AMES, C.E.; WILLIAMS, T.D. *; VEZINA, F.; Simon Fraser University, Burnaby; Simon Fraser University, Burnaby; Simon Fraser University, Burnaby: Rapid regression of a still-functioning linear organ: the avian oviduct.

Egg production in oviparous vertebrates is widely assumed to be costly but the physiological basis of any costs remains unknown. Previously we have shown that, a) the avian oviduct loses 50% of its mass at the end of laying even while it still retains an oviductal egg, and b) oviduct mass is the best predictor of variation in residual RMR in laying birds (r2 = 0.15). The avian oviduct is highly differentiated, with five functionally specific regions. An egg takes c.25 hrs to pass down the entire length of the oviduct but spends most time (c.20 hrs) in the distal shell gland. We tested the hypothesis that this linear organ regresses very rapidly ‘from the top down’ as soon as the more proximal regions have completed their function but while the distal regions are still functional. In zebra finches (Taeniopygia guttata) oviduct mass did not differ between early laying birds at the 1-egg stage and late laying birds with 4+ eggs, one remaining yolky follicle, and an oviductal egg (dry mass, 151-167 mg). However, in birds with no remaining yolky follicles but with an oviductal egg, oviduct mass had already decreased to 94 mg (44%; P < 0.001). This reduction in mass occurred unequally among the different regions of the oviduct, with significant decreases in the proximal infundibulum/magnum and isthmus regions, by 59% and 40% respectively (P < 0.001), but no change in shell gland/vagina mass. Shell gland mass did not regress until after the last egg was laid. Thus, the avian oviduct has a highly regulated size-function relationship; the very rapid onset of regression that we report is consistent with the oviduct being an expensive organ to maintain, and with oviduct function being a significant contributor to the physiological costs of egg production.