Bone histology as an approach to providing data on certain key life history traits in mammals: Implications for conservation biology

The knowledge of the life histories of wild mammals is of crucial importance in the field of conservation management. The endangered status of many species calls for faster data collection that can be used in risk assessment and, ultimately, for designing conservation policies. This study is pioneering the potential of bone histology to provide data on life history traits crucial for conservation biology in long-lived mammals. Long bone cross-sections show pronounced annual cycles of growth arrest allowing application of skeletochronology (counts of lines of arrested growth ‘LAGs’). Consequently, the number of LAGs within the primary fast-growing bone tissue up to the outer cortical slow-growing bone tissue corresponds to the age at first reproduction; whereas the age at death can be estimated by the total number of rest lines throughout the whole of bone cross-section. Furthermore, the diameters of successive growth rings as well as the osteocyte lacuna density may shed light on growth rates. We use the endangered desert dwelling antelope Addax nasomaculatus as a case study. By analyzing different ontogenetic stages in five Addax individuals (three captive and two wild specimens) from a museum collection, we show that bone histology may be a reliable tool for determining certain key life history traits. In our sample, the wild Addax female attained reproductive maturity at three years, whereas the male specimens, both the captive and the wild ones, reached maturity at four years. This is congruent with data from other large antelopes with male-biased size dimorphism, but differs slightly from data on sexual maturity previously published for wild Addax. Moreover, quantification of osteocyte lacunae in both adult males provides a higher cell density in the captive one than in the wild one suggesting the strong effect of constant resources supply in individuals from zoos on growth rates. While age at first reproduction and longevity are essentials parameters to carry out demographic models, growth rates may allow evaluation of the health status of wild populations. This approach may provide useful data on life history traits when applied to bones collected in the wild.