Three-dimensional study of the lberian red deer antler (Cervus elaphus hispanicus): application of geometric morphometrics techniques and other methodologies

Abstract

Over time, many studies about deer antlers, based on classical morphometry, have been done. The present study propose a new perspective on the study of Iberian red deer antlers (Cervus elaphus hispanicus) from Sierra Morena (Spain), introducing new techniques able to provide more detailed information about them. The objectives addressed in this work have been principally focused on the use of geometric morphometrics methods and photogrammetric systems based on CAD-3D technology. Three-dimensional coordinates of 217 antlers concerning both the full and half antler structure, were adapted to each objective of the study and subjected to several statistics analyses. The exhaustive characterization of the shape changes of different antler parts with age was carried out, as well as the exploration of the existence of an optimal age for the obtention of the best antlers. The antlers of youngest individuals (2 to 3 years old) showed smaller but more elongated near the skull, with straighter and vertically oriented tines; while antlers of fully-grown individuals (6 to 9 years old) were shorter near the skull and more arched distally, with longer and curved tines. The allocation of an own antler categorization, based on measures of homologation protocols, made possible the observation of an antler improvement of individuals aged between 4-9 years, as well as some deterioration from 10 to 13 years. This is important for deer management in the area studied, which also might be applicable to other species. The high variability of antlers in terms of asymmetry was studied through the evaluation of the presence of Fluctuating asymmetry (FA) and Directional asymmetry (DA). Generally, previous studies show FA on antlers. However, the presence of FA and DA was significant in the antlers sampled, being the amount of DA higher than FA. The predominance of DA, rejected the possibility of considering FA as individual quality indicator. In addition, the DA presence involved new insights about the antler asymmetry and its development, pointing to to a more intense use of the right antler side. On the other hand, the annual capacity of antlers to regenerate, where multiple pathways interact, led us to the idea that antlers may act as modular structures. Accordingly, five modularity hypotheses of different biological interest, were proposed taking into account the variation among individuals and the variation suffered at direct developmental pathways. Modularity and integration were found, highlighting the tight relationship between both phenomena in antlers. This last fact, reflected the interaction of multiple pathways along the antler growth, as well as the existence of developmental interactions transmitting the effect of random perturbations throughout the whole structure. Thus, antlers generally act as single integrated modules, performing a common function as head and eyes protection during fights. However, the modularity observed suggested the relative independent behaviour of some antler parts, as the trez tines, during antler development. Finally, a Photogrammetric Interactive Measure Method (PhIMM), based on the combination of digital methods and the use of a parametric CAD system, was proposed to complete the study. This methodology, represents advantages from the data-taking point of view, since the use of digital photographs allows to sample in the field in an economic and fast way. Furthermore, the lack of taking measures in the field is not a problem, since the CAD technology allows to create three-dimensional models of the structures on which we can perform all the measurements desired, including volumes and perimeters. In addition, the information extracted from this models can be directly shown in excel sheets, facilitating the data processing and even the introduction in other programs.

La presente investigación estudia astas de ciervo Ibérico (Cervus elaphus hispanicus) mediante técnicas de morfometría geométrica y fotogrametría. Los cambios de forma de 217 cornamentas fueron caracterizados en función de la edad de los individuos. Especímenes de 2-3 años presentaron cornamentas pequeñas y alargadas, con puntas rectas y orientadas verticalmente. En cambio, las cornamentas de especímenes de 6-9 años, eran más cortas y arqueadas, con puntas largas y curvadas. La categorización propia de las astas, permitió observar una mejora de la calidad de las mismas en individuos de 4 a 9 años, y un deterioro en las de 10 a 13 años. La presencia de asimetría fluctuante (FA) y asimetría direccional (DA), también fue estudiada. Tanto FA como DA fueron significativas, siendo DA más abundante que FA. La predominancia de DA, imposibilitó considerar FA como indicador de calidad individual, y sugirió un uso más intenso de la rama derecha. Que las cornamentas pudieran ser estructuras modulares, llevó a proponer cinco hipótesis de modularidad, teniendo en cuenta tanto la variación entre individuos, como la debida a vías directas del desarrollo. La presencia de modularidad e integración en las cornamentas fue significativa. Ello reflejó tanto la interacción de múltiples vías durante el proceso de crecimiento de las cornamentas, como la existencia de interacciones capaces de transmitir perturbaciones aleatorias a lo largo de toda la estructura. Por lo tanto, las astas actuarían como un único módulo integrado. Sin embargo, la modularidad observada sugirió cierta independencia de partes como los candiles. La implementación de un método fotogramétrico interactivo de medida (PhIMM), basado en la toma de fotografías digitales y la utilización de un sistema CAD paramétrico, fue propuesto para completar el estudio. Dicha metodología, es ventajosa en la recolección de datos, puesto que la tecnología digital permite muestrear rápida y económicamente. Además, la falta del registro de ciertas medidas en el campo no es un problema, ya que la tecnología CAD permite crear modelos tridimensionales de las estructuras de interés, donde es posible realizar todas las medidas deseadas.