A wasp (right) and the corresponding 3D-printed replicas. Photo: Christopher Taylor (University of Nottingham)
Whether mimicry works depends above all on who is doing the looking. Birds see through poor imitations with ease, whilst invertebrates are deceived far more readily.
Birds can flawlessly distinguish accurate imitations of dangerous insects from less convincing copies, whereas invertebrates such as spiders and mantises are more easily fooled. This is the finding of experimental research published in Nature on 2 July. Because predators differ greatly in their ability to see through mimics, imperfect mimicry can persist in nature.
Batesian mimicry – in which a harmless animal imitates a dangerous or unpalatable species – remains a classic example of adaptive evolution. Although natural selection would be expected to produce ever more accurate imitations, this is far from always the case. Nature is full of sloppy copies; hoverflies, to the human eye, bear only a passing resemblance to wasps.
Replicas
To find out why imprecise mimicry persists, British biologists produced 3D-printed replicas of wasps and hoverflies, including intermediate forms that do not occur in nature. They manipulated features such as colour, shape and pattern independently, and presented the models to a range of predators, including blue tits, great tits, crab spiders, jumping spiders and mantises. This was done both at feeding stations in the field and in the laboratory.
Lookalikes
The experiments show that birds make far more precise distinctions than invertebrates. They quickly learn which prey items are unpalatable and more often avoid good lookalikes than poor ones. Colour and size in particular prove to be the key factors in recognition – birds respond to these more strongly than to pattern or body shape. Invertebrates, by contrast, struggle to distinguish between mediocre and accurate imitations, suggesting that they exert less selective pressure on mimetic precision.
Left: A jumping spider (bottom right) and a group of 3D-printed insects, including a fly (top right), a wasp (next to the spider) and a series of intermediate variants. Photo: David McMahon. Right: a series of 3D-printed replicas of the common wasp (the front one shown at actual size). Photo: Tom Reader, University of Nottingham. Photos: Christopher Taylor and Tom Reader (University of Nottingham) & David McMahon
This difference can be partly explained by the divergent visual systems of the two predator groups. Birds possess sharp colour vision and greater cognitive capacity for learning, whilst many invertebrates respond primarily to contrast or movement.
Prey selection
Although the focus was on visual perception, the authors do not rule out that scent or movement may also play a role in prey selection for some predators. A second hypothesis tested by the researchers was whether a mimic gains additional protection by resembling several dangerous species simultaneously. This so-called multiple-models hypothesis was not confirmed: mimics that resembled several models were not attacked less frequently than those resembling just one.
Ultimately, the researchers conclude that the visual system of the predator determines how much protection mimicry confers. It appears that a sloppy and less costly disguise offers sufficient protection against invertebrate predators.