Department of Animal Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Wageningen, The Netherlands.
PLoS One. 2012;7(3):e34090. doi: 10.1371/journal.pone.0034090. Epub 2012 Mar 20.
Seasonal timing of reproduction and the number of clutches produced per season are two key avian life-history traits with major fitness consequences. Female condition may play an important role in these decisions. In mammals, body condition and leptin levels are correlated. In birds, the role of leptin remains unclear. We did two experiments where we implanted female great tits with a pellet releasing leptin evenly for 14 days, to manipulate their perceived body condition, or a placebo pellet. In the first experiment where females were implanted when feeding their first brood offspring we found, surprisingly, that placebo treated females were more likely to initiate a second brood compared to leptin treated females. Only one second brood fledged two chicks while five were deserted late in the incubation stage or when the first egg hatched. No difference was found in female or male return rate or in recruitment rate of fledglings of the first brood, possibly due to the desertion of the second broods. In our study population, where there is selection for early egg laying, earlier timing of reproduction might be hampered by food availability and thus nutritional state of the female before egg laying. We therefore implanted similar leptin pellets three weeks before the expected start of egg laying in an attempt to manipulate the laying dates of first clutches. However, leptin treated females did not initiate egg laying earlier compared to placebo treated females, suggesting that other variables than the perceived body condition play a major role in the timing of reproduction. Also, leptin treatment did not affect body mass, basal metabolic rate or feeding rates in captive females. Manipulating life history decisions using experimental protocols which do not alter individuals' energy balance are crucial in understanding the trade-off between costs and benefits of life history decisions.
繁殖季节和每个季节产的窝数是两个关键的鸟类生活史特征，对适应度有重要影响。雌性的状况可能在这些决策中起着重要作用。在哺乳动物中，身体状况和瘦素水平是相关的。在鸟类中，瘦素的作用仍不清楚。我们进行了两项实验，其中我们给雌性大山雀植入了一个能够在 14 天内均匀释放瘦素的药丸，以操纵它们的身体状况感知，或者植入一个安慰剂药丸。在第一个实验中，当雌性正在喂养第一窝雏鸟时，我们发现令人惊讶的是，与瘦素处理的雌性相比，安慰剂处理的雌性更有可能开始第二窝繁殖。只有一个第二窝成功孵化了两只雏鸟，而另外五只在孵化后期或第一只蛋孵化时被遗弃。我们没有发现雌性或雄性的归巢率或第一窝雏鸟的招募率有差异，这可能是由于第二窝的遗弃。在我们的研究种群中，存在着对早产卵的选择，因此繁殖时间的提前可能会受到食物供应的限制，从而影响到雌性在产卵前的营养状态。因此，我们在预期产卵开始前的三周给雌性植入了类似的瘦素药丸，试图操纵第一窝的产卵日期。然而，与安慰剂处理的雌性相比，瘦素处理的雌性并没有更早地开始产卵，这表明除了感知到的身体状况之外，其他变量在繁殖时间上起着重要作用。此外，瘦素处理并没有影响到圈养雌性的体重、基础代谢率或进食率。使用不会改变个体能量平衡的实验方案来操纵生活史决策，对于理解生活史决策的成本和收益之间的权衡至关重要。
