Effects of artificial social stimuli on the reproductive schedule and hormone levels of yellow-eyed penguins (Megadyptes antipodes).

The effects of social stimuli on avian reproductive behaviors such as breeding schedules and courtship behaviors are well known due to numerous field studies. However, studies that have simultaneously examined the effects of social stimuli on reproductive behavior and the mediating endocrine mechanisms have been largely restricted to captive populations, which may not be representative of free-living populations. This study, conducted over two breeding seasons, aimed to simultaneously measure the effects of experimentally increasing auditory stimuli on the breeding schedule and endocrinology (levels of total androgen, estradiol, progesterone and prolactin) on free-living yellow-eyed penguins (Megadyptes antipodes). The yellow-eyed penguin is the least colonial of all penguins, nesting far apart from each other under dense vegetation, and, therefore, is presumed to experience much lower levels of social stimuli than other penguins. Egg laying was significantly more synchronous and tended to be earlier when birds were exposed to playbacks of the calls of conspecifics in 1 year of the study. We also found that levels of total androgen and estradiol of males in 1 year, and prolactin in another year, were proportionally higher among treated birds compared control birds that received no artificial auditory stimuli. These results show that even among supposedly solitary nesters, social stimuli could still play a role in influencing reproductive behavior and physiology. For the first time in free-living seabirds, we have demonstrated that behavioral responses to increased social stimuli are associated with hormonal changes.

Dynamics of the responses in secretion of luteinising hormone, leptin and insulin following an acute increase in nutrition in mature male sheep.

In Merino rams, an increase in the plane of nutrition increases the frequency of luteinising hormone (LH) pulses within a few days and this response is correlated with changes in the circulating concentrations of metabolic hormones. To analyse early dynamic aspects of these responses, we studied mature rams fed with diets that contained either low or high amounts of energy and protein. Jugular blood was sampled every 20 min for 96 h, including a control period of 24 h before the change of diet in the high-diet group. In the high-diet group, a significant increase in LH pulse frequency was first detected 6 h after the increase in nutrition on Day 1 and the frequency remained significantly elevated throughout the 72-h treatment period, except for a 12-h period on Day 2. Following the change of diet, insulin concentrations increased within 3 h and leptin concentrations increased within 7 h, after which time the concentrations of both hormones remained high. Dietary treatment did not affect the concentrations of thyroxine or insulin-like growth factor-I, but the high diet increased the concentrations of tri-iodothyronine. These observations are consistent with insulin and leptin playing a role in the early activation of the gonadotrophin-releasing hormone-LH axis by nutritional inputs.