Davies W James, Saccheri Ilik J
Institute of Integrative Biology, University of Liverpool , Liverpool , UK.
PeerJ. 2015 Jan 6;3:e707. doi: 10.7717/peerj.707. eCollection 2015.
Protandry (prior emergence of males) in insect populations is usually considered to be the result of natural selection acting directly on eclosion timing. When females are monandrous (mate once), males in high density populations benefit from early emergence in the intense scramble competition for mates. In low density populations, however, scramble competition is reduced or absent, and theoretical models predict that protandry will be less favoured. This raises the question of how males behave in heterogeneous landscapes characterized by high density core populations in a low density continuum. We hypothesized that disadvantaged late emerging males in a core population would disperse to the continuum to find mates. We tested this idea using the protandrous, monandrous, pierid butterfly Anthocharis cardamines (the orange-tip) in a core population in Cheshire, northwest England. Over a six-year period, predicted male fitness (the number of matings a male can expect during his residence time, determined by the daily ratio of virgin females to competing males) consistently declined to <1 in late season. This decline affected a large proportion (∼44%) of males in the population and was strongly associated with decreased male recapture-rates, which we attribute to dispersal to the surrounding continuum. In contrast, reanalysis of mark-release-recapture data from an isolated population in Durham, northeast England, showed that in the absence of a continuum very few males (∼3%) emerged when fitness declined to <1 in late season. Hence the existence of a low density continuum may lead to the evolution of plastic dispersal behaviour in high density core populations, maintaining late emerging males which would otherwise be eliminated by selection. This has important theoretical consequences, since a truncated male emergence curve is a key prediction in game theoretic models of emergence timing which has so far received limited support. Our results have implications for conservation, since plastic dispersal behaviour in response to imperfect emergence timing in core (source) populations could help to maintain sink populations in heterogeneous landscapes which would otherwise be driven to extinction by low mate encounter-rates (Allee effects).
昆虫种群中的雄性先熟（雄性先羽化）通常被认为是自然选择直接作用于羽化时间的结果。当雌性为单配偶制（只交配一次）时，高密度种群中的雄性会从早羽化中受益，以便在激烈的配偶争夺竞争中占据优势。然而，在低密度种群中，争夺竞争减弱或不存在，理论模型预测雄性先熟将不那么受青睐。这就引出了一个问题：在以低密度连续区域中的高密度核心种群为特征的异质景观中，雄性的行为会怎样？我们假设核心种群中处于劣势的晚羽化雄性会扩散到连续区域去寻找配偶。我们在英格兰西北部柴郡的一个核心种群中，以雄性先熟、单配偶制的粉蝶Anthocharis cardamines（橙尖粉蝶）对这一想法进行了测试。在六年的时间里，预测的雄性适合度（雄性在其停留时间内预期的交配次数，由未交配雌性与竞争雄性的每日比例决定）在季节后期持续下降至<1。这种下降影响了种群中很大一部分（约44%）的雄性，并且与雄性再捕获率的下降密切相关，我们将其归因于向周围连续区域的扩散。相比之下，对英格兰东北部达勒姆一个孤立种群的标记重捕数据的重新分析表明，在没有连续区域的情况下，当季节后期适合度下降至<1时，很少有雄性（约3%）羽化。因此，低密度连续区域的存在可能导致高密度核心种群中可塑性扩散行为的进化，维持那些否则会被选择淘汰的晚羽化雄性。这具有重要的理论意义，因为雄性羽化曲线的截断是羽化时间博弈论模型的一个关键预测，到目前为止该预测得到的支持有限。我们的结果对保护工作具有启示意义，因为核心（源）种群中因羽化时间不完善而产生的可塑性扩散行为，有助于在异质景观中维持汇种群，否则这些汇种群会因低配偶相遇率（阿利效应）而走向灭绝。
