Field estimates of survival do not reflect ratings of mimetic similarity in wasp-mimicking hover flies

Abstract

The evolution of mimicry, and particularly the persistence of undefended Batesian mimetic forms that are imperfect copies of their defended models, remains a central question in evolutionary biology. Previous work has demonstrated that variation in mimetic fidelity in artificial prey can alter survival. However, no studies have validated the assumption that detailed laboratory-based measurements of mimetic fidelity are actually reflected in survival in natural field experiments. Here, we demonstrate that, in line with previous studies, the mimetic similarity of 77 hover fly (Diptera: Syrphidae) species to the common wasp Vespula alascensis is strongly related to the number of abdominal stripes exhibited by the flies. We then produce three artificial pastry baits: (i) a "model" which is chemically defended and has two stripes, (ii) a one-stripe mimic, and (iii) an unstriped mimic. Based on the ratings study, we predicted that the one-stripe mimic would exhibit survival intermediate between the unstriped mimic and the model. Baits were deployed in experiments each involving 81 baits (27 of each kind), at 3 sites, with experiments replicated 10 times at each site for a total deployment of 2430 baits. Proportional hazards models show that both one-striped and model baits survived equally well and significantly better than the unstriped baits, suggesting categorical prey identification rather than the use of stripe number as a continuous trait, as was suggested by the laboratory study. These findings suggests that, while humans and avian predators can distinguish mimics and models in the laboratory using a range of traits, behaviour in the field may not reflect this ability. This absence of a link between continuous measures of mimetic fidelity and prey selection may contribute to the maintenance of imperfect mimicry, but more studies using near-natural experimental paradigms are needed to investigate the phenomenon further.

Gradual and contingent evolutionary emergence of leaf mimicry in butterfly wing patterns

“…One plausible explanation for survival of poor mimics is found in the concept of imperfect mimicry [61], which maintains that the survival of poor mimics (e.g. hoverflies that are poor wasp mimics [62,63]) is often explained by a trade-off in predator foraging behaviours (e.g. a trade-off between the speed and accuracy of decision-making [64]).…”

Which traits do observers use to distinguish Batesian mimics from their models?

“…It is quite possible that a single mimic species would have reliably distinguishable colors from a single model species; but as more species are encountered, it becomes impossible to adopt a simple rule that separates one taxon from the other. Mimicry that appears "inaccurate" based on a single comparison may in fact be sufficient to provide protection from predation in a scenario where a number of model species are encountered (Ihalainen et al 2012;Easley and Hassall 2014), as in this study and likely faced by many wild predators. It is interesting to note that participants did attack larger insects less frequently during the Size treatment, despite gaining no resulting advantage.…”

“…There is an increasing appreciation that, to understand mimicry, we must take into account the complexity of the prey community (Easley and Hassall 2014) and the limited knowledge of the predator . By comparing results of simple experiments with others that confront predators with more complex prey communities and realistic situations, as done here and in other studies (Ihalainen et al 2012;Easley and Hassall 2014), we can establish the circumstances under which selection will and will not act on inaccurate mimetic signals.…”

Morphological, colour and behavioural mimicry of cuckoo bees by the hoverfly Eumerus tricolor (Fabricius) (Diptera: Syrphidae)

“…This is not a result of the real situation in natural biotopes, but more the fact that they have not been studied very much. Accordingly, we know quite a bit about the mimics of hornets, wasps, bumblebees, honeybees and ants, but very little about those of other groups of aculeates (Maier 1978, Howarth et al 2000, Penney et al 2012, Easley & Hassall 2014. Only one comprehensive study has been done, but this was based only on general appearance (Howarth et al 2000).…”

“…Similarity in morphology and colouration between two unrelated insects represents a very common natural phenomenon, usually being explained as some form of mimicry. Although mimicry was first described in butterflies (Kirby & Spence 1817), which have been intensively studied, more recent work shows that it is more common in dipterans and hymenopterans (Maier 1978, Howarth et al 2000, 2004, Easley & Hassall 2014, and especially in the hoverflies (Syrphidae), a well-known group of Diptera resembling many species of bees, wasps and related aculeate Hymenoptera (Howarth et al 2000, 2004, Rashed & Sherratt 2007, Easley & Hassall 2014. Within this family there are well-known mimics of bumblebees (Volucella bombylans (Linnaeus), see Rupp 1989, Edmunds & Reader 2014, social wasps and hornets (V. inanis (Linnaeus), V. zonaria (Poda), species of Eupeodes Osten Sacken, Helophilus Meigen and other genera: see Howarth et al 2000) and bees (especially members of the genus Eristalis Latreille: see Golding & Edmunds 2000, Golding et al 2001, as well as other species with poorly studied mimicry (Howarth et al 2000).…”