For better sperm, try polygamy, it works for mice

Male fertility of mice may be improved by competition

source: Wendy Zukerman
New Scientist vol 209 no 2797, January 29 2011 p9

Male fertility of mice may be improved by competition. House mice are able to be both polygamous and monogamous. Leigh Simmons, a researcher from Western Australia University, Crawley, created polygamous and monogamous strains of mice. He then mated 16 females with both polygamous and monogamous males. The polygamous males were fathers of 76% of the resulting offspring when mated first, and 58% when mated after the monogamous males. Polygamous males produced greater quantities of sperm, which also had improved motility.