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渡鸦的食腐行为促进狼的群体觅食(2003)
JOHN A. VUCETICH*, ROLF O. PETERSON* & THOMAS A. WAITE†
* School of Forest Resources and Environmental Science, Michigan Technological University
† Department of Evolution, Ecology, and Organismal Biology, Ohio State Univeristy
(Received 19 November 2002; initial acceptance 18 February 2003;
final acceptance 13 June 2003; MS. number: A9487)
大部分食肉动物(85-90%)都是单独行动的(Gittleman 1989)。民间传说中的它们也通常是独行者,因此,研究少数食肉动物为什么会群居、为什么存在社会行为就成了一个持久的课题。众所周知,许多社会性动物生活在一个所谓的觅食经济型的群体中,在群体里个体能够获得更高的觅食收益(reviewed by Giraldeau & Caraco 2000)。普遍认为,社会性(尤其是大型食肉动物的社会性)有助于个体的觅食(e.g. Schaller 1972; Kruuk 1975; Nudds 1978; Gittleman 1989; Fuller & Kat 1990; Packer et al. 1990; Fanshawe & Fitzgibbons 1993; Stander & Albon 1993; Thurber & Peterson 1993; Caro 1994; Creel & Creel 1995; Schmidt & Mech 1997; Hayes et al. 2000)。可对这个假说的验证却导致了相当多的混乱和争论。
觅食经济假说认为,个体的平均收益随着群体规模的增加而增加。但由于个体之间的干扰也会随着规模的增加而增加,因此收益率会达到一个峰值(此时群体规模为G*),随后收益变会随着群体规模的增加而减少。最终甚至会低于个体单独觅食的收益率(此时群体规模为Gˆ)。对于这样的聚群而言,集体觅食显然是存在好处的,但预期的规模上限是多大呢?从表面上看,当群体规模G在峰值G*附近时,每个成员的收益都可以最大化。然而,迄今为止的大多数研究表明,社会性食肉动物往往在规模超过G*的群体中生活(Fuller & Kat 1990; Packer et al. 1990; Fanshawe & Fitzgibbons 1993; Thurber & Peterson 1993; Caro 1994; Schmidt & Mech 1997; Hayes et al. 2000; reviewed by Giraldeau & Caraco 2000; for an exception see Baird & Dill 1996)。这些发现使得研究者们怀疑觅食经济假说可能无法解释大型食肉动物社会性的演化(Packer et al. 1990; Caro 1994; Packer & Caro 1997; but see Creel 1997)。
但是,这些发现也不一定和觅食经济理论相矛盾(reviewed by Giraldeau & Caraco 2000; see also Waite & Field, in press)。实际上,群体的规模很不稳定,因为任何单独的个体都有加入群体的强烈动机。即使一个群体已经达到G*了,它依然会不断地收纳新的成员,除非当规模超过了Gˆ,新的个体才会停止加入,因为加入规模超过Gˆ的群体并不会为自己带来好处。因此,一个平衡的群体可能像G*一样小,或者像Gˆ一样大(见下文),这取决于新个体加入群体的难易和个体之间亲缘关系的程度。
尽管不考虑大于G*的群体规模是否和假说相互矛盾,大型食肉动物群体中的觅食收益如何维持依然存在争议(e.g. Packer et al. 1990; Caro 1994; Creel 1997)。在这里,我们想解释的并不是狼Canis lupus的群体规模为什么会超过G*,而是为什么狼群规模经常超过Gˆ (Thurber & Peterson 1993; Schmidt & Mech 1997; Hayes et al. 2000)。首先,我们建立了一个传统的群体觅食模型(包括领地防御、净摄入率最大化、能量不足的风险最小化和亲属选择),预期的Gˆ远小于真实的狼群规模。然后,我们在模型中纳入了一个先前被忽略、但对狼的觅食生态有显著影响的因素,即渡鸦Corvus corax食腐所造成的损失。如此一来,预期的Gˆ增加了,而真实的狼群规模在它之下。因此,我们认为觅食经济假说可以解释狼群的规模为何如此之大。
狼和渡鸦的相互影响 Behavioural Interactions Between Wolves and Ravens
渡鸦是一种遍布北美地区的分解者,常见于狼杀死的猎物周围(e.g. Crisler 1956; Mech 1966, 1970; Bruggers 1988; Heinrich 1989; Paquet 1991; Promberger 1992; Carbyn et al. 1993; Mech et al. 1998; Hayes et al. 2000; Drummer et al. 2002; Stahler et al. 2002; see also Bjarvall & Isakson 1982)。狼的食物边常有6至25只渡鸦(e.g. Promberger 1992; Heinrich 1999; Drummer et al. 2002; Stahler et al. 2002),有时达50 (Promberger 1992)至80 (Carbyn et al. 1993)只,甚至是上百(Drummer et al. 2002)。在罗亚尔岛(Isle Royale),过去32年间几乎每一只被狼杀死的驼鹿尸体边都会聚集5-15只渡鸦。
但渡鸦对狼吃剩下的猎物却没有兴趣(Mech 1966, 1970; Peterson 1977; Harrington 1978; Allen 1979; Bjarvall & Isakson 1982; Carbyn et al. 1993)。在一些狼和渡鸦共存的地区,渡鸦常和狼同时出现,而很少单独出现在猎物尸体旁(e.g. Stahler et al. 2002)。这种密切的联系可能是渡鸦的一种觅食策略,因为在冬天它们很难找到腐肉。
除了简单的行为联系外,渡鸦也会吃掉相当一部分狼的猎物。一个渡鸦每天可以从大型猎物尸体上带走0.5-2 kg肉(Heinrich & Pepper 1999; see also Magoun 1976)。按平均计算,每只狼每天估计都要消耗2-4 kg的食物用于打发渡鸦(Hayes et al. 2000)。在一个案例中,渡鸦可以吃掉一只300-kg驼鹿尸体的一半肉量(Hayes et al. 2000)。而在另一项研究中,渡鸦群每天都要从狼嘴下夺走37 kg的食物(Promberger 1992)。这些研究为我们提供了数据基础,即狼群每天都有2-20 kg食物喂了渡鸦。
在分析觅食经济假说的时候,我们将这些观察到的狼群规模的估算值作为群体规模的数值。这是合理的,因为在罗亚尔岛(以及其他地方),狼群总是共同参与狩猎,所有的成员都可以享用捕猎的成果,而非成员则会被排除在外(e.g. Mech 1966; Peterson 1977; Peterson et al. 1998; see also Mech 1970; Carbyn et al. 1993; Mech et al. 1998; Hayes et al. 2000; Drummer et al. 2002)。因此,觅食群的大小就等于狼群的大小。因此,我们的分析不仅仅可以说明觅食经济假说是否有利于狼群的觅食,还能说明其是否有利于狼的社会。
我们通过样线航测计算每个冬季、每个狼群的猎杀率(每个冬季平均走44条样线)。沿着样线观察,我们可以发现狼杀死驼鹿时留下的痕迹,错过现场的几率是很低的,因为:(1) 飞行高度足够看到尸体的残留物(例如毛皮、胃容物、血液和骨头),并且这些残留物会存留几天时间;(2) 狼群经常回访旧的猎物尸体。大雪偶尔阻碍我们的观察,这些都不包括在我们的计算中。类似的方法也被用在其他有关猎杀率的研究中(e.g. Mech 1966; Peterson 1977; Peterson et al. 1984; Ballard et al. 1987; Thurber & Peterson 1993; Dale et al. 1995; Schmidt & Mech 1997; Hayes et al. 2000)。
我们估算狼的每日BMR是3724 kJ。这个估算基于哺乳动物特异性的异速生长关系(Kleiber 1947),并假设罗亚尔岛的狼平均体重为31.5 kg (Peterson 1977)。对放养犬科动物的观察研究表明,步行代谢率是BMR的三到八倍(Gorman et al. 1998; R. Peterson, unpublished data),追逐代谢率则可高达BMR的二十五倍(Gorman et al. 1998),其他活动的代谢率则是BMR的1.5到3.5倍(Gorman et al. 1998)。通过观察发现,罗亚尔岛的狼追逐猎物的时间通常从1到10分钟(Peterson 1977)。根据这些观察和估算数据,我们对DEE进行了敏感性分析。
为直接比较DEE和猎物总捕获率,我们将DEE的单位从BMR的单位转换成猎物公斤数(即6862 kJ per kg)(Creel 1997)。为获得群体规模和净摄入率之间的关系,我们用每个群体的平均总捕获率(图1)减去每个群体规模的DEE(公式1)。当只考虑以上因素时,能够让个体觅食率最大化的群体规模为一对狼(G* = 2,图3),当规模超过3时,个体觅食率就会比独狼的觅食率还低(Gˆ = 2),因此对狼而言,稳定的群体规模数是2。但是这无法解释为什么我们观察到的狼群通常都超过了三只。
无论群体规模如何,能量短缺的风险都很高(图4)。除非群体成员之间的食物竞争就是单纯的争夺而非竞赛性质(sensu Nicholson 1955; see also Hassell 1975),那么实际的风险可能比我们预测的要少。而且能量短缺的高风险和罗亚尔岛狼群的高死亡率相一致。1971至2001年,平均死亡率为0.22 (Peterson et al. 1998, unpublished data)。
我们的分析表明,群体规模的增大增加了捕猎频率(Fig. 2b; see also Thurber & Peterson 1993; Hayes et al. 2000)、并减少了渡鸦夺走的分量,因此可以抵消群体增大造成的狼均觅食率降低。进一步扩展,如果渡鸦造成的损失也会随着捕食量的增加而增加,捕食大型猎物(如驼鹿)的狼就会更倾向于形成大群(图6a),而捕食小型猎物(如白尾鹿Odocoileus virginianus,重88 kg)的狼会更倾向于形成小群(图6b)。已有野外调查发现了猎物大小和狼群规模存在类似的正相关关系(Nudds 1978; C. Schiffer, T. A. Waite & J. A. Vucetich unpublished data)。
目前有关偷窃寄生现象的研究主要集中在评估这种行为(e.g. Tuckwell & Nol 1997; Morissette & Himmelman 2000)对寄主物种的负面影响(e.g. Finney et al.2001)、如何降低这种影响(e.g. Lahti et al. 1998; Goss-Custard et al. 1999; Stienen & Brenninkmeijer 1999)、以及寄生物种的适合度(e.g. Whitehouse 1997; Broom & Ruxton 1998; Goss-Custard et al. 1998; Ruxton & Broom 1999)。大多数有关偷窃寄生现象的研究都集中在鸟类和蜘蛛身上(Higgins & Buskirk 1998; Stienen & Brenninkmeijer 1999)。我们的研究丰富了人们对偷窃寄生现象的认识,这是一种像社交行为一样显眼的行为。
狼和渡鸦的互动可能也可以解释其他掠食者和分解者之间的互动。举个例子,非洲的群居食肉动物可能也会被秃鹫(在非洲占主导地位的分解者)夺走大量的食物,因此那些食肉动物从群体中获益的机制可能和这里的狼相似。但是,由于秃鹫比渡鸦更大、机动性也更弱,因此驱逐秃鹫可能比驱逐渡鸦要容易得多。使用和我们相似的建模方法,Carbone et al. (1997)发现群体增大带来的成本增加超过了减少偷窃寄生所带来的好处。但是他们的结论可能并不准确,因为他们没有考虑到群体规模的增大也会导致狩猎频率和成功次数的增加(Creel & Creel 1995)。无论如何,对非洲野犬Lycaon pictus而言,偷窃寄生现象造成的能量损失也相当高,并且成为了非洲野犬的一个重要选择压力(Gorman et al. 1998)。
如果分解者的存在可以构成大型食肉动物的一种选择压力,那么我们就可以更加了解食肉动物在社会性和独居性上的演化机制。捕食小型食草动物的小型食肉动物常在环境中聚集分布,对这些物种而言,群体的力量可以增强觅食能力,并且保护自己免遭敌害(Clutton-Block et al. 1999)。而独居的中小型食肉动物捕食和自己体型差不多的猎物(例如貂),它们的猎物太小了还不足以吸引食腐者。一些大型食肉动物通常吃大型猎物,因此容易通过群居把食腐者造成的损失降到最低(例如非洲野犬、狮子Panthero leo、鬣狗Crocuta spp.、和猎鹿的郊狼Canis latrans)。其他独居的大型食肉动物可能依靠捕食小型猎物来规避食腐者(如鬃狼Chrysocyon brachyurus、狩猎小型猎物的郊狼和狼)(Peterson et al. 1984; Schaller 2000),或者通过其他方式躲避食腐者(如独居的猫科动物会将食物藏在茂密的丛林下、藏在树上、或埋在土里)。
总之,渡鸦的存在有利于维持狼群的社会稳定。狼和其他大型食肉动物的社会是非常复杂的,可能包括亲属选择(Schmidt & Mech 1997)、和/或抵御偷窃寄生现象(Caraco & Wolf 1975; Cooper 1991; Fanshawe & Fitzgibbons 1993)。尽管如此,觅食经济假说似乎也可以解释狼和其他大型食肉动物的社会性,特别是当猎物很大、并很容易被清道夫夺走的情况下。
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