Alternative reproductive tactics are widespread in males and may cause intraspecific differences in testes investment. Parker’s sneak-guard model predicts that sneaker males, who mate under sperm competition, invest in testes relatively more than conspecific bourgeois males who have lower risk. Given that sneakers are much smaller than bourgeois males, they may increase testes investment to overcome their sperm productivity limited by their small body sizes. We here re-examined the mechanism that mediates differential testes investment across tactics in the Lake Tanganyika cichlid fish Lamprologus callipterus. Bourgeois males in the Rumonge population are smaller than bourgeois males in other populations and have a body size close to sneaky dwarf males. Therefore, if sperm competition explains differences in relative testis investment, it should increase from dwarfs > bourgeois males in Rumonge = bourgeois males in the other populations. If body size explains differences in relative testis investment, it should increase from dwarfs > bourgeois males in Rumonge > bourgeois males in the other populations. Comparisons of relative testis investment among the three male groups supported the role of sperm competition, as predicted by the sneak-guard model.