My awesome coauthors and I just had a paper come out in Ethology. In this blog post, I’ll break it down for a non-science audience and give some behind-the-scenes info and photos! Here is the full article (open-access) for those interested.
What do reds, oranges, and yellows have in common across the animal kingdom? Danger! Warning! Think of a coral snake or monarch butterfly, advertising their venom or toxicity to potential predators, warning them to stay back, or else! This is known as aposematism, the use of warning colors and patterns. Many animals (like most of the arthropods (insects, arachnids, etc.)) cannot see into the long wavelengths, so how do they know to stay away? First, bold colors are often paired with bold patterns. Coral snakes and monarch caterpillars/butterflies both have orange and yellows paired with black stripes. This achromatic contrast (contrast of brightness, not color) may help predators without color vision to stay away from dangerous prey. Second, aposematic colors are often paired with chemical defenses, so olfactory cues (smell) can help predators with limited vision avoid toxic prey as well.
Oncopeltus fasciatus, the large milkweed bug, is an excellent example of an animal that employs aposematism with both bold bright colors and conspicuous patterns. Milkweed bugs use a long, piercing mouthpart to feed on the inside of the seeds of milkweed, Asclepias. As they feed, they sequester (store) the cardiac glycosides (alkaloids) in the plant to use for their own defense. This is the same mechanism that monarch caterpillars use to defend themselves. Most animals think milkweed is distasteful, but a few insects have evolved to actually harness its toxicity to their own advantage.
(Note: this does not always work, there are lots of predators that are also well-equipped to eat toxic prey. Check out my photo of these predatory stink bugs eating a monarch caterpillar below)
But when predators hunt, how do they know to stay away from dangerous prey? Is aversion (avoidance) of aposematic prey innate (are they born with it)? Or is it learned from experience? Evidence with predators across the animal kingdom suggests that it can be a combination both. Many naive (with no prior experience) predators will avoid red or striped prey. Some will attack once, but quickly learn from the poor taste or aggressive defensive behaviors, that the prey is unsuitable. Further, these aversions (avoidance) of certain prey characteristics (such as red, or stripes, or the shape of a snake) could potentially be changed with experience. Positive experiences with harmless red prey may extinguish (eliminate) a red bias. Vice versa, negative experiences with toxic red prey can reinforce a red bias.
The way a predator knows or learns aversions (predator psychology) is important for biologists like myself to understand the evolution of warning signals in prey. Elaborate color patterns have evolved over millions of years, yet we still do not fully understand their complexity. In addition to warning signals like aposematism, predator psychology research can help us understand the evolution of camouflage or mimicry (when one animal has evolved to look like another, often more dangerous animal to fool predators).
If predators can change their aversions and preferences for prey based on their experience, one might expect that their habitat will be important in driving these experiences. For instance, if the habitat has more toxic red prey, red aversions could be stronger in the predators in that community. Surprisingly, this idea has rarely been tested, only once in birds!
With this experiment, we investigated whether jumping spiders from different populations had different levels of aversion (dislike) for red prey. We did this by bringing the spiders into the laboratory and presenting them with a choice of several termites, painted black or red with enamel paint (Yes, painting termites is as hard and delicate as it sounds!) (photo credit to Lisa Taylor)
Habronattus is a unique genus of jumping spiders (family Salticidae). Habronattus are one of the only genera of all salticids to have long-wavelength vision, meaning they can see reds, oranges, and yellows. They are quite small (4-8mm in length at maturity). To give you an idea of size, here is H. trimaculatus on a US penny (photo credit to Lisa Taylor).
We chose Habronattus brunneus as our jumping spider predator. H. brunneus occurs throughout Florida and on some islands such as the Bahamas. They are sexually dichromatic (males and females are different colors) and quite small in size (spiders were 4-8mm in length in our study).
Habronattus brunneus (female (left) and male (right) pictured above, photo credit to Colin Hutton) is a relatively drab species of Habronattus, compared to species like Habronattus americanus, H. pyrrithrix, and H. coecatus that have striking red coloration on their faces. The males of H. brunneus do have green and orange markings on their third pair of legs, that they display to females during courtship (see below, again, credit for these stunning photos to Colin Hutton).
When we found that there were indeed differences between four populations around Florida, my coauthor and undergraduate student at the time, Jeff Coco, conducted habitat surveys. He measured the habitat composition (whether it was covered in leaf litter, grass, sand, etc.) and did thorough sampling for arthropods in the habitat using multiple techniques (sweep netting, leaf-litter sifting, and sticky traps).
One very interesting finding was tens of these little red wasps. These are in the family Trichogrammatidae, but we have NO idea whether they are toxic to spiders! Much of this study (and really all studies) have left us with more questions than answers. Unfortunately, tiny arthropods are often barely taxonomically described, let alone well-studied. These tiny red wasps were most abundant in the populations with less or no red aversion. We know Habronattus will eat other tiny wasps (microhymenoptera), but we do not know if they eat these wasps. If they did, this would more evidence that palatable red prey extinguishes (eliminates) red aversion in jumping spider predators.
At the sites where spiders had higher levels of red aversion, we found more Hemiptera in the families Cicadellidae and Delphacidae, the leaf hoppers and tree hoppers. These insects are often chemically-defended and have striking patterns. While this is just speculative, the higher numbers of these insects at red-averse sites could help to reinforce red biases in the jumping spiders.
Here’s a selection of Cicadellidae from around the world, you can see that they can be quite colorful (photos from Wikimedia Commons)
The oak treehopper, Platycotis vittata (below) and the thorn bug, Umbonia crassicornis are two species that occur in Florida with bright coloration. Studies have shown that lizards reject these bugs as prey after tasting them, which lends evidence to these species being chemically-defended, and advertising this with aposematic coloration and patterning.
In addition to these striking variations in the community of potential prey for our jumping spiders, we also found differences in the habitat composition. Habitat composition (leaf litter, grass, sand, etc) could correlate with species in the habitat, but may also be important for visual systems of the spiders. For instance, more shadows in the environment could reduce the ability to see red. The habitat is dynamic and multi-faceted, even when you’re microscopic!
If you’re still interested, please check out the paper which is packed with a lot more data and information.
And a huge thank you to my wonderful coauthors, this paper wouldn’t exist without the contribution of every one of them!