Warning coloration, body size and the evolution of gregarious behavior in butterfly larvae

Many species gain anti-predator benefits by combining gregarious behavior
with warning coloration, yet there is debate over which trait evolves
first, and which is the secondary adaptive enhancement. Body size can also
influence how predators receive aposematic signals, and potentially
constrain the evolution of gregarious behavior. To our knowledge, the
causative links between the evolution of gregariousness, aposematism and
larger body sizes have not been fully resolved. Here, using the most
recently resolved butterfly phylogeny and an extensive new dataset of
larval traits, we reveal the evolutionary interactions between important
traits linked to larval gregariousness. We show that larval gregariousness
has arisen many times across the butterflies, and aposematism is a likely
prerequisite for gregariousness to evolve. We also find that body size may
be an important factor for determining the coloration of solitary, but not
gregarious larvae. Additionally, by exposing artificial ‘larvae’ to wild
avian predation, we show that undefended, cryptic ‘larvae’ are heavily
predated when aggregated but benefit from solitariness, whereas the
reverse is true for aposematic prey. Our data reinforce the importance of
aposematism for gregarious larval survival, whilst identifying new
questions about the roles of body size and toxicity in the evolution of
grouping behavior.

诸多物种通过将群聚行为与警戒色（aposematism）相结合，可获得反捕食益处，但学界对于二者中何者率先演化、何者为次生适应性增强性状仍存在争议。体型大小亦会影响捕食者接收警戒信号的方式，并可能对群聚行为的演化构成约束。据我们所知，群聚性、警戒色与体型增大三者演化间的因果关联尚未完全阐明。本研究借助最新解析的蝴蝶系统发育（phylogeny）与一套规模庞大的全新幼虫性状数据集，揭示了与幼虫群聚性相关的关键性状间的演化互作关系。研究表明，幼虫群聚性在蝶类中已多次独立起源，且警戒色很可能是群聚性演化的必要前提。我们还发现，体型大小可能是决定独居幼虫体色的重要因素，但对群聚幼虫并无此影响。此外，通过将人工“幼虫”暴露于野生鸟类捕食压力下的实验，我们证实：无防御能力的隐蔽型“幼虫”在群聚时会遭受严重捕食，独居则可使其获益；而具有警戒色的猎物则恰好相反。本研究数据进一步凸显了警戒色对群聚幼虫存活的重要性，同时也为体型大小与毒性在集群行为演化中的作用提出了新的研究问题。