ABSTRACT
Almost since the moment animal semen was first viewed under the microscope, it was clear that the shape and size of spermatozoa varied from one species to the next (Leeuwenhoek 1697). Early comparative biologists similarly noted striking differences in the size of the testis across different species (Ray 1678). Although the general features of the male reproductive system are rather conservative across the 10,000 or so extant species of birds, recent work has confirmed large interspecific variation in the sizes of both testes and sperm. For example, the combined mass of the left and right testes of breeding Alpine accentors (Prunella collaris) is 3.0 g and comprises about 8% of adult body mass, while that of the Zebra finch (Taeniopygia guttata) is only 0.05 g or 0.4% of its body mass (Birkhead et al. 1991; Nakamura 1990). Likewise, a single spermatozoon of the Yellow warbler (Dendroica petechia) averages 277 µm in length, while that of the Red-backed shrike (Lanius collurio) is only 43 µm long, a six-fold difference in size (Briskie et al. 1997). Why should testes mass and sperm size differ so much from one species to the next?
