Sperm velocity is one of the main determinants of the outcome of sperm competition Since sperm vary considerably in their morphology between and within species, it seems likely that sperm morphology is associated with sperm velocity Theory predicts that sperm velocity may be increased by enlarged midpiece (energetic component) or flagellum length (kinetic component), or by particular ratios between sperm components, such as between flagellum length and head size However, such associations have rarely been found in empirical studies In a comparative framework in passerine birds, we tested these theoretical predictions both across a wide range of species and within a single family, the New World blackbirds (Icteridae) In both study groups, sperm velocity was influenced by sperm morphology in the predicted direction Consistent with theoretical models, these results show that selection on sperm morphology and velocity are likely to be concomitant evolutionary forces

/pdf/sperm-morphology-and-sperm-velocity-in-passerine-birds-4pf9iqtdxh.pdf

Sperm morphology and sperm velocity in passerine birds

Estimates of the numbers of sperm storage tubules (SSTs) in the utero-vaginal junction of 11 bird species are presented. Numbers of SSTs varied by a factor of 40 between species, and ranged from 500 to 20000. Body mass accounted for over 50% of the variation in SST mumbers. SST length was positively correlated with the length of spermatozoa across species. The duration of sperm storage was not correlated with the number of SSTs or the volume of sperm storage tissue. However, the number of ‘active’ SSTs appears to vary between species and it was not possible to make allowance for this. Sperm storage duration was weakly, positively correlated wth clutch size, but showed a significant positive relationship with the number of days over which laying occurred. The number of SSTs was also positively correlated with the number of sperm per ejaculate. The best predictor of sperm storage duration was a multiple regression equation using the spread of laying and the length of sperm storage tubules. The duration of sperm storage in birds which remain together during the pre-laying period is such that a single insemination immediately before the start of laying could fertilize the entire clutch.

Numbers and size of sperm storage tubules and the duration of sperm storage in birds: a comparative study

Sperm morphology varies considerably across taxa, and postcopulatory sexual selection is thought to be one of the main forces responsible for this diversity. Several studies have investigated the effects of the variation in sperm design on sperm function, but the consequences of variation in sperm design on testis morphology have been overlooked. Testes size or architecture may determine the size of the sperm they produce, and selection for longer sperm may require concomitant adaptations in the testes. Relative testes size differs greatly between species and is often used as an index of sperm competition, but little is known about whether larger testes have more sperm-producing tissue or produce sperm at a faster rate. Using a comparative approach in New World Blackbirds (Icteridae), we found (1) a strong link between testis histology and sperm length, suggesting selection on testis architecture through selection on sperm size, and (2) that species under intense sperm competition had a greater proportion of sperm-producing tissue within their testes. These results support the prediction that sperm competition fosters adaptations in reproductive organs that extend beyond testes size, and raise questions about the trade-offs influencing reproductive investment.

/pdf/sperm-competition-selects-beyond-relative-testes-size-in-16j0d73fqv.pdf

Sperm Competition Selects Beyond Relative Testes Size in Birds

Male passerine birds store sperm in a cloacal protuberance during the breeding season. We consider three hypotheses to explain interspecific variation in relative cloacal protuberance size. The copulation efficiency hypothesis states that a relatively large cloacal protuberance facilitates sperm transfer and predicts more rapid copulation in species with larger protuberances. There is insufficient data to test this idea rigorously, but we found no evidence for such an effect. The spermatozoa size hypothesis is a non-functional hypothesis which states that cloacal protuberance size is merely a consequence of sperm size, and predicts that species with larger spermatozoa have relatively larger cloacal protuberances. Sperm length was positively correlated with protuberance size, providing support for this hypothesis, although it seems unlikely that variation in sperm size alone is sufficient to account for variation in protuberance size because the numbers of sperm stored in the cloacal protuberance were also positively correlated with its size. The sperm competition hypothesis states that the cloacal protuberance is a sperm store and predicts that when sperm competition is intense, as measured by male copulation frequency and or mating system, males will have relatively large protuberances and testes mass. The sperm competition hypothesis was supported: in a comparative study in which we controlled for phylogenetic effects, relative cloacal protuberance size was significantly and positively correlated with copulation frequency. Across all mating systems protuberance size was also positively correlated with the mass of seminal glomera tissue, the number of sperm stored in the seminal glomera, and with relative testes size. These results suggest that where sperm competition is intense, a large cloacal protuberance is required to maintain a large sperm reserve for a high copulation rate.

Male sperm reserves and copulation frequency in birds

Cryopreservation of semen from endangered pheasants: the first step towards a cryobank for endangered avian species.

Semen samples were obtained from budgerigars by applying gentle pressure to both sides of the cloaca. The semen appeared to be stored in the seminal glomera, two structures on either side of the proctodeum. These structures have been described before in Passeriformes. The spermatozoon in budgerigars is similar to the spermatozoon of the domestic fowl, but differs from the spermatozoon of Passeriformes.

JH Samour

Papers

Semen collection and spermatozoa characteristics in budgerigars (Melopsittacus undulatus)