“性侵犯”是一种通过暴力强迫异性和自己交配的行为,通常情况下雄性动物是施暴者 (Smuts and Smuts 1993)。脊椎动物广泛存在这种行为 (e.g. Smuts and Smuts 1993; Clutton-Brock and Parker 1995),但此行为的意义还无人知晓。很多鳍足类动物都具有明显的两性差异 (King 1983),再加上他们通常都是一夫多妻且聚群繁殖,因此雄性之间的竞争压力相当大 (McCann 1980)。性侵犯通常都发生在雌雄差异较大的动物身上,对雄性而言这是一种高效率的繁殖策略 (Clutton-Brock and Parker 1995)。“生殖干扰”的定义则是一种对交配双方有害(或至少对其中一方有害)的种间交互作用,通常发生在外观和行为十分相似难以辨别的两个物种身上 (Gro¨ning and Hochkirch 2008)。类似的事例在很多动物类群里都有发现 (Gro¨ning and Hochkirch 2008)。鳍足类动物中也会出现这类现象,跨种杂交 (e.g. Wynen et al. 2000) 或是跨属杂交 (e.g. Kovacs et al. 1997) 都曾有过报导。而过去,有关鳍足类动物对其他动物种群的性侵犯行为的报导则只出现过一例——De Bruyn et al. (2008) 曾经报道过一只年轻的成年雄性南极海狗 Arctocephalus gazella 对成年王企鹅 Aptenodytes patagonicus 的性侵犯行为。在本篇文章中,我们将描述三起新的性侵犯行为,并尝试解释这些行为的意义,此些行为都发生在亚南极岛屿马里恩岛(46°52′S,37°51′E)上。
三个新实例和过去案例 (De Bruyn et al. 2008)的比较都在表1中称述。所有的四个案例中,侵犯者都是年轻的成年雄性南极海狗(AFSs),而受害者均为健康的成年王企鹅(KPs),性别未知(表1)。所有的一切都是发生在海狗的繁殖期——即十一月末到一月初 (Kerley 1983)——且主要发生在马里恩岛的东侧和南侧(图2),这些区域也是海狗的种群密度最大的区域 (Hofmeyr et al. 2006)。Funk和Trypot海滩上的海狗密度较小(年均繁殖幼仔数<100),而Goodhope海湾则是刚成年或亚成雄性海狗的聚居地。Goodhope海滩和Funk海滩的王企鹅数量也很少,分别是<900和<150繁殖对 (Crawford et al. 2003),Trypot海滩则是小群旅鸟和留鸟的家园。
南极海狗偶尔会捕食王企鹅 (Makhado et al. 2007; Charbonnier et al. 2010),但人们只观察到马里恩岛的南极海狗会在陆地上猎杀王企鹅 (Hofmeyr and Bester 1993)。最近三十五年来马里恩岛上的野外研究人员越来越多 (Bester et al. 2011),但 De Bruyn et al. (2008) 观察到的海狗性侵犯企鹅的行为却是首例,显然这并不是一种常见的行为。在1986年,人们首次观察到了马里恩岛上南极海狗对王企鹅的捕食行为 (Hofmeyr and Bester 1993),但性侵犯行为则仅有2006年的一例 (De Bruyn et al. 2008)。此后六年间,我们又观察到了三次同样的性侵犯行为(表1)——这也是马里恩岛的南极海狗独有的行为。这四起案例中,施暴的雄性南极海狗都属同样的年龄阶层(刚成年)。但由于六年来只观察到这四起,因此这可能是一种非常罕见的偶发行为。雄性的南极海狗可以活15年 (de Magalhaes and Costa 2009),因此六年前和近来的施暴者并不是同一只海狗。我们试着比较了近年拍摄的三组照片,但是无法辨别他们是不是不同的个体。
要确定这些异常行为的动机简直是不可能的。不过我们可以提出一些疑问和假设,待后来的研究帮忙予以验证。De Bruyn et al. (2008) 认为性侵犯行为是海狗捕食行为的一部分,例如在追逐企鹅的过程中激起了海狗的性欲。但在2011年的案例中,海狗杀死并吃掉了被他侵犯的企鹅。其他的新案例倒是可以支持这种假设。在这里我们又提出了两个新的假设。我们认为,这可能是一种学习行为,或者是一个和生殖干扰有关的实例。
海狮科动物具有不错的学习能力 (Rawls et al. 1985; Shapiro et al. 2004; Schusterman 2008),他们对环境的适应力很强,并且在面临环境压力时其行为的可塑性也很高 (e.g. Staniland and Boyd 2003; Holcomb et al. 2009; Kovacs et al. 2012)。例如新西兰海狮能够很快地适应新的环境、捕食新的猎物 (Moore et al. 2008),圈养的海狗也能很快地适应以塘鹅作为食物 (Crawford and Cooper 1996),新西兰海狮会在黄眼企鹅的巢边埋伏 (Moore and Moffat 1992),南极海狗能在陆地上追逐捕食王企鹅 (Hofmeyr and Bester 1993) 等等。Kirkman (2009) 的综述很全面地阐述了南非的鳍足动物和海鸟的种间相互作用。
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