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Figure 3.2 Morphological diversity of mammalian spermatozoa. The acrosome and midpiece of each sperm is shaded. 1. Honey possum (Tarsipes rostratus); 2. Marsupial rat (Dasyuroides byrnei); 3. Short-nosed brindled bandicoot (Isoodon macrourus); 4. Tammar wallaby (Macropus eugenii); 5. Brush-tailed possum (Trichosurus vulpecula); 6. Koala (Phascolarctos cinereus); 7. Hippopotamus (Hippopotamus amphibius); 8. Human (Homo sapiens); 9. Rabbit (Oryctolagus cuniculus); 10. Ram (Ovis aries); 11. Golden hamster (Mesocricetus auratus); 12. Laboratory rat (Rattus norvegicus); 13. Chinese hamster (Cricetulus griseus). Source: After Setchell (1982).

region of the sperm head is hooked, and especially so in species with large relative testes sizes (Immler et al. 2007). Sexual selection may have favoured this specialization because such sperm form cohorts, hooked together and swimming more efficiently as a group than would be possible for each gamete on its own (Moore et al. 2002). In the majority of South American marsupials the sperm are joined together in pairs at the acrosome, and remain so until they reach the oviduct (Rodger and Bedford 1982). These binary sperm are also thought to possess more efficient motility, especially when moving through a viscous medium (Taggart et al. 1993). Thus, in some mammals sperm competition has resulted in specializations of sperm morphology to enhance motility.

The largest spermatozoon among mammals occurs in the diminutive honey possum (Tarsipes rostratus). Indeed, overall sperm length is not correlated with body size among mammals as a whole (Gage 1998; Anderson, Nyholt, and Dixson 2005). Might the observed differences in sperm length observed among mammals also be related in some way to sperm competition? Gomendio and Roldan (1991) proposed that this might be so, and that longer sperm occur in those species of primates and rodents, where females mate with multiple partners during the fertile period. Long sperm may swim more rapidly within the female reproductive tract, and hence they might secure some advantage in sperm competition. However, despite early support for this hypothesis, based on studies of primate sperm lengths and relative testes sizes (Dixson 1993), subsequent analyses of much larger data sets have failed to reveal any correlation between sperm length and sperm competition in mammals (Harcourt 1991; Gage 1998; Dixson 1998a; Gage and Freckleton 2003; Anderson et al. 2005). Examples of Anderson et al.'s (2005) comparative measurements of mammalian sperm are provided in Table 3.1. Linear measurements of the individual compartments of mammalian sperm do not correlate with mating systems or relative testes sizes. However, studies of the volume of the sperm midpiece have provided significant comparative and evolutionary insights, as described below.

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