Measuring sperm swimming performance in birds: effects of dilution, suspension medium, mechanical agitation, and sperm number

Sperm swimming performance, including swimming speed and the proportion of motile cells, may strongly affect fertilization success. However, little is known about how methodological factors affect in vitro measurement of these parameters. We compare the swimming performance of sperm from House Sparrows (Passer domesticus) and Spanish Sparrows (Passer hispaniolensis) in two standard suspension media, at two different dilutions, and with different degrees of cell agitation. Further, we conduct a resampling analysis to investigate sample size effects. Sperm performance was generally reduced when sperm were diluted, or when suspended in phosphate-buffered saline (PBS) rather than a medium containing additional nutrients. Sperm performance was particularly low when they were diluted more and suspended in PBS, suggesting that seminal fluid may provide compounds that enhance performance but that these are less available following dilution. Mechanical agitation of the cells by vigorous pipettin...