Longitudinal covariation of testosterone and sperm quality across reproductive stages in the zebra finch

Birds that breed opportunistically maintain partial activation of reproductive systems to rapidly exploit envi-ronmental conditions when they become suitable for breeding. Maintaining reproductive systems outside of a breeding context is costly. For males, these costs are thought to include continual exposure to testosterone. Males of seasonally breeding birds minimise these costs by downregulating testosterone production outside of a breeding context. Opportunistically breeding birds trade off the need to rapidly initiate reproduction with the costs of elevated testosterone production. One way opportunistically breeding males could minimise these costs is through fine scale changes in testosterone production across discrete reproductive stages which have a greater or lesser requirement for active sperm production. Although spermatogenesis broadly depends on testosterone production, whether changes in testosterone levels across the reproductive stages affect sperm quality and production is unknown. Here, we measured testosterone, sperm quality, and body condition in male zebra finches at discrete stages within reproductive bouts (egg laying, incubation, nestling provisioning, and fledging) and across two consecutive reproductive events in captive male zebra finches (Taeniopygia castanotis). We also examined associations between male testosterone, sperm quality/production, body condition, and nestling body condition. We found that testosterone levels varied across discrete reproductive stages with the lowest levels during incubation and the highest following chick fledging. Testosterone levels were positively associated with sperm velocity and the proportion of motile sperm but were not associated with male body condition. We found no associations between paternal body condition, testosterone levels, or sperm traits with nestling body condi-tion (a proxy for the reproductive quality of a male and his partner). This study is the first to show that opportunistically breeding males vary testosterone synthesis and sperm traits at discrete stages within a repro-ductive event.

Automated summary

Birds that breed opportunistically maintain partial activation of reproductive systems to rapidly exploit environmental conditions when they become suitable for breeding. Males of seasonally breeding birds downregulate testosterone production outside of a breeding context to minimize costs.