Adaptations of the short-beaked echidna Tachyglossus aculateus for sperm production, particularly in an arid environment.

Abstract

A re-examination is made of the old interpretation that the male reproductive system of the short-beaked echidna (Tachyglossus aculateus), and monotremes in general, is ‘primitive’. There is an emerging concept of the significance of the male reproductive system in relation to the role of males in conception. Even though some aspects of the male reproductive system of T. aculateus may be considered primitive and impose limitations on their life style, they nevertheless display significant specializations present in the male tract of other mammals: an epididymis with the unique initial segment and a role in post-testicular sperm maturation and storage, and homologues of the prostate gland and penis. The absence of a descended testis and scrotum has not restricted the survival of T. aculateus in virtually all habitats in Australia. However, it may restrict their capacity to breed throughout the year. It is suggested that T. aculateus is also specialised for sperm competition. An estimate of the rate of spermatogenesis indicates that it is comparable with other mammals. Consequently, the size of their testes and number of extragonadal sperm compared with other mammals are indicative that they are involved in sperm competition. The cooperation involved in the formation of sperm bundles is also indicative of sperm competition. Further, the formation of ‘echidna trains’, in which a number of males will follow in line behind an oestrous female indicates that there is competition between males to mate with a female.