1957) emphasized the physiological and behavioral similarities between all members of the family Varanidae. Thus, Varanus indicus on Guam may differ from iguanid and agamid behavior and population characteristics with a similar extent as Varanus komodoensis. The objective of this study is to document the seasonal changes in the population of Varanus indicus in a limestone forest community on Guam, Marianas Islands.

Materials and Methods

Four areas of limestone forest on Guam where Varanus indicus are known to exist will be selected for the study. These limestone forest communities are:

1) The NCS Beach--Hilaan Point strand to the cliffline forest
2) The Ritidian Point--Northwest Field forest
3) The Mangilao limestone forest
4) The Mt. Lam Lam peak forest.

A sample group of Varanus indicus will be caught in each of the areas. Pressure plate live capture traps baited with carrion (e.g., decaying fish, crushed snails, and road-kill carrion) will be set and checked daily in each area for one week of each month of the study. The monitors will be sexed, measured, weighed, marked with non-toxic fluorescent paint, and then released. Early in the study, selected monitors will have a tracking device developed by Heath (1961) attached to their tails or will be fed an internal telemetric tracking transmitter before being released. This will facilitate the location of egg clutches, dens, and territorial areas.

Some monitors will be trapped monthly in areas outside of the study sites so as to not disturb the study populations. These additional monitors will be taken to the University of Guam Biosciences Department. The monitors will be sexed, measured, weighed, and sample animals of each sex will be killed and dissected for determination of their reproductive status.

Tentative statistical analysis will include the use of two-way analyses of variance without replication (Sokal and Rohlf 1969) for determination of overall, male, female, and juvenile population densities. Natality and survivalship will be treated by birth and death rate analyses described by Andrewartha and Birch (1954). Student's t-distribution (Sokel and Rohlf 1969) will be used to determine any significant deviations within the collected data. Growth rates will be calculated from weekly measurements of captive laboratory specimens on controlled diets and from mark-recapture data obtained in the field.

Literature Cited

Andrewartha, H.G., and L.C. Birch. 1954. The distribution and abundance of animals. Univ. Chicago Press, Chicago. 782 p.
Auffenberg, W. 1978. Social and feeding behavior in Varanus komodoensis. In N. Greenberg and P.D. Maclean (eds.), Behavior and neurology of lizards. NIMH. p. 301-331
Bates, M., and D. Abbott. 1953. Coral island, portrait of an atoll. Scribner, New York. 254 p.
Crampton, H.E. 1921. A journey to the Marianas Islands--Guam and Saipan. Natural History 21:126-145
Dryden, G. 1965. The food and feeding habits of Varanus indicus on Guam. Micronesica 2(1):73-76
Fosberg, F.R. 1956. Military geography of the northern Marshalls. Engineer Intelligence Study, HQ U.S. Army Forces Far East. 325 p.
Fulbeck, J. 1947. Monitors in the Marshalls. Fauna 9:122-124
Gressitt, J.L. 1952. Description of Kayangel Atoll, Palau Islands. Atoll Res. Bull. (14):1-8
Heath, W.G. 1961. A trailing device for small animals designed for field study of the Gila monster (Heloderma suspectum). Copeia (4):491-492
Kaneshiro, S. 1950. Anthropological report on Woleai-Lamotrek field trip. Report to Chief Admin., Field Headquarters, Trust Territory of the Pacific Islands. [Cited in H.J. Wiens. 1962. Atoll environment and ecology. Yale Univ. Press, New Haven. 532 p.
Porter, K.R. 1972. Herpetology. W.B. Saunders Company, Chicago. 524 p.
Schmidt, K.P., and R.F. Inger. 1957. Living reptiles of the world. Doubleday and Company, Inc., New York. 287 p.
Sokal, R.R., and F.J. Rohlf. 1969. Biometry. W.H. Freeman and Company, San Francisco. 776 p.
Uchida, T. 1967. Observations on the monitor lizard, Varanus indicus (Daudin) as a rat control agent on Ifaluk, Western Caroline Islands. Micronesica 3(1):17-18

The hardcopy file of this document also contained the tentative outline for the thesis and the proposed budget (submitted to granting authorities--Sea Grant and the University of Guam).