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

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.

To evolve, and remain adaptive, collective behaviours must have a positive impact on overall individual fitness. However, these adaptive benefits may not be immediately apparent owing to an array of interactions with other ecological traits, which can depend on a lineage's evolutionary past and the mechanisms controlling group behaviour. A coherent understanding of how these behaviours evolve, are exhibited, and are coordinated across individuals, therefore requires an integrative approach spanning traditional disciplines in behavioural biology. Here, we argue that lepidopteran larvae are well placed to serve as study systems for investigating the integrative biology of collective behaviour. Lepidopteran larvae display a striking diversity in social behaviour, which illustrates critical interactions between ecological, morphological and behavioural traits. While previous, often classic, work has provided an understanding of how and why collective behaviours evolve in Lepidoptera, much less is known about the developmental and mechanistic basis of these traits. Recent advances in the quantification of behaviour, and the availability of genomic resources and manipulative tools, allied with the exploitation of the behavioural diversity of tractable lepidopteran clades, will change this. In doing so, we will be able to address previously intractable questions that can reveal the interplay between levels of biological variation. This article is part of a discussion meeting issue ‘Collective behaviour through time’.

Highlights d Birds use flowers as cues to avoid foraging from plants bearing toxic prey d The plant's flowers act as an extended phenotype advertising unpalatability d Non-warningly colored prey on the same plants also benefit d This could help explain how toxicity evolves in non-warningly colored animals

Insect herbivores, such as lepidopteran larvae, often have close evolutionary relationships with their host plants, with which they may be locked in an evolutionary arms race. Larval grouping behaviour may be one behavioural adaptation that improves host plant feeding, but aggregation also comes with costs, such as higher competition and limited resource access. Here, we use the Heliconiini butterfly tribe to explore the impact of host plant traits on the evolution of larval gregariousness. Heliconiini almost exclusively utilise species from the Passifloraceae as larval host plants. Passifloraceae display incredible diversity of form, leaf shape and a range of anti-herbivore defences, suggesting they are locked in an arms race with Heliconiini larvae. By analysing larval social behaviour as both a binary (solitary or gregarious) and categorical (increasing larval group size) trait, we revisit the multiple origins of larval gregariousness across Heliconiini. We investigate whether host habitat, leaf defences and leaf size are important drivers of, or constraints on, larval gregariousness. Whereas our data do not reveal links between larval gregariousness and the host plant traits included in this study, we do find an interaction between larval host specialisation and behaviour, revealing gregarious larvae to be more likely to feed on a narrower range of host plant species than solitary larvae. We also find evidence that this increased specialisation typically precedes the evolutionary transition to gregarious behaviour. The comparatively greater host specialisation of gregarious larvae suggests that there are specific morphological and/or ecological features of their host plants that favour this behaviour.