Copulatory behaviour in the Bonelli´s Eagle: Assessing the paternity assurance hypothesis

We examined copulatory behaviour in the Bonelli´s Eagle (Aquila fasciata) at nesting sites in the eastern zone of the Baetic Cordillera, southern Spain, between 2010 and 2012. We observed the copulatory behaviour of 15 pairs during the pre-laying period. Bonelli´s Eagles commenced sexual activity ca. 69 days before egg-laying. Ninety-six percent of mounting attempts were successful. Bonelli´s Eagle pairs averaged 99.8 copulation attempts per clutch, with an average copulation frequency of 0.86 copulation attempts per day. Pairs displayed a daily bimodal pattern of copulation activity, with copulations occurring most frequently in the evening. We used our data to test three predictions with regard to the paternity assurance hypothesis. Prediction 1, that within-pair copulations increase with local breeding density, was rejected because our models showed no evidence for it. Prediction 2, that within-pair copulations increase during the female fertile period, was marginally supported. Finally, Prediction 3, that mate attendance increases during the female fertile period, was also rejected because mate-guarding did not increase as the fertile period approached. However, mate-guarding was positively correlated with within-pair copulation frequency. Moderate copulation rates compared to other raptors and the absence of mate-guarding suggest that, in the study area, Bonelli´s Eagles exhibit only partially adaptive behaviour to assure their paternity. A possible explanation could be related to the low number of extra-pair encounters observed (opportunities for which appear to be rare), although the gradual increase in within-pair copulations during the female fertile period is consistent with the sperm competition hypothesis. The results are discussed based on the signalling hypothesis, which proposes that raptors signal territory ownership to conspecifics, and possibly to other raptor species, by copulating frequently and conspicuously in the defended nesting area.我们于2010—2012年间，在西班牙南部贝蒂科山脉（Baetic Cordillera）东部的巢址中，针对博内利雕（Bonelli´s Eagle，学名Aquila fasciata）的交配行为（copulatory behaviour）开展了研究。研究期间，我们观测到15对博内利雕在产卵前期的交配活动。博内利雕的性活动始于产卵前约69天，96%的爬跨尝试均取得成功。每窝繁殖周期内，该雕类配对的平均交配尝试次数为99.8次，日均交配频率达0.86次。其交配活动呈现每日双峰模式，且晚间为交配频次峰值时段。 本研究基于父权保障假说（paternity assurance hypothesis）设置三项预测，并利用观测数据进行检验：预测1为配对内交配（within-pair copulations）频次随本地繁殖密度升高而增加，但本研究模型未发现相关证据，该预测因此被推翻；预测2为雌性可育期内配对内交配频次会上升，该预测仅获得边际性支持；预测3为雌性可育期内配偶守卫（mate-guarding）频次会升高，但由于随着可育期临近，配偶守卫行为并未出现增加，该预测同样被推翻。不过，配偶守卫行为与配对内交配频次呈显著正相关。 相较于其他猛禽（raptor），本研究中博内利雕的交配频次处于中等水平，且未表现出配偶守卫行为，这表明在该研究区域内，博内利雕仅通过部分适应性行为来保障父权。尽管雌性可育期内配对内交配频次的逐步上升符合精子竞争假说（sperm competition hypothesis），但观测到的配对外交配（extra-pair encounters）次数较少，即配对外交配机会稀缺，这或许可以解释上述现象。本研究还基于信号假说（signalling hypothesis）对结果展开讨论：该假说认为，猛禽会通过在受保护的巢区频繁且醒目地进行交配，向同种个体乃至其他猛禽物种传递领地所有权信号。