Life is beset with competition, from the sprint for victory to a tasty morsel to stylish preening to attract mates, but one of the most fundamental contests must occur in the microscopic realm where fish sperm battle to penetrate an egg first. ‘Males often exhibit different tactics to increase their reproductive success’, says Ian Butts, from Auburn University, USA. Some male fish pass themselves off as females to outsmart rivals guarding harems, while others try sneaking behind opponents’ backs. Small Chinook salmon males – known as ‘jacks’, which return to the river of their birth a year or two before older and more aggressive hooknose males – invest more energy in their sperm and gonads in a bid to outcompete their larger counterparts. Might the young males’ increased investment also be reflected in their sperm's athleticism? In addition, ovarian fluid released when female Chinook salmon expel their eggs alters the viscosity, pH and salt concentration of the water into which the males’ sperm is released. Working together as part of Trevor Pitcher's long-term study of Chinook spawning based at the University of Windsor, Canada, Butts, Galina Prokopchuk and Vojtěch Kašpar caught male and female fish from the Credit River in Canada to compare the performance of jack and hooknose male sperm in water and dilute ovarian fluid.
Analysing the tail beat patterns and calculating the speed at which the sperm could swim, their efficiency and the power required to propel the sperm through the water, it was clear that the jack sperm outperformed the sperm of the more robust older fish. The length of the wriggling wave that passes along each beating sperm tail was longer in the jack sperm than the hooknose, and Jacky Cosson calculated that the jack sperm beat their tails faster to propel them through the water at 161 μm s−1, while the hooknose sperm only reached 155 μm s−1. In addition, the jack sperm swam more efficiently in dilute ovarian fluid, with the hooknose sperm using 45% more energy. It was also apparent that the fluid released with the females’ eggs impacted the performance of both males’ sperm, slowing the beat of the sperm's tail while increasing the sweep from side to side. ‘Each female creates a unique spawning environment by simultaneously expelling ovarian fluid along with an egg batch’, says Butts.
So although it might look at first glance as if the reproductive odds are stacked against smaller younger jack salmon males, they have a stealthy card up their sleeves in the form of their super-competitive sperm, which outperform the sperm of apparently more superior hooknose males. The team also suggests that ‘sperm biomechanics may be driving divergence in competitive reproductive success between alternative reproductive tactics’.