When you’re in the business of making nearly a thousand clay caterpillars by hand, there are a few must-have investments: Spotify premium (for hours’ worth of podcasts), ludicrous amounts of alcohol wipes (to scrub your stubbornly ink-dyed hands), more Copic marker refills than you think you would ever need (so you don’t have to drive back and forth to Sacramento multiple times to buy more) and, most importantly, someone willing to give you a long hand massage at the end of the day. Anyone will do, but fellow PhD students that understand the bizarre things one must do for science are probably your best bet. Pressing lumps of clay into 3D-printed caterpillar molds is satisfying after the first few – think Play-Doh Fun Factory  – but after the 10^th, 20^th, or 100^th time you might think that your wrists will give out. Yet, the risk of carpal-tunnel is well worth it. After weeks of meticulous shaping, trimming, airbrushing and painting, you’re rewarded with an army of adorable fake caterpillars. If you’re lucky, you’ll have made them realistic enough to scare the janitor. 

Beyond the practical factors of materials and labor, I found that making artificial prey also required something a bit more philosophical – a deep understanding of the link between art and science. For me, these two passions have become almost indistinguishable. To achieve an appropriate level of biological realism in my models, I had to consider design elements like shape, color, proportion, and texture. It’s strange to me that art and science are often considered opposites, when I find myself drawing on the same sources of inspiration for both; nature abounds with bizarre and beautiful organisms, provoking scientific inquiry and aesthetic appreciation in equal measure. To me, the most obvious connection between these two worlds is animal coloration.

Color is perhaps the most striking manifestation of natural and sexual selection we can directly observe, from the vibrant displays of birds of paradise (family Paradisaeidae) to the cunning disguises of leaf-mimicking katydids (subfamily Pterochrozinae). By its nature, color is both fascinating and vexing to study. Research on this subject requires a deep understanding of how light, environmental conditions, and the sensory systems of the eye interact to produce what we perceive as “color.” It’s sort of like trying to study an optical illusion – color really is in the eye of the beholder! Additionally, animal coloration is rarely one-note. Many cephalopods like squid and octopuses can change color in seconds using specialized chromatophore cells [1], while some animals communicate secretly with ultraviolet patterns that are invisible to their predators [2]. For this research project, I was interested in another dynamic color strategy known as ontogenetic color change (OCC), which refers to animals that change color as they age. My current research addresses a deceptively simple question: why switch colors?

Possible reasons for switching colors over time are easy to speculate, but rarely tested. Previous studies have found that species with OCC often move from drab to conspicuous color patterns[3], perhaps because it’s advantageous for young animals to avoid being seen until they’re less vulnerable [4,5,6]. Yet, this general trend doesn’t fully encompass the diverse range of OCC patterns we see in nature. I became especially interested in swallowtail caterpillars (genus Papilio) because several of these species don’t follow the drab-to-conspicuous track. Species such as the western tiger (Papilio rutulus) and pale (Papilio eurymedon) swallowtail are masters of disguise throughout their larval stages, first masquerading as bird poop and then mimicking snake eyes as they get older! Both feces-mimicry and eyespots serve to protect the larva, just in different ways; potential predators aren’t too keen on eating bird poop, and are likely intimidated by the big snake-like eyes [7]. I was curious as to why these species switch color strategies rather than sticking to one or the other. Also… swallowtails happen to be extremely adorable.

As they age, swallowtail caterpillars get bigger – and size seems to play a role in the effectiveness of both masquerade [4] and eyespots [7] in other animals. Essentially, I thought it might be hard for a giant caterpillar to look convincingly bird-poopy; meanwhile, a caterpillar with tiny eyespots probably wouldn’t seem that intimidating to predators. Thus, I hypothesized that there must be a certain size where developing caterpillars have to make the switch, because the effectiveness of their initial coloration wears off. To help figure this out, my caterpillar army comes into play. Since a lot of the swallowtail eyespot research done previously used artificial prey (usually clay or dyed pastry), I decided that it would be a good technique for my purposes, too [7]. The models allowed me to test specific hypotheses related to larval development and coloration, without having to keep track of hundreds of live caterpillars in the field. Plasticine clay is soft enough to show bite marks, yet sturdy enough to travel well. So, when you put the artificial prey outdoors, you can get a rough estimate of predation rates! This let me look at how effective each color strategy was at preventing predator attacks, depending on the size of the model. Initially, I figured that predation would increase with size for the bird-poop models, but decrease with size for the eyespot models. This would reflect the fact that we see small bird-poop larvae and large eyespotted larvae in nature, rather than the other way around. It would also confirm my hypothesis: that different colors are best for duping predators at different stages of caterpillar growth, leading to the color change we see in these species.

With the help of the TEAM engineering lab at UC Davis, I created 3D-printed molds based on a general swallowtail caterpillar body shape. I made molds for three different sizes (2, 4, and 6 cm long) representing the typical growth of a swallowtail caterpillar. I then borrowed my advisor’s airbrush – which he usually used to make fishing lures – to paint half the models like bird poop, and the other half green with eyespots. I painted the eyespots on the models one at a time, with the tiniest brush I could find. Once I had enough models to set up the experiment, I was ready to put my caterpillar army to work at my field site: the Putah Creek Reserve, a stretch of riverside land about 10 minutes from UC Davis’ campus.

For an entire week, I alternated between prepping the models for transport, setting them up along the creek, and then checking them for predation after two days of exposure. This required a highly regimented schedule – I planned my days meticulously, typically starting with a 6:30am wakeup call. Despite field sites chock-full of poison oak and thorny, wild blackberries, prepping the models for transport was the most painful part of the process. I used insect pins to stabilize each model in a container lined with foam core, and ended up with bruises on my thumbs from pushing so many pins in! Trekking around Putah Creek in the hot summer sun (strategically placing the models out for predators to munch on) was both challenging and satisfying – I liked being able to think of the outdoors as my temporary “office.” Without the help of some amazing undergraduate students, my advisor, and the manager of the reserve, it would have been impossible to get all the models out there!

Every two days I would eagerly check on these little models to see if predators tried these not-so-tasty clay snacks. Seeing the beak- and tooth-marks on my prey for the first time was thrilling! Occasionally, the smallest models would disappear entirely, likely carried off by a fooled predator. Sometimes I would find their chewed up little forms meters from where I set them out. Other mishaps included the fact that many of the big models melted in the sun, turning valuable data points into piles of plasticine goo. Ultimately, the experiments worked incredibly well. As an ecologist I often prepare for total failure, as nature rarely likes to go along with best-laid plans, so it was all the more exciting that my experiments went as smoothly as they did. 

So, what did I find? One of my predictions was correct: I saw more attack marks on the bigger bird-poop models than the smaller bird-poop models. This made sense to me in the context of how size can often influence the effectiveness of masquerade – the unrealistically large “bird poops” got nailed by discerning predators. However, unexpectedly, I found that the same pattern applied to the eyespot models too! Despite previous evidence showing that eyespots are typically more intimidating the larger they are [7], my bigger eyespot models were actually attacked far more than the smaller eyespot models. Even more curiously, the eyespot models were attacked at higher rates than their bird-poop counterparts overall.

I have a few ideas of why this might be. As convincing as my models may be in terms of shape and color, they’re missing one important variable: movement. In reality, swallowtail caterpillars have a lot of cool defensive behaviors that they begin to exhibit as they grow. For example, they will often puff up their thoraxes – and thus puff up their eyespots – when they feel threatened. So, perhaps eyespots on a static model act like a target for keen-eyed predators, but eyespots on a living, moving caterpillar are a formidable intimidation tactic. I hope to explore this hypothesis, among others, in future research. That’s the fun part about unexpected results; they often open up doors to new experiments!

Building and deploying my caterpillar army took a ton of time and effort, but I really appreciate how it allowed me to tie together my scientific thinking with my artistic predilections. Creativity is often an experimenter’s best tool…. along with a lot of spare time. While I may be done with this initial part of my research, and I hope to move on to live caterpillars next, I haven’t put away my caterpillar molds yet. Who knows what else they might be useful for – perhaps an army of fancy, caterpillar-shaped chocolates?

Elizabeth Postema is a second-year PhD student in the Animal Behavior Graduate Group at UC Davis, in the Yang Lab. She currently studies the relationships between color, behavior, and ecology, particularly among insects. Her research is driven as much by scientific curiosity as her love for cute bugs!

All photos taken by Elizabeth unless otherwise specified.

References:

[1] Zylinski, S., How, M. J., Osorio, D., Hanlon, R. T. & Marshall, N. J. (2011). To Be Seen or to Hide: Visual Characteristics of Body Patterns for Camouflage and Communication in the Australian Giant Cuttlefish Sepia apama. Am. Nat. 177, 681–690.

[2] Siebeck, U. E., Parker, A. N., Sprenger, D., Mäthger, L. M. & Wallis, G. (2010). A Species of Reef Fish that Uses Ultraviolet Patterns for Covert Face Recognition. Curr. Biol. 20, 407–410.

[3] Booth, C. L. (1990). Evolutionary significance of ontogenetic colour change in animals. Biol. J. Linn. Soc. 40, 125–163.

[4] Valkonen, J. K. et al. (2014). From deception to frankness: Benefits of ontogenetic shift in the anti-predator strategy of alder moth Acronicta alni larvae. Curr. Zool. 60, 114–122.

[5] Magnhagen, C. (1991). Predation risk as a cost of reproduction. Trends Ecol. Evol. 6, 183–186.

[6] Sandre, S.-L., Tammaru, T. & Maend, T. (2007). Size-dependent colouration in larvae of Orgyia antiqua (Lepidoptera : Lymantriidae): A trade-off between warning effect and detectability? Eur. J. Entomol. 104, 745–752.

[7] Hossie, T. J., Skelhorn, J., Breinholt, J. W., Kawahara, A. Y. & Sherratt, T. N. (2015). Body size affects the evolution of eyespots in caterpillars. PNAS.