PATTERNS OF SPERM STORAGE IN RELATION TO SPERM COMPETITION IN PASSERINE BIRDS

Using a technique for observing sperm storage tubules (SSTs) in whole-mount preparations of oviduct tissue, we estimated the number and length of SSTs across 20 species of passerine birds in seven subfamilies. The average number of SSTs per female varied nine-fold across the species examined and was correlated positively with body mass and relative egg size, suggesting that much of the interspecific variation in the number of SSTs may be a consequence of allometry. However, after controlling for body mass, the number of SSTs per female was also negatively correlated with the average length of SSTs and we suggest this pattern could result from selection for greater sperm length when access to sperm storage sites is limited by females. SST length varied eight-fold across the species examined but unlike the number of SSTs, SST length was not correlated with body mass. Instead, SST length was strongly and positively correlated with sperm length, suggesting a history of coevolution between male gametes and sperm storage sites in females that is independent of body size. Neither mating system nor any of four other morphological variables (testis length, oviduct length, clutch size, sperm storage capacity) was consistently correlated with either the number or size of SSTs. Variation between species in the potential for sperm layering within an SST, however, indicates that the pattern of sperm precedence (i.e., the pattern of paternity relative to mating order) may vary from species to species. We propose that a conflict of interest between the sexes with respect to sperm storage could lead to an arms race between sperm length and SST length and may help explain both the diversity in sperm storage patterns among species and the influence of sperm storage on the evolution of mating behavior in birds.