Many animals use bright colours to warn a potential predator that they can defend themselves. Wasps, for instance, are armed with a harmful sting and advertise this fact via their distinctive yellow and black stripes. Predators often learn to heed such warnings and avoid these unpalatable animals in future. As a result, animals that mimic another animal’s warning signals can reap the benefit of being left alone by predators even if they are otherwise undefended.

Textbooks on evolution are typically full of different examples of mimicry. However, the specifics of these examples are often poorly understood. Ninety years ago a famous Australian entomologist, Alexander Nicholson, suggested the existence of large groups of mimics in the Australian wildlife. More of these so-called “mimetic complexes” have recently been recognized among several species of insect, but not previously in ants.

Now, Pekár et al. have looked at all known ants and ant-like mimics in Australia and discovered over 140 species that use gold and black colours as a warning signal. Most of the species were ants, but the collection of mimics also includes wasps, spiders, true bugs and insects called treehoppers. Some of the mimics were less palatable than others, and they possessed a range of defences, including spines and foul-tasting chemicals.

Pekár et al. then looked in the guts of 12 species of predators in the wild, and found that very few of them ate the mimics. When mimics were offered to three different predators (specifically a lizard and two species of spider), most avoided the mimics regardless of whether they were palatable or unpalatable. Instead, the predators preferred to eat a spider that was not a member of the group of mimics because it lacked the gold colouration.

Further studies are now needed to continue to document the details of this and other mimetic complexes. For example, this includes revealing how the different defences protect the members of the complex from predators do not use vision to recognize their prey and so cannot see the warning colouration. All this is needed to understand evolutionary processes that have fascinated biologists for decades, and explain how such large mimetic complexes evolved and persisted in spite of the influence of the community of predators.

Mimicry complexes (i.e. a composite of several Müllerian and Batesian mimics) have long fascinated biologists for the theoretical and empirical challenges they pose. Classic Müllerian mimicry was thought to only include equally defended species (Müller, 1878), while Batesian mimicry (Bates, 1862) describes undefended species resembling defended models. In reality, this simplistic dichotomy (sensu Speed, 1999) represents the end-points of a mimetic spectrum that contains species that vary in their mimetic fidelity and/or their degree of defensiveness. A topic of considerable debate (Sherratt, 2008) is the putative influence that moderately defended mimics have on the stability of mimicry rings (i.e. a composite of several similar Müllerian mimics). Discrepancy in protection between two mimetic species may dilute the protection of a more defended species (Speed, 1999). Great tits (Parus major, Paridae, Passeriformes) for instance, attacked artificial models more often when less defended mimics were frequently encountered and food was restricted (Rowland et al., 2010). In direct contrast, associative learning mechanisms predict a mutualistic relationship between moderately and highly defended members of the ring (Rowland et al., 2007): wild caught great tits equally avoided four species of true bugs with varying levels of defence but which shared similar warning colours (Hodová Svádová et al., 2013).

Müllerian mimicry rings typically consist of a few closely related species (Symula et al., 2001; Williams, 2007; Alexandrou et al., 2011), with the exceptions of the species-rich mutillid wasp (Wilson et al., 2012, 2015), Heliconius butterfly (Mallet and Gilbert, 1995) and bumblebee (Williams, 2007) complexes. These complexes, despite being speciose, are taxonomically limited because their members often belong to only a single family or genus (Lepidoptera or Hymenoptera). The mutillid wasp complex is suspected of including species from other orders but this has not been formally presented (Wilson et al., 2012, 2015). A mathematical model suggests that mimicry complexes should be common in multispecies communities with distantly related taxa (Beatty et al., 2004). The general lack of data on multi-order mimicry complexes may represent an observer bias. For example, ants have been largely overlooked as Müllerian mimics, yet they are diverse, typically well defended through a range of defensive measures including a sting, noxious chemicals, spines, a thickened cuticle, mandibles and communal attack (Hölldobler and Wilson, 1990). There is also a rich diversity of invertebrate taxa that mimic ants.

Species within Müllerian rings thus far described use the same defensive traits. Heliconius butterflies, for instance, rely on their distastefulness while mutillid wasps have a venomous sting. The degree of distastefulness or level of venom toxicity within a mimicry ring is expected to vary among members. Experimental investigations, that started a hundred years ago (e.g. McAtee, 1912, 1932), have tended to focus on a single visually-oriented predatory guild and yet most environments contain a wide range of potential predators of mimics. For example, species that mimic ants may encounter several ant-averse and ant-eating predators (Pekár et al., 2011). In some microhabitats, such as leaf litter, the ant-eating predators (which often use chemical cues to recognise prey) and ant-adverse nonvisually-orienting predators are indeed more common than visually-orienting ones, thus exerting strong selection pressure on mimics to signal their unpalatability in sensory modalities other than visual. Thus one of the major outstanding questions regarding the contribution of mimics to the function of mimetic rings is evaluating the success of Müllerian and/or Batesian mimicry by analysing the selection pressure exerted by all predators in a given environment (Blumstein, 2006).

Here, we describe a novel diverse and species rich mimicry complex consisting of golden ant, wasp, true bug, treehopper, and spider mimics and we investigate hypotheses concerning their unpalatability and resulting efficacy against predation. Mimetic complexes are often composed of rings – groups of species that draw advantages from similar aposematic signals (Williams, 2007; Wilson et al., 2015). Thus, our first step was the identification of separate rings to understand how the mimicry complex evolved and is reinforced. We therefore tested whether the rings have a sympatric distribution, which would indicate a mutualistic relationship between mimics and whether mimics within a ring share common ancestry.

Our second step was to quantify the relationship between the proposed aposematic signal (golden/black colouration) and the level of noxiousness or distastefulness of the mimics that display the signal. Aposematic warning signals and distastefulness are expected to evolve together within a species (e.g. Ruxton et al., 2004) and thus, the strength of the distasteful stimulus should coincide with the conspicuousness of the warning signal (Speed and Ruxton, 2007). In this mimicry complex, we investigated whether there is a positive relationship between signal intensity and unpalatability across species as has recently been found among lady-bird beetles (María Arenas et al., 2015). This would mean that truly Müllerian mimics are expected to honestly signal their distastefulness but truly Batesian mimics display a conspicuous warning signal without the noxious stimulus. We tested these assumptions by correlating the size of the warning signal (golden/back colouration) with a measure of distastefulness derived from the sum of defences found in the mimics.

Our critical third step was to extend the scope of previous studies of mimicry complexes by experimentally testing the efficacy of the aposematic signal, and indeed the validity of the Müllerian mimicry claim, through staged predator-prey trials. Based on the broad diversity of defences used by mimics in this complex, we predict that predators should learn quickly to avoid mimics (Beatty et al., 2004), and that mimics should be protected from a range of predators.

Finally, we examined how the aposematic signal and the level of defensiveness interplay with a community of predators that rely on multiple sensory modalities. Mimicry complexes are typically characterised by a distinct colour pattern and hence experiments are often biased towards visual-oriented predators and usually vertebrates. In reality, the members of a complex are exposed to predators that use a variety of sensory modalities and foraging modes to detect and subdue their prey. We combined staged predatory trials with a dietary analysis of wild populations of a community of predators to test the hypothesis that more unpalatable species are attacked less frequently by a community of predators.

Ants have not been considered in Müllerian mimetic rings, which is perhaps surprising given their diversity and the number of known ant-mimics. The lack of ant fauna in mimetic rings possibly reflects an overall lack of conspicuous defensive coloration in ants. However, our study supports a species rich mimicry complex of golden coloured ant models and mimics, adding to the small number of documented large-scale complexes. Mimetic complexes (among several families of beetles) were first recognised by Nicholson (1927) for Australian insect fauna. Such complexes are presumably far more widespread than appreciated, but have only been subject to more intensive study relatively recently. Many more large-scale mimetic complexes among ants and other animals (particularly arthropods) are likely to be described in future.

The species richness of this golden complex exceeds those of fish or millipedes (Marek and Bond, 2009; Alexandrou et al., 2011) but is currently smaller than in mutillid wasp (Wilson et al., 2012, 2015), heliconid butterfly (Mallet and Gilbert, 1995) and bumblebee (Williams, 2007) complexes. The key members in the golden complex are ants but the complex is taxonomically broad and includes species with distant phylogenetic origins. Beside the species mentioned, it may include a few other groups, such as bees, flies, and cerambycid beetles, in which some species also have a golden colouration. This is in contrast to complexes of similar scale that only include members from a single order.

The distribution of the majority of the ten putative mimetic rings in the complex overlapped with at least one other ring. Although the cluster analysis distinguished rings, the distances between the rings were not large suggesting that the entire complex is just a single ring. Our findings are not unlike those from other complexes that include rings of different colour pattern, which overlap in geographic distribution (Williams, 2007; Wilson et al., 2012, 2015). An overlap of golden rings or actually their non-existence potentially suggests that the variable golden colour patterns produce similar aposematic signals.

Large mimetic complexes include taxa of close and distant phylogenetic origins (e.g., Linsley et al., 1961; Rodriguez et al., 2014) in which the colour pattern is not only a product of common ancestry but also of convergence. A shared ancestry is apparent for two species of Myrmarachne spiders (Pekár et al., 2017), three species of Daerlac bugs (Casis and Symonds, 2012), and several species of Polyrhachis ants (Robson et al., 2015). The majority though, including Camponotus, Diacamma, Pachycondyla, and Iridomyrmex ants, other spider species, Eurymela treehopers, and Ephutomorpha wasps, have converged on the same phenotype as evidenced by their isolation within lineages of non-mimetic colour patterns. The diversity and geographic pattern of the golden phenotype is likely a result of co-divergence as found in other complexes (e.g., Rodriguez et al., 2014).

Species in Müllerian rings described to date, e.g. heliconid butterflies, use a single defensive trait to teach predators that they are distasteful (e.g., Van Zandt Brower, 1958). The lesser-defended individuals in this context are less distasteful. Species in our golden complex use multiple defensive traits including a variety of structural and chemical defences. To understand how these traits interplay with the aposematic signal we created an index that summed all traits known to serve a defensive function. We acknowledge this approach is somewhat simplistic as various traits are differentially effective against various predators. For example, a thick cuticle is effective against arthropod predators but ineffective against vertebrates (Evans and Sanson, 2005). On the other hand, spines are effective against vertebrates but ineffective in the defence against arthropod predators (Mikolajewski et al., 2006). Different species of closely related predators may also use a variety of adaptations to deal with the defences of prey. For example, some spiders are able to catch ants even with a thick cuticle while others are not (Pekár and Toft, 2015). A true index of unpalatability requires an estimation of the total effectiveness of a combination of defensive structures, such as spines and thick cuticle and the toxicity of chemical compounds for each predator-prey combination. Estimates including spines and cuticle thickness may be relatively straight forward but information on the toxicity of chemical compounds is largely not available (see http://toxnet.nlm.nih.gov). Although our approach is simplistic, the positive relationship between the index of unpalatability and the response of skinks strongly suggests that the measure is a good proxy for unpalatability.

Some of the defensive traits were strongly correlated with body size. Body size alone is an anatomical constraint that may prevent predation, thus it is difficult to separate the effect of pure body size from the relative size of some defensive traits (e.g., spines). A strong correlation between body size and the size of defensive traits is natural, because larger spines, for example, require a larger body to support them. It is, however, the absolute size of defences not their relative size that is ecologically important (e.g., a skink will swallow larger spines with greater difficulty than smaller ones). Whatever their single effect is, all these traits together, add to an overall unpalatability of mimics.

Our study describes a new complex with a golden/black colour combination. This combination is somewhat similar to the classic aposematic yellow/black phenotype (Ruxton et al., 2004; Williams, 2007). Accordingly, our findings support the idea the coverage of golden colour acts as an honest signal of aposematism. All the predators in staged trials exhibited an aversion to the prey with the greatest coverage of golden colour. Furthermore, golden individuals were rarely found in the diet of natural populations of predators.

Honest signalling is further supported by the positive correlation between the extent of the golden colour and unpalatability, controlling for body size in the analyses via phylogenetic correlation or as a covariate. The theory of honest signalling, however, predicts such a relationship within rather than across species. However, recent evidence shows that this relationship can also exist across closely related aposematic species (Cortesi and Cheney, 2010; María Arenas et al., 2015). It remains to be tested what speciation process and selection forces have driven this pattern. Given the fact, that the complex is composed of many unrelated taxonomic groups that occur in different micro-habitats, the processes are likely to vary. It is necessary to investigate whether such a relationship between species holds for other known mimetic complexes.

Unlike the classic yellow/black combination, members of the golden complex are defined by a metallic golden sheen. This sheen reflects brightly under direct sunlight making the animals very conspicuous, at least to a human observer. Just how important this sheen is in deterring a predator over and above just a matt colour has yet to be determined for this system. In other systems though, reflective iridescence has a significant additive effect (e.g. Fabricant et al., 2014) so it is likely that the sheen enhances the detectability of the signal. A possible additional benefit, or even an alternative explanation, is that the sheen is important for reflecting harmful radiation and dissipating heat (Shi et al., 2015). Irrespective of the explanation the importance of this colour variant, further experimental investigation is warranted.

The aversion to prey with golden colour was apparent irrespective of the predator’s visual sensitivity. Visually- and non-visually orienting predators equally avoided the golden prey suggesting that golden mimics advertise their unpalatability via multiple sensory modalities. The non-visual predator still proportionally avoided the mimics despite not being able to detect the visual signal. This is an area of promising future investigation. Some non-visual predators potentially recognize mimics via the patterns of vibrations generated by their movements (Pekár et al., 2011). Predators typically use multisensory modalities to identify prey (Bro-Jørgensen, 2010) such as movement, shape and defensive chemicals they emit.

Our measure of unpalatability suggested that the highest proportion of individuals was moderately defended and defensiveness is distributed from highly defended to weakly defended. This is in agreement with previous claims that mimicry rings are dominated by moderately defended species (Sherratt, 2008). With a predominance of moderately defended prey, there is an expectation that their presence will erode the effectiveness of the model. Our findings, based on a limited number of mimics, multiple predatory guilds when taking into account, did not support this notion. Instead, all mimics investigated in our predator-prey trials experienced significantly lower mortality than the non-mimetic species, and this was irrespective of the coverage of gold. We found the skinks were more likely to attack the lesser-defended individuals but when the suite of predators was taken into account there was little variation in the overall predation pressure. This is potentially because some predators may not use the visual signal as a reference and are cuing in on other signals.

Diet analyses of several potential predators also showed predators of all guilds, including non-visually oriented predators, captured mimics with very low frequency. The predators that we used are known to feed on non-mimetic prey too (Jackson and Harding, 1982; Daniels, 1987; Platnick, 2000; Higgins et al., 2001; McGinley et al., 2015). Indeed, they fed on prey related to the mimics as evidenced by the positive results of designed primers targeting the mimics. We did not measure the availability of alternative prey so we do not have an absolute indication of relative encounter rates. However, birds learn to avoid both Müllerian models and mimics irrespective of the availability of alternative prey (Lindström et al., 2004). In addition, there are multiple species of ant and ant models distributed across the sampling habitat occupying both terrestrial and arboreal habitats. The majority of the species in the complex are also ants and ants are typically super abundant due to their sociality. So it seems likely the predators would see these animals on a regular basis.

Using a multi-disciplinary approach here, we faced several constraints. For example, measurements of mimic colour were based on photographs as we were unable to collect all mimics. To correct for inherent differences in illumination used during photographing mimics, the obtained RGB values were standardized but not linearised, and thus should be interpreted with caution. Similarly, the results of experiments with skinks must be interpreted with caution due to constrained randomisation used (Müllerian mimics first). The response of skinks could be affected by the constrained prey order (due to some kind of habituation). Yet, other predators used showed similar response to mimics as skinks, though the order of offered prey was fully randomised.

Here we present the first account of the golden mimetic complex, which we believe is an exciting model for future research. Our work to date is mainly descriptive, and it requires more explicit experimental approaches to generate robust conclusions about the evolution of this golden aposematic signal. For example, the relationship between the level of unpalatability and the quality of the golden signals needs rigorous testing with a greater number of mimics. This would also help in deciphering the role of less-defended mimics in the protection of the entire complex. Similarly, exploring the function of non-visual signals in deterring predators will elucidate how this mimicry ring works.

In conclusion, we present the first account of the golden mimicry complex. It consists of species that show considerable variation in their level and mode of defence, ranging from undefended Batesian mimics to highly defended Müllerian mimics, with most members of the complex somewhere in between. Our data suggest that all members of the golden mimicry complex receive quite similar net protection from a variety of predators presumably because they have a variety of defensive traits. We predict that mimetic complexes that incorporate a range of different forms of defence are not only more common than previously believed, but also more likely result in mutualism among the unequally defended members of the complex.

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Author details

1. Stano Pekár

   Department of Botany and Zoology, Masaryk University, Brno, Czech Republic

   Contribution

   SP, Formal analysis, Supervision, Investigation, Methodology, Writing—original draft, Writing—review and editing

   For correspondence

   pekar@sci.muni.cz

   Competing interests

   The authors declare that no competing interests exist.

   "This ORCID iD identifies the author of this article:" 0000-0002-0197-5040

2. Lenka Petráková

   Department of Botany and Zoology, Masaryk University, Brno, Czech Republic

   Contribution

   LP, Formal analysis, Methodology, Writing—original draft

   Competing interests

   The authors declare that no competing interests exist.

3. Matthew W Bulbert

   Department of Biological Sciences, Macquarie University, North Ryde, Australia

   Contribution

   MWB, Resources, Methodology, Writing—original draft

   Competing interests

   The authors declare that no competing interests exist.

4. Martin J Whiting

   Department of Biological Sciences, Macquarie University, North Ryde, Australia

   Contribution

   MJW, Methodology, Writing—original draft

   Competing interests

   The authors declare that no competing interests exist.

5. Marie E Herberstein

   Department of Biological Sciences, Macquarie University, North Ryde, Australia

   Contribution

   MEH, Conceptualization, Writing—original draft, Writing—review and editing

   Competing interests

   The authors declare that no competing interests exist.

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