Immature and mature sperm morphometry in fresh and frozen-thawed falcon ejaculates

•Falcon ejaculates are very pleiomorphic.•Ejaculates show a high incidence of spermatocytes and spermatids.•Four sperm subpopulations were identified in falcon ejaculates.•Morphometric variables were smaller in the frozen-thawed samples.

Sperm morphometry is one characteristic which may be useful in prediction of fertility and sperm freezability in a species. Knowledge of the sperm characteristics of the ejaculate and the morphometric descriptors is necessary to effectively develop sperm cryopreservation. The aim of the current study was to provide a general description of the sperm from two falcon species (Peregrine falcon Falco peregrinus peregrinus/brookei and Gyrfalcon Falco rusticolus) including immature sperm, sperm head morphometric descriptors, and the existence of mature sperm subpopulations.Semen samples were collected by massage and voluntary false copulation and diluted with Lake and Ravie medium. Smears were prepared of the diluted samples, stained with Hemacolor®, and subjected to: 1) morphological analysis (bright field optical microscopy), and 2) computerised morphometric analysis; each sperm head was measured for length, width, area and perimeter. In addition, in the Gyrfalcon, pooled semen was frozen in pellets using DMA as a cryoprotectant and the analyses repeated after thawing. The mean percentage of immature sperm (spermatocytes and spermatids) was similarly high in all species/subspecies: Brookei Peregrine falcon (F. p. brookei) 55.5%, European Peregrine falcon (F. p. peregrinus) 65.5% and Gyrfalcon 64.7%. Clustering analyses identified four subpopulations of mature spermatozoa with different morphometric characteristics (P < 0.001). The relative proportions of these subpopulations were similar in all three species. The mean values recorded for the morphometric variables of the four subpopulations were, however, lower (P < 0.001) in the thawed Gyrfalcon samples than in fresh samples. The results support the idea of pleiomorphy as a characteristic of raptor mature sperm. This finding, plus that of the existence of four sperm subpopulations with different morphometric characteristics, may be important in the future development of cryopreservation protocols for falcon sperm.