Geographic variation in sperm production by Trinidadian guppies

Population comparisons yield important insights into adaptive differentiati on. However, despite the current interest in sperm competition and spermato genesis, geographic variation in these traits has received little attention . We tested the hypothesis that sperm production covaries with risk by comp aring five natural populations of Trinidadian guppies (Poecilia reticulata) : two that inhabited dangerous Crenicichla localities, two from low-risk Ri vulus sites and a 'transplant' population comprised of the descendants of g uppies moved from one of the high-risk sites to a, low-predation environmen t. As predicted, males from the three low-risk sites performed significantl y more courtship displays and had larger sperm reserves than their high-ris k counterparts. This result implies higher rates of sperm competition in th e low-risk sites.