Structure and allometry of genitalia in males and females of a social African ground squirrel with high polygynandry

Abstract

The few studies that have looked at genital allometry in mammals have typically shown a positively allometric relationship with body size and high coefficients of variation. Cryptic female choice, sexual conflict or sperm competition are mechanisms underlying genital evolution and as these are not mutually exclusive, they are often difficult to disentangle. In addition, these mechanisms are affected by both male and female social structure and/or mating strategies and, as such, pre-and post-copulatory behaviours have been shown to alter selection on genitalia. We examined genital traits and allometry in a polygynandrous and social ground squirrel Xerus inauris. We found that male testes are positively allometric and account for 1.5% of their body weight, one of the highest percentages known for sciurids. The penis, at 42.4% of head/body length, was isometric while the female reproductive tract, 22.4% head/body length, demonstrated no such relationship. Based on the allometric relationships of both males and females presented here, in conjunction with high levels of competition for females and lack of male aggression and territoriality, we suggest that sperm competition is the most likely mechanism for the evolution of the extremely large genitalia in this species.

Sexual selection and genital allometry in the Hottentot golden mole (Amblysomus hottentotus)

“…This is in agreement with the findings of previous studies of genital allometry in mammals conducted by Manjerovic et al (2008), Kinahan et al (2007Kinahan et al ( , 2008, Tasikas et al (2007) and particularly Lüpold et al (2004) where the role of the penis as a good genes indicator was mooted. We support the sexual selection hypothesis for several reasons (adapted from Kinahan et al, 2007), even though positive allometry of both male and female genitalia was not found for A. hottentotus.…”

“…In this study we found positive allometry and a high CV in the genitalia of male Amblysomus hottentotus, which is typical of sexually selected traits and has been supported by previous studies on mammals (Miller & Burton, 2001;Lüpold et al, 2004;Kinahan et al, 2007;Manjerovic et al, 2008). In the first study of allometry performed on female mammals, Kinahan et al (2007) found positive allometry and a high CV for female reproductive tract length in Bathyergus suillus.…”

“…For the small, but increasing, number of studies on mammals the opposite pattern has generally been observed (e.g. Tasikas et al 2009;Manjerovic et al 2008;Kinahan et al 2008;Kinahan et al 2007;Lűpold et al 2004;Miller & Burton, 2001). Kinahan et al (2007), one of the first studies to include females in the studies of mammal genital allometry, found positive allometry and high coefficients of variation for both male genitalia and female reproductive tract length 4 in the African Cape dune mole-rat (Bathyergus suillus) implying an active role of sexual selection in the evolution of that species" genitalia.…”

“Failure to launch”: is there a reproductive cost to males living at home?

“…Condition also has been linked to variations in sperm quantity and/or quality with the assumption that better body condition leads to producing more sperm (Pitnick et al 2009). Although no differences in size of testes have been detected between tactics (Scantlebury et al 2008), male X. inauris show high amounts of sperm competition (Manjerovic et al 2008), which is common in species that multiply mate. Given the fact that the most estruses fail to produce any offspring and those that do average a single offspring, the likelihood of multiple paternity is low.…”

“…Instead, males compete predominantly via precopulatory competitive searching (Waterman 1995(Waterman , 1998 and sperm competition (Manjerovic et al 2008) although additional behaviors might influence reproductive success. Male X. inauris reach reproductive maturity between 8 and 10 months, at which point they either disperse and join a roving all-male band or delay dispersal and remain in their natal burrow for up to 4 years (Waterman 1995(Waterman , 1997Scantlebury et al 2008).…”