Tag / iridescence

by Roslyn

SICB Portland

Here is the poster we presented at SICB Portland last week on the biomechanics of peacock displays (click to enlarge):

SICB poster

I think it turned out pretty well, although I’m not sure it could stand alone without an interpreter.

We had a constant stream of awesome visitors. My coauthor Suzanne brought feathers and a model peacock to demonstrate what we were talking about – brilliant! We also had a touchscreen mounted to the left of the poster to display the supplemental videos, but to my surprise we didn’t use it much. It was too slow to load for every new visitor, although it did come in handy for people who wanted an in-depth look. I realize now that videos should really be integrated spatially with the poster content. This could be done if whole display was a touchscreen, for example.

One of the highlights of the meeting was seeing how folks in Stacey Combes’ lab are tracking the movements of individual bees by gluing tiny QR codes onto the bees’ backs (the codes are automatically recognized on video of the bees entering and exiting their hives by tracking software). Another highlight was Ken Dial’s talk about the influence of predation on the development of flight in nestling birds. Portland had lots of good food and drink and exciting views of 1000s of crows roosting late at night downtown.

Thanks to Owen, Suzanne, Jim and Bob for such a fun project!

by Roslyn

Masters of illusion

It can be easy to see intelligence in the animals we spend a lot of time with. Everyone has their pet example, one of the most common being dogs who can anticipate the precise time of their owners’ return. But what does this really say about the mental life of dogs? Some birds are capable of even more impressive mental stunts – only they often go unnoticed in the wild. Two recent field studies in Africa and Australia provide a nice illustration. The results challenge our notion of limited animal intelligence, but as we will see, the way we interpret them might say more about our own minds than it does about the birds.

Fork-tailed drongos are masters of deception. These small, glossy black birds from southern Africa are known for their ability to mimic the calls of other bird species – much like the mockingbirds found throughout the US and parts of southern Canada. Most of the time, drongos forage alone hunting insects, but occasionally they get other animals to do the hard work for them. Drongos will follow groups of meerkats and pied-babblers – mammals and birds known for their highly social lifestyles – and steal their food, a process known as kleptoparasitism. It might not be a complete loss for the victims, either. Drongo thieves give plenty of alarm calls along the way, and these may help the meerkat and babbler groups avoid predation1.

Perhaps not surprising for an accomplished mimic, the fork-tailed drongo has a diverse alarm call repertoire that includes its own unique warning “chink” as well as the calls of several other bird species. On the savannahs of the Kalahari Desert, birdwatchers noticed that drongos often seem to use these mimicked calls during kleptoparasitism, swooping in to steal food from pied-babblers immediately after sounding a false alarm1. For Tom Flower at the University of Cambridge, this was fascinating anecdotal evidence, so he set out to test whether these alarm calls are used by the drongos in a deceptive way2.

The first thing Flower needed to do was eliminate the possibility that the drongo false alarms are coincidental. If the drongos are truly deceptive, the calls should only occur when the birds are attempting to steal food. Flower also had to establish that the drongos sound the same, regardless of whether they are using their alarm calls in an honest or deceptive context. Finally, he had to show that the meerkats and babblers respond similarly in both cases.

Fork-tailed drongo

Fork-tailed drongo.

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by Roslyn

Deep archives: Irreducible beauty

Peacocks and audience at the Toronto Zoo

Were peacocks designed with this kind of audience in mind?

A while back I was searching for images of peacock feathers on Google, and I stumbled upon this article. It’s a piece by Stuart Burgess, an engineer who is head of the Department of Mechanical Engineering at Bristol University, and apparently also quite an opinionated creationist.

Burgess’ idea is that the peacock’s train feathers “contain an extremely high level of optimum design”, so much so that they provide evidence against Darwinian evolution. He thinks that the aesthetic features of the peacock are so complex, so contingent upon each other, that no step-by-step process of evolutionary change could have produced them. He’s right that these ornaments are highly complex, and that selection for this kind of extreme aesthetic feature presents a bit of a puzzle for evolutionary biology. To claim that the extraordinary complexity must be “irreducible”, however, is a big assumption.

The article provides a lot of amusing examples of twisted logic along the way. For example, one of the features that Burgess finds irreducibly beautiful is the fact that the peacock’s train forms a fan-like shape. This is because “the axis of every feather can be projected back to an approximately common geometrical center” – indeed, the body of the bird that grew them!

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by Roslyn

Deep archives: Iridescence: From insect crystals to nature’s transformers

Iridescent cuttlefishNow that I’ve given my talk at the iridescence conference, I feel like I can relax and reflect on the last few days.

The biggest success of this trip so far was undoubtedly making it across the border. When I arrived at the airport, I was informed by a rather sour US customs lady that because I didn’t already have a return ticket, she didn’t have to let me into the country. This was news to me, but in retrospect my story that I was a grad student attending a conference and then traveling indefinitely in California probably wasn’t the best choice. Customs lady expressed her disbelief, handed my passport back to me in a bright yellow folder and directed me to take my warning-beacon folder into a special room for suspicious types. I was terrified that she would would somehow find and confiscate the stuffed peahen or any of the other bizarre (but critical) items in my suitcase full of field equipment. Somehow, however, I was able to get by without having to provide any more details about my plans. As for the peahen, I waited until I was settled into my Arizona hotel room before anxiously opening the suitcase to survey the damage. I’m relieved to report that she survived the trip without confiscation or serious injury. Although she does look a little worse for wear I’m confident that the males will still find her an acceptable target.

The conference has been amazing – I’ve heard talks from the world’s experts on the physics, development, evolution, and behavioural display of nanostructural colours in animals. The very first talk was on ways to manufacture macro-scale versions of the colour-producing nanostructues found in butterflies, and then use microwaves to measure optical properties of these models in a scaled-up way. The advantage of this technique is that it lets you get around the problems of performing accurate optical experiments with very small things (such as single butterfly scales).

I’ve also heard about how to model the optical properties of animal nanostructures, how to improve my goniometer for feather colour measurements, how insects build crystal-like cuticular nanostructures with exquisite control at the cellular level, and how some butterflies are similar to peacocks in terms of orienting themselves for optimal reflectance of their iridescent wing colours during display flights. We’ve had serious discussions about the developmental differences between “squishies” (vertebrates) and “crunchies” (arthropods), the meaning of the word “crystal”, and the use of dark framing around bright colours in both art and animals.

By far the most interesting talks, however, were about work from the Hanlon lab on the cuttlefish colours. Cephalopods have evolved an incredible ability to control the hue, patterning and texture of their skin – including what one speaker referred to as “changeable iridescence” – for use in predation and camouflage as well as in conspicuous signaling. Cuttlefish skin contains a base layer of bright blue-green iridophores covered by a layer of pigment sacs called chromatophores. All of these structures are laced together in a network of muscle cells allowing the cephalopods to actively control their appearance by essentially expanding or contracting the sacs of colour. Amazingly, the iridophores can reflect polarized signals that cuttlefish can perceive but that predators cannot. By producing this polarized iridescence from beneath the layer of pigmented chromatophores these animals can accomplish camouflage and signalling simultaneously. For those interested in camouflage, I’ve also learned that a number of sea creatures produce a countershading effect by producing iridescence or even bioluminescence on the undersides of their bodies.

Apart from the mystery font-size changes and video problems on the conference laptop, my talk went tremendously well. I managed to draw a crowd of questioners afterwards and received a great deal of feedback (and encouragement) – sweet. Time to start worrying about how to draw a crowd of peacocks instead.