Aposematic uniformism in butterflies as adaptive to the mob-mentality of forest birds

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Is Muellerian mimicry (https://en.wikipedia.org/wiki/M%C3%BCllerian_mimicry) a misnomer and a non-concept in Lepidoptera?

Coexisting butterflies with warning colouration tend to resemble each other even where all the mimicking species are genuinely noxious to predators. These Muellerian mimicry rings – which have been regarded as a textbook example of deceptive colouration in animals – would seem to have been adequately explained for more than a century.

One example consists of the following coexisting butterflies in tropical Africa:

• Papilio cynorta (Papilionidae, https://www.inaturalist.org/taxa/359407-Papilio-cynorta),
• Elymnias bammakoo (Nymphalidae: Satyrinae, https://www.inaturalist.org/taxa/830770-Elymnias-bammakoo),
• Pseudacraea eurytus (Nymphalidae: Nymphalinae, https://www.inaturalist.org/taxa/348769-Pseudacraea-eurytus),
• Bematistes epaea (Nymphalidae: Acraeinae, https://www.inaturalist.org/taxa/607603-Acraea-epaea), and
• Acraea jodutta (Nymphalidae: Acraeinae, https://www.inaturalist.org/taxa/358150-Acraea-jodutta).

These species share a recognisable pattern on their wings despite being unrelated to each other phylogenetically.

Because recognition of aposematic patterns by birds is learned rather than instinctive, it is advantageous to both the avian predators and the vulnerable butterflies if the signal of one model species of butterfly is copied by several noxious species. Such mimicry enables each species to advertise its identity as a dangerous food item.

However, 'Muellerian mimicry’ may be too poorly named and explained to deserve its current status in biological textbooks.

Mimicry is deception by definition, yet in ‘Muellerian mimicry’ all the butterflies in the ring (particularly exemplified by Nymphalidae: Heliconiinae in Amazonia) display gaudy warnings of real toxicity. This means that the butterflies’ communication with their potential predators is not deceptive but honest.

A conundrum in the term mimicry in ‘Muellerian mimicry’ is that of identifying any one species as the ‘model’ in the evolution of these patterns. This requires an arbitrary view of the comparative efficiencies of different aposematic patterns, plus assumptions about which lineage had priority in the evolutionary chronology.

I suggest a better term, ‘aposematic uniformism’, which drops the concept of mimicry from both the name and the explanation.

Existing explanations of ‘Muellerian mimicry’ hardly account for the real aposematic uniformism.

Warning colouration by definition promotes identification, but aposematic uniformism blurs distinctions between species of butterflies and hence types of toxicity. This would compromise the communication between butterfly and predator, which boosts the risk of mistaken attacks on butterflies by birds.

Thinking laterally in this re-assessment leads me to the following novel explanation:
Aposematic uniformism among butterfly species is adaptive to predation by mixed-species foraging flocks of birds (https://www.allaboutbirds.org/news/like-chasing-tornadoes-the-fun-and-challenge-of-mixed-species-flocks/ and https://jlrexplore.com/explore/from-the-field/friendship-networks-mixed-hunting-flocks and https://en.wikipedia.org/wiki/Mixed-species_foraging_flock and https://avianres.biomedcentral.com/articles/10.1186/s40657-015-0023-0 and https://www.researchgate.net/publication/286831034_Mixed_flock_composition_and_foraging_behavior_of_insectivorous_birds_in_undisturbed_and_disturbed_fragments_of_high-Andean_Polylepis_woodland and https://www.frontiersin.org/articles/10.3389/fevo.2020.537816/full and https://www.sciencedirect.com/science/article/abs/pii/S0003347217303585 and https://twitter.com/amornith/status/1229797726323384326?lang=cs).

The crucial observation is that, worldwide, unrelated insect-eating bird species tend to forage collectively. This includes all the woodlands and forests in which ‘Muellerian mimicry’ of butterflies occurs. The aggregations of insect-eating birds pass like waves through the vegetation, flushing insects – both noxious and harmless – into flight.

Under such pressure, the butterflies run a gauntlet in which the birds in the 'mob' have minimal time to gauge whether any particular insect is harmful.

Some of the birds are likely to belong to species relatively tolerant of certain toxins, or to be individuals naive with regard to warning signals. Therefore, even the most noxious butterfly species – with conspicuous warning colouration – risks being the victim of an error that the bird, too, will regret.

Because the gauntlet is one of hasty decisions under pressure, both the prey and the aggregated predators would benefit if various species of noxious butterflies were to share a common pattern - as opposed to the pattern of one particular species being copied by mimicry.

Aposematic uniformism allows every member of the mixed-species foraging flock to desist from attacking any butterfly thus coloured, despite the hectic confusion of the encounter and the competitive urgency of the birds.

This new explanation is based on a trade-off of quality of communication for quantity of communication.

In summary:
The minimal opportunities for discrimination by members of mixed-species foraging flocks are hypothetically compensated by the immediate recognition of an aposematic uniform. This would apply even in the case of ignorant or naive individual birds, which can inform their decisions by the immediate reactions of others nearby.

Posted on June 13, 2022 10:19 PM by milewski