Aspectos morfológicos associados ao dimorfismo sexual em filhotes de Melanosuchus niger (Spix, 1825) (Crocodylia: Alligatoridae) na reserva de desenvolvimento sustentável Mamirauá, Amazonas

Abstract

The shape and size of the body are characteristics that differ between adult males and females of crocodilians; however, there is no report about sexual dimorphism for Melanosuchus niger hatchling. The present study had as objective to analyze the existence of sexual dimorphism in of M. niger hatchling based on morphological data. The specimens analyzed in this study came from an artificial incubation experimente. The eggs were collected at Lake Tracajá on Mamirauá Sustainable Development Reserve. The gonads were analyzed with macroscopic parameters (shape, texture, color and presence / absence of Müller duct) to verify if there was sexual dimorphism. The sexual identification of the hatchling was confirmed by the histological analysis of Gonad-Adrenal-Mesonefro (GAM) complex according to routine procedures. For the analysis of the variation of the form the quantified morphology of the epidermal plates of the hatchling' head was used from 13 landmarks submitted to a geometric morphometry analysis. For this, the heads of the specimens were photographed in dorsal view and the TPSUtil, TPSDig2 and MorphoJ software were used for analysis. Histological analysis served to confirm the animal´s sex of this study. There were identified, 38 females and 12 males. In the testes it was possible to observe the presence of the tunica albuginea associated with a little differentiated marrow, where the presence of seminiferous tubules was not observed, only the presence of vacuolated cells. Associated with the mesonephro we observed the presence of the remaining Müller duct. In the ovaries, the presence of an evident germinative epithelium, formed by simple cuboid epithelial tissue was observed. In the medullary region the presence of ovarian follicles and marrow gaps were observed. The Müller Duct was connected to the mesonephro by the mesosalpinge. It was observed that the macroscopic characteristics of the gonads differ between the sexes. They showed elliptical, smoothly, yellow testes. The ovary presents granular structure with light beige coloration and associated with the presence of Müller's Duct. Principal component analysis (PCA) showed variation in the shape of epidermal scales of M. niger hatchling, indicating the presence of sexual dimorphism. The Principal Component 1 (PC1) explained 52.6% of the shape variation, as the Principal Component 2 (PC2) explained for 14.4%, and the overall variation explained by these two main axes was 67.0%. The analysis of discriminant function confirmed the presence of sexual dimorphism (p = 0.01). It was observed that the females have the posterior region of the head longer and wider, and the jaw shorter and narrower. While the posterior region of the males head is slightly shorter and narrower, and the jaw longer and wider. The macroscopic and histological characteristics of the gonads were similar to those found in other species of crocodilians. Macroscopic analysis was sufficient to identify the sex of M. niger hatchling. In adult crocodilians, sexual dimorphism may be present in the region of the skull related evolutionarily to visual recognition of the genus. However, sexual selection would not be as relevant to young people as to sexually mature individuals. The variation of the shape in both the skull and mandible found in M. niger hatchling suggests that males and females may have undergone different ontogenic processes. The lack of work involving the study of sexual dimorphism using a geometric morphometry technique in hatchling of other populations of M. niger and/or other species of crocodilians makes difficult direct comparison. However, it is clear the importance of the study of sexual dimorphism in gilts of crocodilians to estimate the sexual proportion, especially if applied in natural populations. Geometric morphometric models can help to develop graphic representations of sexual dimorphism of the head in crocodilians, which makes these results important for conservation programs under natural conditions, since it represents a non-lethal alternative to the techniques used for sexual identification in M. niger.