Age-related changes in size, bone microarchitecture and volumetric bone mineral density of the mandible in the harbor seal (Phoca vitulina)

Abstract

Detailed knowledge of age-related changes in the structure and mineralization of bones is important for interpreting osseous changes in wild mammals caused by exposure to environmental contaminants. This study analyzed mandibular size, microarchitecture and volumetric bone mineral density (vBMD) in harbor seals (n = 93, age range 0.5 months to 25 years) from the German North Sea. Bone microarchitecture and vBMD were assessed using high-resolution peripheral quantitative computed tomography (HR-pQCT). Significant differences were observed between the analyzed age classes (i) young juveniles (0.5–10 months), (ii) yearlings (12–23 months), and (iii) adults (12–25 years) for several of the variables, indicating an overall increase in cortical and trabecular area, cortical thickness and total and cortical vBMD with age. Furthermore, for juvenile animals (≤ 23 months), significant positive correlations with age were observed for mandible length and perimeter, cortical area, cortical thickness, trabecular separation, and total and cortical vBMD. The findings demonstrate a rapid increase in overall size, cortical dimensions and the degree of mineralization of the harbor seal mandible during the first two years after birth. Negative correlations with age existed for trabecular number and thickness as well as for trabecular bone volume fraction in the juveniles. The findings suggest a reduction in trabecular bone volume fraction with age, due to the bone trabeculae becoming thinner, less numerous and more widely spaced. Given the strong age dependence of most analyzed parameters, it is recommended to standardize samples with respect to age in future studies comparing microarchitecture and mineralization of harbor seal mandibles from different populations or different collection periods.

Bone mineral density and microarchitecture change during skeletal growth in harbor seals (Phoca vitulina) from the German coast

“…Previously, in-depth skeletal examinations of specific anatomic sites in harbor seals were limited to the temporomandibular joint 39 , 40 . Specifically, one report using HR-pQCT showed an increase in cortical thickness but a decrease in BV/TV and Tb.Th during growth for a portion of the mandibular corpus 8 . However, little is known about age-related changes in vertebral aBMD and the microarchitecture of harbor seals.…”

“…Multiple studies report on different anthropogenic impacts and their effects on the immune, endocrine, and reproduction system of harbor seals in the North and Baltic Sea 2 , 4 – 6 . However, only a limited number of studies have investigated the skeletal system and potential changes due to various stressors 7 , 8 .…”

“…Wildlife studies on the skeleton are comparably limited, but in the last years, an increase in the number of reports has been noticeable 8 , 15 , 24 , 25 . One marine species is the common bottlenose dolphin ( Tursiops truncatus ), measuring aBMD at different ages in this dolphin 17 , 23 .…”

“…As the skeleton undergoes a remarkable development during growth, including rapid growth in the first years of the life of mammals 34 , defining age-specific characteristics of harbor seals may be of central relevance. Whereas one study investigated age-associated changes in the microarchitecture of the mandible of harbor seals on the German coast 8 , no reports of vertebral aBMD or bone microarchitecture parameters are available. As the spine is of importance for the functional integrity of the skeleton and thus the survival of the animal, in this study we examined nine harbor seals of three different age groups using DXA and HR-pQCT to determine the skeletal features and their association with age.…”