Yolk testosterone levels and offspring phenotype correlate with parental age in a precocial bird

Parents, and particularly mothers, can influence their offspring's development in non-genetic ways. Maternal effects can occur during the mothering phase as well as during the embryonic phase. Prenatal maternal effects in birds can be mediated by yolk steroid hormones that influence subsequent offspring development. Studies have focused mainly on the influence of laying females' living conditions on yolk hormonal contents, and rarely on the effects of individual characteristics. Here, we investigated prenatal influence of parent age on yolk steroid levels and on offspring phenotype. We compared Japanese quail at two different ages: at the beginning of their reproductive cycle (11 weeks old: age 1) and six months later, after egg production peak (37 weeks old: age 2). Egg composition, reproductive outcomes, and offspring growth, sexual development and behaviour were studied at both ages. We found that laying rate, fertility and chick survival rates declined between age 1 and age 2. Age 2 eggs had relatively lighter shells and higher yolk plus albumen contents; they also had lower testosterone contents. Age 2 offspring weighed more at hatching than did age 1 offspring; subsequently their growth patterns differed and their sexual development was more precocious. Age 2 offspring were less emotional than age 1 offspring when encountering a novel environment, and they appeared more sensitive to social separation. Our study shows, for the first time in a bird species, a strong impact of parental age on offspring phenotype, and especially on behaviour, an impact that is possibly mediated via modulation of yolk testosterone content.