Bats are difficult to age once they reach adulthood, and the currently available methods of age determination have several limitations. To begin with, bats show no visible markers of aging, and the few traits that have been correlated with age to establish reference standards show significant variation due to genetic structuring of populations and environmental variation which impacts development. Additionally, available reference standards lack data from very old bats, likely because the oldest individuals are few and seldom caught. Finally, several of the more robust methods are invasive, making repeated measures impossible, and their accuracy is questionable (Anthony, 1988).
Undoubtedly, the best method for age determination is capturing and tagging individuals during their birth year. Thereby, at every subsequent recapture exact chronological age can be easily calculated. However, long-term monitoring of marked individuals can be very time consuming, and recapture rates are often low. A researcher must assess the level of estimate precision necessary for his or her investigations. If exact chronological age is necessary, then permanently marking individuals is the appropriate method. Otherwise, one of the following methods may suffice.
After birth, bats grow quickly, achieving almost full adult size prior to fledging (Barclay, 1994; Jones and
MacLarnon, 2001). During this time, long bones grow linearly and many researchers use forearm length measured with calipers to determine age of growing juveniles. In fact, this method provides very accurate estimates of age (Anthony, 1988). Its usefulness, however, is restricted to the first 2 to 6 weeks after birth, depending on species (Brunet-Rossinni and Wilkinson, in review). During this growth phase, the cartilaginous epiphyseal growth plates of the phalanges in the wing first expand to generate the phalangeal growth necessary for wing development and then become increasingly calcified. This process results in a linear increase and subsequent linear decrease in total length of the cartilaginous region between the boney diaphysis of a metacarpal and the boney diaphysis of the proximal phalanx (a.k.a. total gap). Epiphyseal growth plates are easily visualized by trans-illuminating the wing with a light source, as cartilaginous tissue allows more light to pass through than bone. Calipers can be used to measure total gap, though many researchers use a dissecting microscope with an ocular micrometer and a sub-stage light source to increase measurement accuracy (Kunz and Anthony, 1982). This linear change in total gap is a robust and accurate method for determining age of juveniles well beyond the valid age estimates provided by forearm length. The ease of obtaining measurements and the minimal equipment requirement are advantages of this aging method. However, once the epiphyseal growth plates are calcified, age estimates are no longer possible.
Nonetheless, the presence of cartilage in and the shape of phalangeal joints can be used to classify bats into general age categories such as infant, juvenile, and adult. If when trans-illuminating a wing cartilage is visible, the bat is a juvenile. Once the epiphyseal plate calcifies, one can still distinguish juveniles from adults as juvenile phalangeal joints are more tapered and less knobby than adult joints (see Figure 1 in Anthony, 1988). External reproductive traits can also be used to distinguish adults and juveniles. The presence of scrotal testes and well-developed teats are characteristic of reproducing adults (Racey, 1988).
The methods of age determination for adult bats involve assessing tooth wear and counting incremental dentin and cementum lines in teeth. Both methods are based on traits that vary tremendously and are difficult to measure, so researchers should exercise caution when using them for age estimations.
Bats have a permanent set of teeth by the time they can fly and feed independently, and mastication over a lifetime results in tooth cusps wearing down and becoming dull. Assessing the extent of tooth wear can be used to place bats into relative age categories, and standards have been developed for a few species (Brunet-Rossinni and Wilkinson, in review). However, noticeable differences in tooth wear occur over long time periods, which necessarily results in broad age categories. Additionally, scrutiny of some established reference standards attests to the limited predictive power of this method due to extensive variation in tooth wear associated with diet and behavior (Hall et al., 1957). Due to this variation, it is critical that estimates be based on reference standards developed for the particular study population and that these estimates be made by investigators with extensive experience and knowledge about tooth wear patterns in the particular species.
Dental incremental lines or "annuli" have been used to determine age of adult bats under the assumption that one new layer of dentine and cementum is laid on pre-existing dental tissue each year (Klevezal' and Kleinenberg, 1967). Thus, by extracting and sectioning a tooth, the lines can be visualized and counted under a microscope, giving an estimate of a bat's age in years. However, incremental lines may be difficult to count, especially in small species, the number of incremental lines counted may depend on the tooth extracted, and several factors can result in nonannual cycles of deposition (Batulevicius et al., 2001; Cool et al., 1994; Phillips et al., 1982). The aforementioned problems put in question the accuracy of this method. An additional drawback is the laborious process of preparing the sectioned tooth, which can only be extracted from a dead specimen, making repeated measures impossible.
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