Thin sections from femur, tibia, metatarsus and metacarpus were analyzed in an ontogenetic series of 9 specimens of
E. hemionus (
Table 1). Only specimen IPS83154 lacks metacarpal bone, totaling 35 the cross-sections studied. As shown in
Table 1, the sample includes individuals from different habitats, sex and ages. Sex data were provided by curators while age at death was estimated according to dental eruption pattern of the species (Lkhagvasuren et al., 2013 (
link)) and corroborated with the analysis of cementum layers in adult individuals (R Schafberg, pers. comm., 2014). Wild specimens (IPS83876–IPS83877) were collected during the Mongolian-German Biological Expeditions in the Gobi desert (Schöpke et al., 2012 ) and are housed at the Natural History Collections of the Martin-Luther-University Halle-Wittenberg (Halle, Germany). Captive individuals (IPS83149–IPS83155) lived in the Hagenbeck Zoo (Hamburg, Germany) and belong to the collections of the Zoological Institute of Hamburg University (Hamburg, Germany).
From the mid-shaft of each bone, we prepared histological slices following standard procedures in our laboratory (Nacarino-Meneses, Jordana & Köhler, 2016 (
link)). After measuring and photographing each bone, three centimeters of its mid-shaft were cut and embedded in an epoxy resin (Araldite 2020). This block was later cut into two halves (ISO Met, Biometa) and the exposed surface was polished with carborundum powder to be fixed to a frosted glass with an UV curing glue (Loctite 358). Afterwards, it was cut with a diamond saw (Petrothin, Buehler) up to a thickness of 100–120 microns and polished again with carborundum powder. Finally, a mix of oils (Lamm, 2013 ) was spread over the slice before being sheltered with a cover slip. Longitudinal sections were also prepared from several blocks to corroborate that the identification of bone tissue types does not rely on the orientation of the cutting plane (Stein & Prondvai, 2014 (
link)). All thin-sections were observed in a Leica DM 2500P microscope under polarized light with a 1/4
λ filter and photographed with the camera incorporated in the microscope. The use of a retardation filter that colors the cross-section, which is not mandatory in this kind of studies, was used to improve the visualization of BGMs and to facilitate the description of bone histology and skeletochronology (Turner-Walker & Mays, 2008 ).
To analyze the histological variability between skeletal elements, bone tissue types and BGMs were studied. The histological descriptions follow the classification of Francillon-Vieillot et al. (1990) and De Margerie, Cubo & Castanet (2002) (
link). The terminology proposed by Prondvai et al. (2014) (
link) was employed to describe the different components of the fibrolamellar complex (FLC) (a special case of woven-parallel complex for this authors): “fibrous” or woven bone (WB) and “lamellar” or parallel-fibered bone (PFB). Because the femoral bone histology of the Asiatic wild ass has previously been described in detail (Nacarino-Meneses, Jordana & Köhler, 2016 (
link)), only descriptions of the bone tissue of tibiae, metacarpi and metatarsi will be detailed in the present work. Regarding growth marks, we have generally used the term “bone growth mark—BGM,” interchangeably for LAGs or annuli, instead of “cyclical growth mark—CGM” because not all the marks identified in the samples have proved to be periodical. Double LAGs or LAGs that split were considered as a single event. BGMs were traced along the cross-sections and superimposition of individuals was performed to identify growth marks that have been erased by the remodeling process or the expansion of the medullary cavity (Woodward, Padian & Lee, 2013 ). Each BGM circumference was measured with ImageJ
® software to estimate the bones’ perimeter at different times during ontogeny and the results were plotted to obtain growth curves for each sample (Bybee, Lee & Lamm, 2006 (
link)). The perimeter of the cross-section was also calculated with ImageJ
® software in those animals that are still growing (subadult individuals) to estimate its bone perimeter at the time of death. The perimeter of adult individuals was not determined and only the length of the BGMs identified within the EFS is shown. Because it is generally considered that the presence of EFS indicates the cessation of radial growth in long bones (Huttenlocker, Woodward & Hall, 2013 ), the length of the BGMs located in this bone tissue and the perimeter of the cross-section are almost the same value. Thus, the estimation of the cross-section’s perimeter in adult specimens does not provide relevant information about the growth of the animal. Furthermore, we calculated the size variation per year of each bone in yearling and adult specimens as the difference of BGMs’ perimeters of consecutive annual growth cycles and interpreted it as a proxy of growth rate. Finally, several life history traits were calculated in each bone from the study of CGMs. Age at death of the specimens was determined as the total number of CGMs present in the bone cortex (Castanet et al., 2004 (
link)) and compared with the age estimated from teeth. Age at maturity was calculated by counting the CGMs before the deposition of the EFS (Chinsamy & Valenzuela, 2008 ; Marín-Moratalla, Jordana & Köhler, 2013 (
link)) and contrasted with literature data.
Nacarino-Meneses C., Jordana X, & Köhler M. (2016). Histological variability in the limb bones of the Asiatic wild ass and its significance for life history inferences. PeerJ, 4, e2580.