Velvet ants share warning signals with the neighbours | @GrrlScientist
North American velvet ants are one of the world’s largest species complexes of mimics. Although these beautiful insects produce an intensely painful venom, neighbouring species still mimic each other’s many warning signals, a trait that effectively protects them all from predators
A team of American scientists report they’ve discovered of one of the world’s largest complexes of mimics, New World velvet ants. These brilliantly-coloured insects produce an intensely painful venom, yet neighbouring species of velvet ants still resemble each other so closely that they are barely distinguishable — an unusual trait known as Müllerian mimicry.
Warning signals are directed at specific predators
Aposematism is an evolutionary phenomenon that is more commonly known as a “warning signal”. Aposematic signals are actually beneficial for both predator and prey, because both rely upon them to avoid potential harm. For this reason, warning signals are directed at a specific type of predator and are intended to prevent attack by advertising the bearer’s unpalatability or noxiousness.
There are a variety of warning signals, including conspicuous colours or colour patterns, sounds, odours or other traits that are difficult for would-be predators to overlook. Some types of warning signals appeal more strongly to the senses of one sort of predator than to another. For example, a visual warning, such as brilliant colouring, appeals more strongly to a bird than to a nocturnal colour-blind mammal. Since warning signals are such an effective anti-predator system, other creatures that share the same predators may also mimic them.
There are two forms of mimicry. The first form, which is better studied, is known as Batesian mimicry. Batesian mimics are harmless species that evolve similar warning signals as those of a noxious species so would-be predators are deceived into avoiding them. Basically, they are cheaters. Generally, Batesian mimics are close relatives. This phenomenon was discovered by the English naturalist, Henry Walter Bates, whilst studying butterflies in the rainforests of Brazil.
The second form, discovered by the German naturalist, Fritz Müller, is Müllerian mimicry. This is a system where two or more noxious species that share predators also mimic each other’s warning signals. Müllerian mimics may or may not be closely related, and this phenomenon is less common in nature than is than Batesian mimicry.
Velvet ants form one of the largest known Müllerian mimicry rings
Velvet ants are members of Hymenoptera (the wasps, bees, ants and sawflies). Although they look a lot like fuzzy ants, velvet ants (family: Mutillidae) are actually wasps. Unlike ants (family: Formicidae), velvet ants are not social insects and do not form large complex societies built upon a division of labour carried out by drones, workers, and queens. Yet, unlike wasps, adult females are uniformly wingless, although adult males have wings, which they use to fly over long distances in search of females to mate with. Meanwhile, the females keep themselves busy by looking for ground-nesting bee nests and wasp burrows, where they lay their eggs, so the developing velvet ant larvae can eat the host larvae or pupae alive.
“Velvet ants fascinated me, not only because they are diverse and many are brightly colored and are quite attractive, but also because the family as a whole (Mutillidae) has not been studied very thoroughly so there is a lot of research left to be done”, said first author and evolutionary ecologist Joe Wilson, an assistant professor in the biology department at Utah State University’s Tooele campus.
Velvet ants possess several spectacular traits. Most obviously, many velvet ants are covered in a dense, glossy velvet-like fuzz — hence their common name. This body fuzz, which is particularly common in desert-dwelling species, may help them retain body moisture. Their fuzz typically comes in a wide variety of eye-catching colours and patterns, including black, reddish orange, gold, pale yellow, white and silver. These brilliant colours serve as a visual warning signal to would-be predators.
In today’s Current Biology study, Dr Wilson and his colleagues report on the extent of the North American velvet ant Müllerian mimicry complex by examining the geographic distributions and colour patterns for all of the 21 North American velvet ant genera that are active during the day. They report that this mimicry complex is extensive: it includes 302 of the 361 named species (nearly 84 percent), as well as 16 polymorphic colour forms and an additional 33 species that have not yet been formally described (Figure 1A):
To determine which species formed a mimetic cluster, the team measured several morphological characters (fuzziness and colours on different parts of each velvet ant species’s body). Basically, this figure shows that, of the 351 species and colour forms analysed, 336 share at least some morphological similarities, whilst only 15 (just four percent) were distinct (Figure 1B).
This finding led Dr Wilson and his colleagues to propose that North American velvet ants form eight recognisable mimicry rings — two of which (the red-headed Timulla and black-headed Timulla) are newly described (doi:10.1038/ncomms2275). In fact, this is one of the largest known Müllerian mimicry systems worldwide.
Why is velvet ant venom so ridiculously painful?
“[V]elvet ants are one of the most highly defended wasps that we know of, which makes them even more interesting. They are defended not only with their warning coloration, they have a painful sting that can be delivered through a stinger (officially known as a “sting”) that is nearly half the length of their body”, said Dr Wilson in email.
Is it possible that velvet ants evolved their many defences, particularly their intensely painful venom, as protection from some long-ago predator that is now extinct?
“I like the way you are thinking”, said Dr Wilson.
“In fact, we have suggested that same thing in conversations with colleagues. Unfortunately we don’t yet know what an extinct predator would have looked like”, said Dr Wilson.
It is also possible that velvet ants evolved their variety of aposematic signals in response to a wide array of potential predators, many of which are likely to be extinct.
“[A]t this point we just know that no extant animals are very successful at eating velvet ants, but experiments are ongoing. It could also be that the seemingly inordinately protective defenses are a byproduct of other things that we just haven’t measured yet”, added Dr Wilson.
But only female velvet ants possess a sting. The highly manoeuvrable sting is a modified ovipositor, or egg-laying organ. Velvet ant stings deliver a venom that is so intensely painful that they are often referred to as “cow killers”.
Both male and female velvet ants produce an auditory warning signal — a loud squeak — when harassed. I can easily recall when, as a child, I discovered (and poked) one member of the Western Mimicry ring as she ran across the hot sand, and I was immediately intimidated by her desperate-sounding squeaks. Fortunately for me, the squeaking creature escaped before I could conduct further “tests”.
This video captures a female velvet ant as she squeaks loudly and displays her impressive sting:
Could velvet ant venom really kill a cow?
“They kill cows as much as earwigs burrow into brains, camel spiders gobble down camel stomachs, milk snakes suck cow’s udders, and green darners sew up kids’ lips … that is … probably never”, said Paul Marek, an assistant professor in the department of entomology at Virginia Polytechnic Institute and State University and curator of the Virginia Tech Insect Collection, who was not part of this study.
“In reality, the velvet ant sting is not very toxic (in terms of lethality), even when compared to other wasps. While velvet ant stings won’t kill, they are widely known as being one of the more painful stings a person can get”, agreed Dr Wilson.
“The pain depends a lot on how large the stinging individual is”, said co-author and taxonomist Kevin Williams, who works as an identifier and curator of insects at the State of Florida’s Division of Plant Industry.
“There’s a moderate to intensely painful burning/shocking sensation for about 30 seconds, then itching for another minute or so”, said Dr Williams, who mentioned that he’s been stung “about 50 times”. Surely, so many stings probably makes Dr Williams the world’s only connoisseur velvet ant venoms, a peculiarity that is probably required before one can be considered one of the world’s foremost authorities of velvet ants.
“Generally, after two minutes there’s no sensation or sign that I was even stung. Interestingly, velvet ant venom is less lethal (has higher LD50 values) than honeybee or ant venom, even though the initial pain is more intense”, added Dr Williams.
“It makes sense because the velvet ant’s venom has the sole purpose of shocking/distracting its potential predator long enough to [make an] escape”, explained Dr Williams.
“Moral of the story; painfulness of a sting isn’t necessarily related to toxicity of a sting”, said Dr Wilson.
As if visual and auditory signals aren’t enough to warn away hungry predators, grabby children and inquisitive scientists, velvet ants have several other warning signals, too. For example, they have an extremely strong cuticle (skin) that is difficult for predators to crush — and for insect collectors push a pin through.
“My least favorite experience with velvet ants, and highlights their hard exoskeleton, was early on as an entomologist, I tried to pin one and I pushed so hard on the pin head in order to drill into the hard shell that it went straight out through the other side of my finger instead of [through] the insect”, said Professor Marek.
“We think the physical similarities across species benefit the wasps by quickly and effectively training their predators to avoid velvet ants displaying the local [warning] color pattern. This tells us that mimicry, rather than close genetic ties, may explain why the majority of velvet ants in a particular region [are] the same color,” said Dr Wilson.
Why are velvet ants so extremely sexually dimorphic?
Similar to other Hymenopterans, velvet ants employ haplodiploid sex determination. This sex determination system is where females develop from fertilised eggs (and thus are diploid), whilst males develop from unfertilised eggs (and are haploid). But adult males and females show other dramatic differences.
“Male velvet ants are completely different than the females”, said Dr Williams in email.
In fact, the physical appearances of adult male and female velvet ants are so strikingly different that it is quite challenging to identify whether two individuals are of the same species, unless they are captured whilst mating.
“It’s a really exciting challenge to try to match up the sexes and recognize the species limits. There’s twice as much variation and diversity to study in each of the 4000 velvet ant species”, said Dr Williams.
“Dasymutilla gloriosa females fit in the Desert Mimicry ring (i.e., they are white). The male of this species is most similar to the Western Mimicry ring (i.e., it has orange hair on the head, thorax and abdomen),” said Dr Wilson.
Such extremes between the appearance of adult male and female velvet ants reflects the very different lives that each leads — females are terrestrial whilst males are primarily creatures of the air.
“Basically, the female’s whole life is taken up with walking and searching for something rare and hidden”, said Dr Williams.
“My hypothesis is that the most important characteristic that influences a velvet ant’s reproductive success is time spent searching. Therefore, any defense that increases longevity would be selected for”, said Dr Williams.
Velvet ants: aposematic masters?
“Many aposematic organisms possess a nasty taste and a conspicuous appearance. Sometimes a foul taste, weird noise, and noxious taste. But velvet ants are an “aposematic one-man band” (in this case “one-woman band” since they’re all females) in a sense because they have all manner of noxious and raucous features”, said Professor Marek.
“It’s wonderful to think about velvet ants with their garish conspicuousness and suite of no less than five cues (coloring, odor, squeak, hard shell, and painful sting) that stimulate just about all of the senses of the predator”, said Professor Marek.
“It makes me wonder if the other features have converged as a result of this Müllerian mimicry just as their appearances have. Do co-mimics also stridulate or squeak with the same wave-form? Since birds and lizards have been mentioned as predators, do they appear different according to the visual system of their predators (e.g. UV colors that we cannot see as humans)?” said Professor Marek.
In short, velvet ants comprise a large and mostly unexamined system where scientists can test hypotheses about warning signals and mimicry, especially regarding the evolution of imperfect mimicry (doi:10.1371/journal.pone.0061610). Further, since Müllerian mimicry is an unusual natural phenomenon, there’s plenty more to learn about this, too.
“[T]hese animals are easy to collect (they’re here in our backyards in the US), possess a spectacular diversity of unique mimicry rings, and are exhilarating to try to scoop into a collecting jar without getting stung”, Professor Marek pointed out helpfully.
“Maybe this project will catch the attention of students who are interested in mimicry, but don’t want to write the ten-thousandth paper about monarch butterflies”, added Dr Williams.
Joseph S. Wilson, Joshua P. Jahner, Matthew L. Forister, Erica S. Sheehan, Kevin A. Williams, and James P. Pitts (2015). North American velvet ants form one of the world’s largest known Müllerian mimicry complexes, Current Biology, published online on 17 August 2015 ahead of print | doi:10.1016/j.cub.2015.06.053
Joseph S. Wilson, Kevin A. Williams, Matthew L. Forister, Carol D. von Dohlen and James P. Pitts (2012). Repeated evolution in overlapping mimicry rings among North American velvet ants, Nature Communications 3:1272 | doi:10.1038/ncomms2275 (OA)
Joseph S. Wilson, Joshua P. Jahner, Kevin A. Williams, and Matthew L. Forister (2013). Ecological and Evolutionary Processes Drive the Origin and Maintenance of Imperfect mimicry, PLoS ONE 8(4): e61610 | doi:10.1371/journal.pone.0061610 (OA)
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Originally published at The Guardian on 17 August 2015.