As we take a look back at research articles published so far in PLOS ONE in 2014, we realize we have no shortage of images to terrify our readers, or at least sufficiently creep them out long enough to last through … Continue reading
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From snakes that look like they have two heads to color-shifting chameleons, deception is at the heart of many animals’ survival strategies. Both visual and chemical predator deterrence are well-documented phenomena in the animal world, but new research on ant-mimicking spiders, published in PLOS ONE, may be the first documented case of a species that uses visual deception to elude one group of predators, and chemical deception to escape another.
Ant mimicry, or myrmecomorphy, is a tactic used by numerous spider species, and with good reason, since many predators steer clear of preying on ants due to their aggressive tendencies and often unpleasant taste. Ant-mimicking spiders can have body shapes that closely resemble those of ants, as well as colored patches that look like ant eyes. Combine these characteristics with behaviors such as waving their front legs in the air to resemble probing ant antennae, and these spiders can successfully convince predators to look elsewhere for their next meal. The jumping spider Peckhamia picata is one such ant mimic whose visual signals are an effective deterrent for visually focused predators, such as other species of jumping spiders. The picture below shows a jumping spider on the left and the ant it imitates on the right.
The PLOS ONE study shows that the ant-mimicking spider can also elude predators that rely heavily on chemical signals to identify their prey. In the current study, the spiders successfully eluded spider-hunting mud-dauber wasps (pictured below), and received significantly less aggression from the ants they mimic than other non-mimicking jumping spiders. The researchers presented wasps with a choice between freshly killed ant-mimicking and non-mimicking spiders. In all of the trials conducted, the wasp probed both types of spiders with their antennae, but every time the wasps chose to sting and capture a spider (seven out of eight times), it chose the non-mimicking spider.The researchers also staged encounters between Camponotus ants and live ant-mimicking and non-mimicking spiders. After probing them with their antennae, the ants were significantly less likely to bite the ant-mimicking spiders than non-mimicking ones. These results demonstrate that the jumping spider has a remarkably effective ability to deceive potential predators who focus on chemical cues when selecting prey.
The researchers point out that the spider is not a chemical mimic of the ant species it emulates. Insects rely heavily on hydrocarbons secreted from their cuticles (the hard outer covering of invertebrates) to identify and signal one another. It turns out that ant-mimicking spiders have very low levels of these molecules, only a small fraction of the amount found in non-mimicking spiders and the ants themselves. While further research is required to fully explain the jumping spider’s chemical mechanism for predator evasion, a likely explanation is that the low level of these chemicals does not register as significant to a probing ant or wasp, and the chemical evasion is accomplished in this way.
This study may be the first to describe an animal using a “double-deception” strategy: visual tricks and a deceptive chemical signature, both intended for different audiences. The authors hypothesize that this kind of chemical deception is likely widespread among other visual mimics in the animal kingdom.
Related links:
Video of a ramblin’ ant-mimicking jumping spider (great music)
Spiders gather in groups to impersonate ants
Citation: Uma D, Durkee C, Herzner G, Weiss M (2013) Double Deception: Ant-Mimicking Spiders Elude Both Visually- and Chemically-Oriented Predators. PLoS ONE 8(11): e79660. doi:10.1371/journal.pone.0079660
Images: Images come from Figure 1 of the manuscript
Spiders are everywhere (Arachnophobes, stop reading now). They’re among the most successful predators on earth today and colonize nearly every terrestrial habitat (that is, not just ceiling corners and under beds), and occasionally do so in numbers large enough to take over small islands. Spider silk may be strong enough to stop a speeding train and some webs, ten times stronger than Kevlar, can be large enough to cross rivers in tropical rainforests.
But more than half of today’s spider species don’t rely on webs or silk to capture their prey. Instead, these hunting spiders have evolved hairy adhesive pads on their legs to grab and hold struggling prey down, according to the results of a recently published PLOS ONE study. The adhesive pads, called scopulae, were commonly seen in many spider species but what wasn’t clear until now was whether they were found in all species, or more likely to occur in hunting spiders.
In this study, researchers used a phylogenetic analysis of spider family trees to correlate different species’ prey capture strategies with the presence or absence of adhesive pads on their legs. They found that the majority of spiders were either web builders or free-ranging hunters, and the latter were most often found to have adhesive hairs on their legs (Apart from these two, at least one rare variety may be mostly vegetarian). Nearly 83% of hunting spiders had adhesive bristles on their legs (compared with 1.1% of web-building varieties). Most of these hunters had either not developed silk-dependent strategies to capture prey, or abandoned web-building for hunting.
Why would so many spiders abandon an obviously successful way to catch prey? Web-building is a useful way to trap insects and some small mammals, but even to a spider, silk is expensive. Creating a web requires work, damages caused by prey or people need frequent repairs, and certain kinds of webs can require large amounts of silk to be effective. The classic orb-web (seen in the picture here) radically reduced these costs, which may be why the spiders that make these are particularly common. However, this new study reveals that hunting has proved at least as successful a strategy as web-building to more than half of today’s spiders.
Bristly scopulae on hunting spiders’ legs have played a big part in this, enabling spiders to grasp and hold on to struggling prey. The thin bristles on scopulae come in many shapes and forms, and also contribute to these spiders’ mad climbing skills. Read more about which spiders evolved these bristles or learn about other arachnid research published in PLOS ONE here.
Citations: Gregori? M, Agnarsson I, Blackledge TA, Kuntner M (2011) How Did the Spider Cross the River? Behavioral Adaptations for River-Bridging Webs in Caerostris darwini (Araneae: Araneidae). PLoS ONE 6(10): e26847. doi:10.1371/journal.pone.0026847
Rogers H, Hille Ris Lambers J, Miller R, Tewksbury JJ (2012) ‘Natural experiment’ Demonstrates Top-Down Control of Spiders by Birds on a Landscape Level. PLoS ONE 7(9): e43446. doi:10.1371/journal.pone.0043446
Wolff JO, Nentwig W, Gorb SN (2013) The Great Silk Alternative: Multiple Co-Evolution of Web Loss and Sticky Hairs in Spiders. PLoS ONE 8(5): e62682. doi:10.1371/journal.pone.0062682
Nyffeler M, Knörnschild M (2013) Bat Predation by Spiders. PLoS ONE 8(3): e58120. doi:10.1371/journal.pone.0058120
Images: Foot of the little jumping spider Euophrys frontalis, credit Jonas Wolff; varied shapes and sizes of bristles on scopulae from pone.0062682; spider web on plant by mikebaird
To continue our spooktacular posts this October, we bring you a study which may have some arachnophobes rethinking their next vacation destination.
The island of Guam is home to one of the densest spider communities in the Pacific. In a recent study published with PLOS ONE, researchers investigated this region to discover how the demise of insectivorous birds inhabiting the island has affected one of the most widely feared creepy crawlers.
The downfall of Guam’s native insect-eating birds began in the 1940’s when the infamous brown tree snake was introduced. To investigate the effects this loss had on the landscape, the authors of the recent paper analyzed the spider population on several Pacific islands.
The team compared the neighboring islands of Rota, Tinian and Saipan, to Guam. These islands do not have any known snake populations, and also have similar native bird species to that of Guam. The researchers were then able to assess whether the bird presence correlated with spider web numbers, in addition to what impact bird presence had per season.
What the authors found might send chills right down your spine: The spider web densities in Guam were 40 times higher than those of the other islands during the wet season. Guam had an average of 18.37 spider webs per 10 meters, as compared to the other islands, which only had 0.45 webs per 10 meters. In addition, the bird loss had even increased the web size for a certain spider species.
Whether you suffer from arachnophobia, ophidiophobia (fear of snakes) or ornithophobia (fear of birds), I think we can all agree this is a terrifying case showing the effects the removal of an essential predator can have to a landscape.
Citation: Rogers H, Hille Ris Lambers J, Miller R, Tewksbury JJ (2012) ‘Natural experiment’ Demonstrates Top-Down Control of Spiders by Birds on a Landscape Level. PLoS ONE 7(9): e43446. doi:10.1371/journal.pone.0043446
Image Credit: Anders B on Flickr CC-by license