Sexual size dimorphism, testes size and mating system in two populations of Ctenomys talarum (Rodentia: Octodontidae)

Sexual size dimorphism and relative testes size were correlated to the mating system of two allopatric populations of the subterranean rodent Ctenomys talarum (tuco-tuco) which differed in demographic traits. Both populations, Mar de Cobo and Necochea (Buenos Aires Province, Argentina), showed significant sexual dimorphism in both body weight and length, with males being larger than females. Such intersexual dimorphism was not reflected in differences in burrow diameter between sexes. Small testes size in males from both populations are in accordance with single-male mating systems. Relative testes lengths were 0.057 +/- 0.007 (n = 70) at Mar de Cobo and 0.063 +/- 0.007 (n = 110) at Necochea. C. talarum testes represented 0.18 +/- 0.05% (n = 13) and 0.29 +/- 0.09% (n = 11) of their total body weight for Mar de Cobo and Necochea, respectively. Males from Necochea proved to have larger testes, both in length and in weight, than those from Mar de Cobo. Proposed resource defence polygyny would not be manifested to the same degree for the populations analysed. Lower relative testes size and higher sexual size dimorphism in length in the Mar de Cobo population would indicate that polygyny may be more extreme there than in Necochea.