Ontogenetic study of the mechanical properties and architecture of the radius (appendicular bone) of two species of odontocetes Stenella coeruleoalba and Pontoporia blainvillei = Estudio ontogenético de las propiedades mecánicas y la arquitectura en el radio (hueso apendicular) de dos especies de odontocetos Stenella coeruleoalba y Pontoporia blainvillei

Abstract

This work is the result of extensive research on sexual and ontogenetic variation of parameters with mechanical significance in a radius bone in two species of odontocetes: Stenella coeruleoalba (Sc) and Pontoporia Blainvillei (Pb) (Cetacea, Mammalia). Our assumption was that the proportions of the bone structures were not only limited by the constraints of mechanical nature, but also by factors of adaptive type, including those arising from the selective pressure of possible modes of transport. In Cetacean, it has been reported that the size and shape of the pectoral limbs are related to the ecology of each species. Allometric scaling relationship of the bones of the fins in response to the ecological requirements has also been reported. Our results show negative allometric patterns in the radius of Sc, as in other oceanic dolphins and conversely, positive allometric patterns in Pb. These biomechanical differences in radius in both species might be due to an adaptation to the respective means (oceanic vs. estuarine). Gender differences in Sc are associated to the corresponding effort made males and females according to their complex social structure, in which males have greater physical activity than females. In Sc, the mechanical parameters grow more and faster in females than in males, reaching peak faster. It may be due to a compensation of loss of bone mass relative to males, caused by hormonal changes and lactation. Pb growth is slower and no such compensation is observed. Pb has a hypermineralized bone compared to what normal mineral content is considered, among which is Sc. As a result, their bones offer less resistance against transverse forces. The Ca / P ratio found for both species is greater in sexually mature animals than in other vertebrates and is similar to those cases where the bone has undergone a metabolic diseases This result suggests a non-pathological secondary osteoporosis in these dolphins, which agrees with the already describe Delphinus delphis. Bone microarchitecture is a spatial and temporal function. From the spatial point of view, according to the results of microhardness and nanohardness, bone is harder from the center outward. From the point of view of time, the average hardness of the bone (microhardness) changes with age, increasing in the whole bone in Sc and in the cortical bone in Pb whereas it decreases in the trabecular Pb bone. This observation, along with the fact that the bone is denser, would lead to a bone with more strength.