Aggregation, defence and warning signals: the evolutionary relationship

Abstract

In a seminal contribution, Fisher argued how distastefulness could incrementally evolve in a prey species that was distributed in family groups. Many defended prey species occur in aggregations, but did aggregation facilitate the evolution of defence as Fisher proposed or did the possession of a defence allow individuals to enjoy the benefits of group living? Contemporary theory suggests that it can work both ways: pre-existing defences can make the evolution of gregariousness easier, but gregariousness can also aid the evolution of defence and warning signals. Unfortunately, the key phylogenetic analyses to elucidate the ordering of events have been hampered by the relative rarity of gregarious species, which in itself indicates that aggregation is not a pre-requisite for defence. Like the underlying theory, experimental studies have not given a definitive answer to the relative timing of the evolution of defence and aggregation, except to demonstrate that both orderings are possible. Conspicuous signals are unlikely to have evolved in the absence of a defence and aggregated undefended prey are likely to be vulnerable to predation in the absence of satiation effects. It therefore seems most likely that defence generally preceded the evolution of both aggregation and signalling, but alternative routes may well be possible.

Anti-predator defence mechanisms in sawfly larvae of Arge (Hymenoptera, Argidae)

“…Bright colouration and gregariousness are generally linked traits in insects (Fisher, 1930;Guilford, 1990;Ruxton and Sherratt, 2006), which is observed as a trend in Arge species. Two of the three brightly coloured species were truly gregarious, whereas the cryptic species were generally aggregated or encountered in groups composed of a few individuals only.…”

Functional interactions among tortoise beetle larval defenses reveal trait suites and escalation

“…For example, theoretical predictions are conflicting about whether a caterpillar defense should precede, or follow the evolution of either a warning signal or gregariousness. Phylogenetic analyses using independent contrasts suggest that caterpillar defenses generally precede the evolution of aposematism and gregariousness (Tullberg and Hunter 1996; reviewed by Ruxton and Sherratt 2006). However, neither trait precedence, multi-trait interactions nor suite assembly have not been rigorously examined in other major herbivore lineages with externally feeding (i.e., ectophagous) immature stages, such as those of the Coleoptera (e.g., Chrysomelidae), epilachnine Coccinelidae, Heteroptera (Membracidae), or Orthoptera (Acrididae).…”

High metabolic and water-loss rates in caterpillar aggregations: evidence against the resource-conservation hypothesis

“…The reasons proposed for aggregation are wideranging, including a reduction in predation risk (e.g. Ruxton and Sherratt, 2006), increased sexual signalling or mating success (e.g. Sullivan, 1981), enhanced foraging success, increased growth rates (e.g.…”