Familiarity and body mass effects on mate choice in a mammal with female-biased sexual size dimorphism (Notomy alexis)

Inter-male competition is typically used to account for the male-biased sexual size dimorphism (MSSD) frequently found in mammals. In contrast, although they are not rare, mammalian systems exhibiting female-biased sexual size dimorphism (FSSD) have been poorly accounted for in sexual size dimorphism (SSD) theory across the mammalian spectrum. In addition to demonstrating unambiguous FSSD, male Tarrkawarra (Notomys alexis) have one of the lowest known mammalian testis to body mass ratios. These features make Tarrkawarra an ideal system for further investigating theories of mammalian SSD and alternative social organisation. In the current study we assessed evidence of sexual selection for FSSD in this Australian arid-zone rodent. Controlling for familiarity and relatedness, affiliative preference experiments were run in conjunction with assays of female reproductive state. We found evidence for mate choice in changed patterns of time allocation and visit frequency to stimulus conspecifics over the course of the female reproductive phase. Preferences associated with increased familiarity and size of affiliative partners also indicate a complex interaction between natural and sexual selection processes in the evolution of FSSD and have implications for better understanding the social system of this little studied species.