Histological variability in the limb bones of the Asiatic wild ass and its significance for life history inferences

Abstract

The study of bone growth marks (BGMs) and other histological traits of bone tissue provides insights into the life history of present and past organisms. Important life history traits like longevity or age at maturity, which could be inferred from the analysis of these features, form the basis for estimations of demographic parameters that are essential in ecological and evolutionary studies of vertebrates. Here, we study the intraskeletal histological variability in an ontogenetic series of Asiatic wild ass (Equus hemionus) in order to assess the suitability of several skeletal elements to reconstruct the life history strategy of the species. Bone tissue types, vascular canal orientation and BGMs have been analyzed in 35 cross-sections of femur, tibia and metapodial bones of 9 individuals of different sexes, ages and habitats. Our results show that the number of BGMs recorded by the different limb bones varies within the same specimen. Our study supports that the femur is the most reliable bone for skeletochronology, as already suggested. Our findings also challenge traditional beliefs with regard to the meaning of deposition of the external fundamental system (EFS). In the Asiatic wild ass, this bone tissue is deposited some time after skeletal maturity and, in the case of the femora, coinciding with the reproductive maturity of the species. The results obtained from this research are not only relevant for future studies in fossil Equus, but could also contribute to improve the conservation strategies of threatened equid species.

Keywords: Bone growth marks; Bone histology; Equus hemionus; External fundamental system; Intraskeletal histological variability; Life history; Limb bones; Longevity; Reproductive maturity; Skeletochronology.

Figure 1. Tibial bone histology of the Asiatic wild ass.

(A) Detail of the lateral cortex of the foal IPS83153, showing two areas that differ in the proportions of the parallel-fibered component (PFB) of the bone matrix. (B) Haversian systems in the anterior cortex of the yearling IPS83150. (C) Anterior cortex of the newborn individual (IPS83152) with a high proportion of parallel-fibered component (PFB) in its bone matrix. (D) Packages of lamellar bone within the fibrolamellar complex in the anterior cortex of the wild male (IPS83877). HS, haversian systems; LB, lamellar bone; PFB, parallel-fibered bone. Scale bars: 1 millimeter. All images were obtained under polarized light with a 1/4λ filter.

Figure 2. Metapodial bone histology of the Asiatic wild ass.

(A) Anterior metatarsal cortex of the yearling IPS83149, showing a fibrolamellar complex with primary osteons oriented in circular rows. (B) Radial canals in the metacarpus of the yearling IPS83150. (C) Circular canals in the metatarsus of the foal IPS83153. (D) Detail of the external fundamental system in the metatarsus of the wild female IPS83876. EFS, external fundamental system; FLC, fibrolamellar complex. Scale bars: 1 millimeter. All images were obtained under polarized light with a 1/4λ filter.

Figure 3. Bone growth marks in foal kulans.

(A) BGM in the lateral side of the tibia (IPS83154). (B) BGM in the anterior cortex of the metacarpus (IPS83153). (C) BGM in the anterior side of the metatarsus (IPS83153). White arrows indicate bone growth marks. Scale bar: 1 millimeter. All images were obtained under polarized light with a 1/4λ filter.

Figure 4. Bone growth marks in yearling kulans.

(A) Femoral bone cortex of IPS83151 showing one BGM in its anterior side. (B) Tibial bone cortex of IPS83150 showing two BGMs in its lateral side. (C) Metacarpal bone cortex of IPS83151 showing one BGM in its lateral side. (D) Metatarsal bone cortex of IPS83149 showing two BGMs in its anterior side. White arrows indicate bone growth marks. Scale bar: 1 millimeter. All images were obtained under polarized light with a 1/4λ filter.

Figure 5. Bone growth marks in the juvenile kulan (IPS83155).

(A) Femoral bone cortex showing one BGM in its anterior side. (B) Tibial bone cortex showing two BGMs in its lateral side. (C) Metacarpal bone cortex showing one BGM in its anterior side. (D) Metatarsal bone cortex showing two BGMs in its anterior side. White arrows indicate bone growth marks. Scale bar: 1 millimeter. All images were obtained under polarized light with a 1/4λ filter.

Figure 6. Bone growth marks in adult kulans.

(A) Femoral bone cortex of the wild female (IPS83876) showing five BGMs in its anterior side. (B) Detail of the most external BGMs identified in the femur of IPS83876. Fifth BGM is located within the external fundamental system. (C) Tibial bone cortex of the wild male (IPS83877) showing five BGMs in its lateral side. (D) Detail of the most external BGMs identified in the tibia of IPS83877. (E) Metacarpal bone cortex of the wild female (IPS83876) showing five BGMs in its anterior side. (F) Detail of the most external BGMs identified in the metacarpus of IPS83876. Fourth and fifth BGMs are located within the external fundamental system. (G) Metatarsal bone cortex of the wild male (IPS83877) showing six BGMs in its anterior side. (H) Detail of the most external BGMs identified in the metacarpus of IPS83877. Fifth and sixth BGMs are located within the external fundamental system. White dashed rectangles indicate areas of image magnifications. White arrows indicate bone growth marks. White scale bar: 1 millimeter; black scale bar: 500 microns. All images were obtained under polarized light with a 1/4λ filter.

Figure 7. Bone growth of the Asiatic wild ass.

From (A–D), bone perimeter (mm, ordinate axis) is plotted against estimated age (years, abscissa axis) to obtain growth curves. From (E–F), variation of bone perimeter (mm, ordinate axis) is plotted against estimated age (years, abscissa axis) as a proxy of growth rate. (A) Growth curves obtained from the femora. (B) Growth curves obtained from the tibiae. (C) Growth curves obtained from the metacarpi. (D) Growth curves obtained from the metatarsi. (E) Femoral growth rate. (F) Tibial growth rate. (G) Metacarpal growth rate. (H) Metatarsal growth rate. Legend is shown in the bottom of the figure. In the graphs, filled characters represent females, unfilled ones correspond to males and linear ones indicate animals with unknown sex. Dashed lines indicate wild animals while continuous lines represent captive ones. Male and female symbols indicate the time of deposition of the external fundamental system (EFS) in each wild adult respectively. It could be noted that this moment does not match with the decline in periosteal growth rate.