Behav Ecol Sociobiol (1993) 32:221-228.
Mate choice and fitness in a hybrid frog: Rana esculenta females prefer Rana lessonae males over their own

Gaby Abt and Heinz-Ulrich Reyer Zoologisches Institut, UniversitŮt Zörich, Winterthurerstrasse 190, CH-8057 Zörich, Switzerland

Received April 15, 1992 / Accepted December 20, 1992

Summary. The evolution and maintenance of female choice based on purely genetic differences is still a con-troversial issue, not only for theoretical reasons, but also because of the practical difficulty of demonstrating the fitness consequences of preferences and heritability of and genetic variability in the chosen traits. We argue that hybrid systems (broadly defined) offer suitable models for studying mate choice according to genetic differences. We present such a study for European waterfrogs of the " hybridogenetic " Rana lessonae/Rana esculenta complex (L/E complex). R. esculenta, originally a hybrid between R. Iessonae and R. ridibunda, eliminates the L genome premeiotically and only produces eggs and sperm containing only the R. ridibunda (R) genome. Consequently, the hybrid will only persist when it lives and mates with R. Iessonae in mixed populations where it can regain the lost L genome. In such mixed populations, there is strong selection against E x E matings. because these will produce no viable offspring. We tested whether females of the hybrid R. esculenta do indeed avoid their own R. exculenta males and choose males of the parental species R. Iessonae instead. Eleven E females were offered a simultaenous choice between one L and one E male. Females exhibited a significant preference for L males that was determined by the type of male, rather than by its size or activity. This choice is in the direction predicted from genetics. The question of why L males "agree" to mate with E females, but L females only rarely mate with E males, is answered by a sexual asymmetry in the cost/benefit ratios of mating with the wrong type and the right size. Our results are consistent with the mating pattern found in natural populations, but further studies are needed to show that female choice really causes this pattern.