Developmental changes in bone mechanics from Florida manatees (Trichechus manatus latirostris), obligate swimming mammals

Mammals living in aquatic environments load their axial skeletons
differently than their terrestrial counterparts. The structure and
mechanical behavior of trabecular bone can be especially indicative of
varying habitual forces. Here, we investigate vertebral trabecular bone
mechanical properties (yield strength, stiffness, and toughness)
throughout development in Florida manatees (Trichechus manatus
latirostris), obligate undulatory swimmers. Thoracic, lumbar, and caudal
vertebrae were dissected from manatees (N=20) during necropsies. We
extracted 6 mm3 samples from vertebral bodies and tested them in
compression in three orientations (rostrocaudal, dorsoventral, and
mediolateral) at 2 mm min-1. We determined variation in mechanical
properties between sexes, and among developmental stages, vertebral
regions, and testing orientations. We also investigated the relationships
between vertebral process lengths and properties of dorsoventrally and
mediolaterally-tested bone. Rostrocaudally-tested bone was the strongest,
stiffest, and toughest, suggesting that this is the principle direction of
stress. Our results showed that bone from female subadults was stronger
and stiffer than their male counterparts; based on these data we
hypothesize hormonal shifts at sexual maturity may partially drive these
differences . In calves, bone from the posterior region was stronger and
tougher than from the anterior region. We hypothesize that since animals
grows rapidly throughout early development, bone in the posterior region
would be the most ossified to support the rostrocaudal force propagation
associated with undulatory swimming .

水生环境中的哺乳动物对其轴骨的载荷方式与陆地同类存在差异。骨小梁的结构与力学行为尤其能反映习惯性受力的变化。本研究探讨了专性波动游泳者——佛罗里达海牛（Trichechus manatus latirostris）在整个发育过程中椎骨骨小梁的力学特性（屈服强度、刚度及韧性）。

胸椎、腰椎及尾椎是在尸检过程中从20头海牛（N=20）体内解剖获得的。我们从椎体中提取6立方毫米的样本，并在三个方向（头尾向（rostrocaudal）、背腹向（dorsoventral）、内外侧（mediolateral））以每分钟2毫米的速率进行压缩测试。

我们分析了性别间、发育阶段间、椎骨区域间及测试方向间力学特性的差异。此外，我们还探究了椎突长度与经背腹向、内外侧测试的骨骼特性之间的关系。

经头尾向测试的骨骼强度最高、刚度最大且韧性最强，表明这是主要应力方向。结果显示，雌性亚成体的骨骼比雄性亚成体更强韧、更坚硬；基于这些数据，我们推测性成熟时的激素变化可能是导致这些差异的部分原因。

在幼崽中，后部区域的骨骼比前部区域更强韧。我们推测，由于动物在早期发育阶段生长迅速，后部区域的骨骼会最充分地骨化，以支撑与波动游泳相关的头尾向力传递。