Penile morphology of African mole rats (Bathyergidae): structural modification in relation to mode of ovulation and degree of sociality A. Parag 1 , N. C. Bennett 2 , C. G. Faulkes 3 & P. W. Bateman 1 1 Department of Zoology & Entomology, University of Pretoria, Pretoria, South Africa 2 Department of Zoology and Entomology, Mammal Research Institute, University of Pretoria, Pretoria, South Africa 3 Biological Sciences, Queen Mary, University of London, London, UK Keywords mole rat; penis; ovulation; baculum; mating systems; sperm competition. Correspondence Philip W. Bateman, Department of Zoology & Entomology, University of Pretoria, Pretoria 0002, South Africa. Email: pwbateman@zoology.up.ac.za Received 24 May 2005; accepted 14 February 2006 doi:10.1111/j.1469-7998.2006.00141.x Abstract The penile morphology and bacular structure of five species of African mole rat are described in relation to the method of ovulation, degree of sociality and polygynandry. We predicted that, with an increase in sociality, and a concomitant decrease in polygynandry and sperm competition, there would be a decrease in penis ornamentation (spinosity) and baculum size. In solitary species of African mole rat with marked seasonal reproduction and induced ovulation (Bathyergus suillus and Georychus capensis), males have numerous epidermal spines on the penis. Social, seasonally breeding, induced ovulating mole rats Cryptomys hotten- totus natalensis possess less elaborate ornamentation in the form of small protru- sions that are rounded at the apex. Two aseasonally breeding eusocial species with spontaneous ovulation Cryptomys damarensis and Heterocephalus glaber have ridges on the penis but lack any elaborate ornamentation. Baculae, however, showed a trend to become proportionally smaller in the solitary species. Our prediction that, with an increase in sociality and a move from induced to spontaneous ovulation, the amount and degree of penile ornamentation declines was, therefore, generally supported. Introduction Three non-mutually exclusive mechanisms have been pro- posed for the evolution of genital morphology in mammals: sperm competition, female cryptic choice and sexual conflict (Thornhill, 1983; Clutton-Brock & Parker, 1995; Eberhard, 1996; Birkhead & Parker, 1997; Larivie`re & Ferguson, 2002). In polyandrous mating systems, there is often male– male competition for access to females either through direct interactions such as fighting or through more cryptic meth- ods such as sperm competition, where gametes from differ- ent males compete for fertilization of the same set of ova (Parker, 1970, 1984). Increased efficiency of copulations through optimal tim- ing of copulation should theoretically increase the probabil- ity of paternity (Parker, 1990; Stockley & Purvis, 1993). Sperm competition acts as a selective force that shapes male morphology to enable optimal deposition of the ejaculate (spatially and temporally) in the female reproductive tract. Males may also develop more elaborate penile morphology to assist in stimulation (Eberhard, 1985). It has been postulated that penile spines and other penile ornamenta- tions may induce ovulation through vaginal stimulation (Altuna & Lessa, 1985) and, therefore, will have an adaptive value in solitary animals that only have brief periods to mate. For example, studies on the penile morphology of solitary Tuco tucos Ctenomys pearsoni (Ctenomyidae) show that the glans and shaft are covered with small spikes that presumably are involved in either stimulating the vagina or ensuring lock between the male and female (Altuna & Lessa, 1985). Stimulation may encourage the acceptance of the male sperm by the female and penises may become increasingly elaborate and ornamented with increasing sperm competition. Sexual selection, therefore, leads to an increase in complexity of penile structure, especially in polyandrous species as compared with monogamous species (Eberhard, 1985). The level of sociality and degree of skew in male repro- duction in a taxon could, therefore, predict the intensity of sperm competition and sexual selection and, as a conse- quence, the mating strategy exhibited by a particular spe- cies. For an optimal mating strategy, female ovulatory patterns essentially need to be compatible for successful fertilization to occur during the generally short life of sperm (c. 48 h for eutherian mammals, especially rodents; Gomen- dio & Roldan, 1993). Sociality may therefore dictate the pattern of ovulation, be it induced or spontaneous. Elabo- rate and ornamented penises, including the relative size of the os penis or baculum, may act as an effective female stimulant to induce ovulation (Altuna & Lessa, 1985; Bennett, Faulkes & Molteno, 2000; Larivie`re & Ferguson, Journal of Zoology 270 (2006) 323–329 c  2006 The Authors. Journal compilation c  2006 The Zoological Society of London 323 Journal of Zoology. Print ISSN 0952-8369