Halloween, one of the few times a year where people are allowed to dress up and perform the mating dance of their favorite spider species. While most people think of costumes as a fun way to show off their creativity or love of monsters, animals, or tv and movie characters, many animals use costumes as a way of staying alive. These costumes can be classified as either camouflage (when an animal is disguised as leaves, tree bark, sticks, or dirt) or mimicry (when an animal is disguised as another animal). A good blog to look at to learn about some of the incredibly diverse forms of camouflage and mimicry found in nature is Not Exactly Rocket Science from Discover Magazine.
Batesian mimicry is a form of mimicry where an animal that can easily be eaten by a predator has evolved to look like another animal that is either poisonous if eaten (such as viceroy butterflies that mimic monarch butterflies) or can harm the predator if provoked (such as hoverflies that mimic bees or wasps). Some disguises are incredibly elaborate, such as those found in caterpillars in the genus Hemeroplanes, which can alter the shape of their bodies to resemble snake heads. Since this form of mimicry relies entirely on fooling potential predators, then it should stand to reason that the more a mimic resembles the animal it mimics, the better a chance they will have to survive. However, many species only mimic superficial traits such as coloration, leading scientists to ask: why are there so many inaccurate mimics?
This question was investigated by a team of researchers from universities in the Czech Republic, Germany, and Australia in the article Is the Evolution of Inaccurate Mimicry a Result of Selection by a Suite of Predators? A Case Study Using Myrmecomorphic Spiders. The paper starts with a run-down of several hypotheses attempting to explain this phenomenon, such as mimics attempting to mimic several model species at once (such as hoverflies mimicking both bees and wasps) or the costs associated with matching the exact shape or size of the model species (some body shapes have reduced space for egg production, which can reduce the number of offspring an individual can produce).
The authors point out that these hypotheses only considers predators that would avoid potentially dangerous prey. If predators prefer to eat the species that the mimics are attempting to resemble, then these mimics have a higher chance of being eaten the more closely they resemble their models. Ant-mimicking spiders such as those in the genus Myrmarachne are almost indistinguishable from ants, which reduces their overall risk of predation but greatly increases their risk of being eaten by ant-eating predators. They suggest that mimics that in order to counteract this change in predation risk, mimics may evolve alternate methods for survival, such as fleeing from certain predators.
To test these ideas, the authors looked at three species of spiders (Liophrurillus flavitarsis, Phrurolithus festivus, and Micaria sociabilis) that resemble ants. In addition to coloration patterns, these spiders mimic their ant models by lifting their front legs into the air and waving them around in a way that resembles the movement of ant antennae. One of each spider was placed into a container, along with an ant of the species each spider was mimicking and either a predator that preferred spiders (spiders of the genera Palpimanus and Drassodes) or a predator that preferred ants (spiders of the genus Zodarion). The researchers looked at which potential prey each predator attacked as well as how each potential prey responded to the presence of a predator.
They found that ant-eating predators attempted to capture both the ants and the ant-mimicking spiders, with greater capture rates when attacking ants, whose response to the predator was to turn towards it and take a defensive pose, than spiders, whose response to predators was to lower their front legs and flee. Conversely, spider-eating predators didn’t attempt to capture ants or ant-mimicking spiders, but would quickly capture other spiders that did not mimic ants. The authors suggest that the form of the spiders not fully replicating the form of the ants being mimicked is a result of a lack of evolutionary pressure. Since they were similar enough to ants to avoid spider-eating predators, there was no reason for the spiders to create a more convincing disguise. The authors suggest that this lack of pressure once mimicry is good enough to fool predators is an important reason for the high amounts of inaccurate mimics, while stressing the importance of looking at the evolution of other mimics in systems containing multiple predators.
This paper emphasizes two major themes of biology that I have become increasingly aware of throughout my college career. The first theme is that evolution only cares about getting an organism to be good enough to survive and reproduce. These spiders have evolved to mimic ants to reduce their predation risk. Once the spiders resembled ants enough to fool their predators, there was no reason to continue evolving to look more like ants. They were good enough to survive, so there was no reason to keep pushing in that direction.
The second theme is that everything in nature is more complicated than it seems at first. Mimicry is generally accepted as being an attempt by organisms to fool predators, which increases their survival. But, as this paper points out, if there are predators that eat the animals being mimicked, then the animals doing the mimicking are still at risk of being eaten. Every part of an ecosystem is linked, and the longer you look at one set of links, the more links you notice.