Age Determination in the Grey-Headed Flying Fox

Abstract

Historically the grey-headed flying fox (Pteropus poliocephalus), the only endemic flying fox in Australia, had a wide distributional range with numbers estimated in the millions (Ratcliffe 1932). However, since European settlement, this species has faced increased mortality (Martin and McIlwee 2002). Loss of both habitat and natural food resources from logging and monocultural timber forests has had a deleterious effect on bats (Eby 1995, Hall and Richards 2000). This loss of habitat and food resources has resulted in species movement into urban areas, where the increase in numbers can be falsely interpreted as population explosion due to rapid breeding (Eby and Lunney 2002). Urban populations are prone to increased mortality due to death of individuals on power lines or aerial wires. Additionally, in the absence of natural food resources, grey-headed flying foxes feed in orchards where they are considered serious pests. Consequently, they are being culled in large numbers (Hall and Richards 2000). Martin and McIlwee (2002) reported an estimated 35% decrease in numbers of P. poliocephalus in the last decade. This decline has caused concern for the species and has lead to the nationwide listing of P. poliocephalus as vulnerable (Eby and Lunney 2002). A method to age flying foxes is urgently needed to assist in managing this vulnerable species. A lack of age-based data on mortality and fecundity of animals in the wild has hindered efforts to construct adequate life tables and to estimate population declines from population models (Martin and McIlwee 2002). Obtaining the ages of individuals in the population generates the age structure and therefore some information related to patterns of recruitment (e.g., proportion of young vs. adults). However, information about age-specific survival and births is more important for understanding population dynamics and thus provides a detailed assessment of population vulnerability. For example, if disease or a change in an environmental factor is causing high mortality, agebased data are needed to determine whether the source of mortality affects age classes or genders equally because these differences have consequences for population growth rates. Accurate age determination depends on the relationship between the chronological and the physiological age (i.e., the changes in the recording structures) of an organism. The growth layers in hard tissues of animals (e.g., teeth and bones) are examples of these recording structures. Information concerning the morphology of these layers has increased since their discovery by Scheffer (1950) and Laws (1952; e.g., Carlson 1990, Lieberman and Meadow 1992, Lieberman 1993). However, the explanation of the mechanism’s underlying layer formation remains hypothetical; nonetheless, counting layers in the teeth of mammals can provide undeniably useful results (e.g., Mitchell 1967, Naylor et al. 1985, Moffitt 1998). One difficulty in aging individuals using growth layers is the lack of known-age individuals on which to check the method’s accuracy (Morris 1972). Obtaining individuals of known age is difficult and often involves capture, marking, and recapture. Unfortunately, the aerial, highly mobile lifestyle of grey-headed flying foxes makes them difficult to capture and recapture (Hall and Richards 2000). Therefore, captive bats with known histories are crucial for studying the age structure of these animals. We are aware of only 1 study on absolute age determination in megachiropterans. Cool et al. (1994) looked at a combined known-age sample (n 1⁄4 14) of 2 Pteropus species: P. poliocephalus and P. alecto. The results of their study confirmed the presence of the growth layers in the bone, as well as the cementum and dentine in teeth. The authors suggested that cementum, in particular, could be used for accurate aging of flying foxes but that more studies were needed. Our objective was to improve the cementum-layer aging technique by examining the annuli of the known-age and unknown-age P. poliocephalus. We then applied this technique to determine the age of the animals that have died as a result of the human-induced causes of electrocution on power lines and entanglement on barbed wire. Such information is important for assessing the impact of this change in the mortality patterns on the population persistence for this vulnerable species.