Elevating Testosterone and Androstenedione Produces Temporary Sex‐Dependent Variation in Japanese Quail (<i>Coturnix japonica</i>) Embryonic Development

Vertebrate growth rates may respond to maternally supplied steroids, yet injections of individual hormones into avian eggs of different species have produced inconsistent results. This might be because diverse steroids' concentrations are naturally correlated in yolks, injections are not elevating hormones at critical times in development, or injected hormones are rapidly metabolized by the developing embryo. To explore these alternatives, we injected naturally occurring levels of androstenedione (A4) and testosterone (T) or an oil control into fertile Japanese Quail (Coturnix japonica) eggs at the onset (Day 0) and/or on Day 6 of incubation. We measured the size of the embryos at two points in development (Days 6 and 15), testing for an effect of the hormone injections and the embryos' sex determined by molecular markers. Additionally, we quantified the concentrations of 27 yolk steroid hormones at five points in development (Days 0, 1, 6, 7, and 15). Ultimately, the A4 + T injection did not elevate yolk concentrations of these hormones 24 h after treatment, nor did it influence concentrations of other steroids in the yolk. Still, initial (Day 0) A4 + T injections reduced the eye diameter and leg length of female embryos and reduced the mass of male embryos on Day 6. The double hormone treatment (A4 + T on Days 0 and 6) produced embryos with larger beaks and wings but reduced mass measurements on Day 15 relative to the oil-oil control, regardless of sex. This suggests that embryonic growth responds to elevated levels of A4 and T at the beginning of incubation and in a sex- and structure-specific manner.