Why do some adult birds skip breeding? A hormonal investigation in a long-lived bird

Skipping reproduction is often observed in long-lived organisms, but proximate mechanisms remain poorly understood. Since young and/or very old snow petrels (Pagodroma nivea) commonly skip breeding, we tested whether they are physiologically able to breed during the pre-laying stage. To do so, we measured the ability of known-age (11-45 years old) petrels to release luteinizing hormone (LH, a crucial driver for breeding), by injecting exogenous gonadotropin-releasing hormone (GnRH). Although young petrels exhibited low baseline LH levels, they were able to elevate LH levels after a GnRH challenge. Moreover, young and very old petrels showed a stronger decrease in LH levels after the 10 min post-GnRH injection compared with middle-aged petrels. Birds that skipped breeding were as able as breeders to release LH after a GnRH challenge, indicating that they had functional pituitaries. However, the decision to skip reproduction was linked to a strong LH decrease after the 10 min post-GnRH injection. Our result suggests that the youngest and the oldest petrels fail to maintain elevated baseline LH levels, thereby do not initiate reproductive activities. Skipping reproduction in long-lived birds probably results from age-related changes in the dynamics of the hypothalamic-pituitary-gonadal (HPG) axis function.