Conforming with stereotypes of urban white educated progressives (UWEP*), I listen to a lot of NPR. As such, I’ve recently subscribed to the Invisibilia podcast, created but the marvelous Lulu Miller and Alix Spiegel.
If you haven’t already given the show a listen, I definitely recommend it. Ms. Miller & Ms. Speigel do a wonderful job exploring that fuzzy intersection of science, psychology, philosophy, and humanism which they classify as Invisibilia: invisible, abstract forces that influence human behavior.
In honor/admiration/parody of their work, this post is about forces that shape animal behavior–forces that are invisible to us, but not to creatures that see ultraviolet radiation. I now christen a neologism to describe such phenomena: U-Visibilia.
Quick summary of electromagnetism: Homo sapiens primarily perceive wavelengths of light between 400nm-700nm. Wavelengths that are a tad longer than 700m are referred to as infrared (longer than visible red wavelengths). Those wavelengths that are shorter than 400nm are called ultraviolet, and are invisible to the human eye.**
Unlike humans, some animal species do perceive ultraviolet wavelengths, and signals in the ultraviolet can play an important role in their natural history. Let’s look at some examples of how animals use ultraviolet color in fighting, friending, and foraging.
Our study begins with the Florida Scrub-jay (Aphelocoma coerulescens). Endemic to patchy oak scrub in Florida, Scrub-jays are sassy, violent, and handsome animals. Adult plumage is sexually monomorphic in the visible spectrum, as seen below.***
These jays are cooperative breeders; young birds often hang around the nest for several years after fledging and help raise their siblings. This family set-up elicits dominance structure among breeders and helpers .
Some of this hierarchy is vetted in the first year of life, before jays develop the blue hood of their parents. Young “brown-heads” will squabble over resources, with some individuals consistently winning out, while others hastily yield.
How do these birds determine who’s stronger than whom? Is it body size? Parental favoritism? Some invisible power? Or is it…U-Visibilia?
Turns out that while we see their heads as brown and wings as light blue, both have reflectance in the UV, which the jays definitely can see. My brilliant friend Angela Tringali tested whether the strength of reflectance in UV-Blue spectra predicted the outcomes of dominance interactions . To do this, she measured UV-blue reflectance in hatch-year siblings, and assessed dominance by quantifying who “won” or “lost” in fights over a feeder of peanuts. Unsurprisingly, individuals with brighter UV-blue colors ranked higher than their duller siblings. Angela took this exploration a step further by altering the plumage of more dominant jays with two different treatments. The first treatment group got their feathers colored with a Sharpie© marker, which heavily depressed their UV-blue reflectance. The second group received a sham treatment–a marker that mussed up feathers without diminishing UV reflectance.
After altering their plumage, the Sharpie-d bird’s odds of winning interactions diminished, while control and sham-treamtent birds retained their dominance in feeder trials.*** So there you go. U-Visibilia.
We now turn to damselfish. Ambon and lemon damselfish (Pomacentrus amboinensis and P. moluccensis) are two similar species that are moderately difficult to distinguish with untrained human eyes.
However, males actively chase members of their own species (conspecifics) off territories, while they are more tolerant of males of the sister species (heterospecifics). This makes sense, as conspecifics pose a greater competitive threat to resources and mates than do heterospecifics.
How do males quickly and successfully identify friend from foe?§ Is it scent? Is it secret fish finshakes? Or is it…U-Visibilia?
Siebeck et al.  demonstrated that both species of damselfish come equipped with the ability to see UV light, and that males have strong UV reflective patterns on their faces. The shape of these blotches is specific to species, with Ambon damselfish showing heavier blotches and stripes than the lemon damselfish.
So if the visiting fish’s face is a different the resident’s fish’s face, they can be fish-friends! Maybe they’ll fish-friend each other of Fish-facebook. Fishbook? Fishbookfriendface? Ugh. Okay, they’ll just not be antagonists.
Lastly, we investigate predatory stylings of Thomisus spectabilis, an Australian crab spider that specializes in snatching honey bees (Apis mellifera) when they visit daisy flowers.
To the human eye, these spiders blend in pretty well with the flower petals, so you may expect this camouflage aides them in hunting bees.
However, Heiling et al.  demonstrated that bees are more likely to visit flowers with spiders than flowers without spiders. This suggests that bees do perceive the spiders, often to mortal consequence.
How do they these crab spiders attract bees? With inter-species pheromones? Mind powers? Tractor beams? Or is it…U-Visibila?Surprise! T. spectabilis is strongly reflective in the UV spectrum. Many flowers have UV ornaments to help attract and guide pollinators . Our spiders “boost” this UV signal with their own colors, increasing the attractiveness of the flower to passing pollinators. The summary of this research gets Heiling et al. a page in Nature. Happy ending for the scientists, if not the bees.
Well, that’s all for now. I won’t be posting for another two weeks–I’ve gotta finish some logistic stuff for Panama, and get Program MARK to cooperate with our new analyses. But check back on February 16, when I’ll write and doodle about heartbreak and break-ups in the animal world in the wake of Valentine’s Day.
*Don’t confuse UWEPs with Yuppies. Yuppies were upwardly-mobile professionals. We’re professionally stagnant .
**Human’s photorecepting cones do respond to ultraviolet wavelengths, particularly blue cones. However, the lenses of our eyes filter out these wavelengths; people who have received artificial lenses through surgery perceive UV as blue-white or violet-white.
***Angela’s experiment is more detailed than my summary makes it out to be; there’s good stuff regarding inter-sexual conflict, too. Go read it.
§Okay, so maybe “friend from foe” isn’t as accurate as “not-a-big-competitor from definitely-a-competitor“, but whatever.
1. GE Woolfenden & JW Fitzpatrick. 1977. Dominance in the Florida Scrub-jay. The Condor 79(1): 1-12.
2. A Tringali & Reed RJ Bowman. 2012. Plumage reflectance signals dominance in Florida Scrub-jay, Aphelocoma corulescens, juveniles. Animal Behaviour 84(6): 1517-1522.
3. UE Siebeck, AN Parker, D Sprenger, LM Mäther, & G Wallis. 2010. A species of reef fish that uses ultraviolet patterns for covert face recognition. Current Biology 20(5): 407-410.
4. AM Heiling, ME Herberstein, & Lars Cittka. 2003. Pollinator attraction: crab-spiders manipulate flower signals. Nature 421: 334.
5. CE Jones, SL Buchmann. 1974. Ultraviolet floral patterns as functional orientation cues in hymenopterous pollination systems. Animal Behavior 22(2): 481-485.