Interindividual variation in yolk mass and the rate of growth of ovarian follicles in the zebra finch (Taeniopygia guttata)

The amount of resources invested in an individual egg yolk must be determin ed by its rate of growth and/or the duration of growth. We examined interin dividual variation in the growth rate of yolks by injecting radiolabeled am ino acid into breeding female zebra finches and measuring the activity asso ciated with protein in the yolks of eggs laid subsequently. We predicted th at (1) there would be a positive correlation between yolk mass and the rate of uptake of activity into the yolk; and (2) there would be a negative cor relation between clutch size and the amount of activity taken up by each of the follicles due to competition between follicles for circulating yolk pr ecursors. The rate of uptake of activity by the yolks was positively relate d to yolk mass (r(2)=0.24, 0.35 and 0.50 for the yolks of the third-, fourt h- and fifth-laid eggs, respectively), suggesting that interindividual vari ation in yolk mass is due, at least in part, to variation in the rate of fo llicle growth. However, we found no evidence of a trade-off between yolk si ze and number. The uptake of activity was generally repeatable between bree ding attempts (repeatability = 0.23-0.44), as was mean yolk mass (repeatabi lity = 0.35), suggesting that these traits are characteristics of individua l females.