Do mating strategies determine genital allometry in African mole rats (Bathyergidae)?

Abstract

Allometry describes the relationship of components of an organism with change in overall body size and has become the focus of numerous studies on the evolution of genitalia. Typically, negative allometry is observed in insects and is explained by stabilizing selection whereas the very few studies on mammals have shown a positive allometric relationship of genitalia in the body size, thought to have arisen as a result of sexual selection. However, all mammal species studied to date are thought to use mainly post-copulatory mating strategies. Across mammals, however, both pre-and post-copulatory strategies occur (although the two are not mutually exclusive). We propose that where pre-copulatory strategies are mainly used, no reproductive benefits would result from evolving positively allometric genitalia. As such, mammal genitalia are not typically positively allometric but rather allometry will, to a certain degree, be determined by mating strategy. We tested this prediction using four species of African mole rats (Bathyergidae) exhibiting variation in their life histories and mating strategies. Although generally supported, in that positive allometry did not occur in species that we assumed use mainly pre-mating strategies, positive allometry did not occur in either of the promiscuous species thought to use post-copulatory strategies. We suggest, therefore, that while mating strategies may tentatively determine genital allometry, whether positively allometric genitalia occur also depends on a number of complex interacting factors. In addition, this study provides further evidence and empirical support for the co-evolution of male and female genitalia in mammals.

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.…”

“…Body measurements recorded were body length (nose to the base of the tail), length of skull (tip of bony part of the nose to the back of the skull), width of skull (widest part of the zygomatic arch), front right foot (tip of the longest toe to the tip of the heel of the foot [SU]) and hind right foot (tip of the longest toe to the tip of the heel of the foot [SU]). Body length was used as an indicator of body size, since body mass varies 6 with individual body condition and across the seasons (see Kinahan et al 2007Kinahan et al , 2008.…”

“…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.…”

“…The positively allometric testes of B. suillus also imply a role for sperm competition (Kinahan et al 2007). Kinahan et al (2008) extended this hypothesis to other mole-rat species, and predicted that positive allometry of penises at least may not necessarily be a general pattern for mammals but would be found in those species with the least likelihood of demonstrating any pre-copulatory mate choice. Qualified support for the hypothesis was found in that the social mole-rats species with more complex courtship did not demonstrate positive allometry of the penis as predicted; interestingly neither did another solitary species (Georychus capensis) that was assumed to have a similar mating system to B. suillus, but in G. capensis male seismic drumming during the breeding season may have a role in female assessment of potential mates (Bennett & Jarvis, 1988).…”

“…Very little is known about its behaviour except that females are polyoestrous and aseasonal breeders 5 (Skinner & Chimimba, 2005;Schoeman et al 2004). We hypothesised that if mating strategies are a major predictor of genital allometry (as suggested for bathyergid molerats; Kinahan et al, 2007Kinahan et al, , 2008, and if this applies generally to subterranean animals with limited visual acuity then predictions could be made about the mating strategies of the phylogenetically very different golden moles that fill a similar subterranean niche.…”