Abstract

BONILLA-RUZ, Carlos; CINTA MAGALLON, Claudia  and  SANTOS-MORENO, Antonio. Determination of sexual secundary dimorphism in military macaw (Ara militaris): morphometry complementary with ocular index. Acta Zool. Mex [online]. 2011, vol.27, n.2, pp.245-255. ISSN 2448-8445.

Most American psittacids do not show sexual dimorphism. Finding differences between sexes allows getting practical biological information about populations of threatened or endangered species. This work shows the significant results of a morphometric study to indicate significant secondary sexual differences on Ara militaris. The hypothesis tested on this analysis is that there are secondary sexual differences on this species that can be visually detected; we show that the size and proportions and layout of certain parts of the head can be used consistently and reliably to differentiate males from females. To test the hypothesis, we used 18 specimens in captivity, 15 of them sexed by DNA analysis (11 females, 4 males plus 3 of unknown sex). We used 11 measurements taken from pictures of the head of each specimen from different parts of the head. A Principal Components Analysis was performed and most significant measurements were identified from these results, therefore an ocular index was built. The ocular index correlates two involved characteristics in the ocular area of the bird, providing information to make an easy differentiation of the sexes. Predictions made with this ocular index matched the DNA results; the sex from the three unidentified specimens was predicted by using the index. The results represent a very practical, complementary, and inexpensive way to differentiate sex among these birds for field management and monitoring purposes.

Keywords : Ara militaris; intrapopulation variation; secondary sexual dimorphism; sex determination.