What do jumping spiders find sexy? How DIY tech is offering insights into the animal mind.

In his quest to understand the hermit crab housing market, biologist Mark Laidre of Dartmouth College had to get creative. Crabs are always looking to move into a bigger, better shell, but having really nice digs also comes with risks. Sometimes crabs gang up to pull an inhabitant out of an especially desirable shell. If they succeed, the shell is quickly claimed by the largest gang member, leaving another open shell for a slightly smaller crab to grab, and on down the chain until everyone has upgraded.

To better gauge the trade-offs between shell size and defensibility, Laidre collaborated with an engineer to create the hermit crab eviction machine, a device that holds onto an occupied shell and measures how much force it takes a scientist to pull the crab out (crabs are not harmed or left homeless). It's essentially a portable load cell that can survive the sun, sand, and humidity of the field.

The force required to evict a hermit crab is an important measurement, because hanging on to their homes is a matter of life and death for crabs. If you are evicted, there's a real strong probability that what is left at the end of one of those chains is something that's too small for you to even enter," Laidre says. In his field area on a beach in Costa Rica, a homeless crab can quickly succumb to predators or heat: You're really dead meat in a sense."

Studying the minds of other animals comes with a challenge that human psychologists don't usually face: Your subjects can't tell you what they're thinking. To get answers from animals, scientists need to come up with creative experiments to learn why they behave the way they do. Sometimes this requires designing and building experimental equipment from scratch.

The DIY contraptions that animal behavior scientists create range from ingeniously simple to incredibly complex. All of them are tailored to help answer questions about the lives and minds of specific species, from insects to elephants. Do honeybees need a good night's sleep? What do jumping spiders find sexy? Do falcons like puzzles? For queries like these, off-the-shelf gear simply won't do.

The eviction machine was inspired by Laidre's curiosity about crabs. But sometimes new questions about animals are inspired by an intriguing device or technology, as was the case with another of Laidre's inventions: the hermit crab escape room (more on that below). The key, Laidre says, is to be sure the question you're asking is relevant to the animals' lives.

Here are five more contraptions custom-built by scientists to help them understand the lives and minds of the animals they study.

OLY DEMPSTERThe falcon innovation box

The brainy birds in the parrot and crow families are the stars of scientific studies on avian intelligence. Now these smarties have a surprising new rival: a falcon. Raptors are not known for creative problem-solving, but behavioral ecologist Katie Harrington of the University of Veterinary Medicine Vienna suspected the striated caracara falcons she had observed on a remote Falkland Island were different. They're really interested in investigating things," she says. They're very intelligent birds in general."

HARRINGTON, ET AL.

To test their smarts, Harrington took inspiration from an innovation arena" (left), designed for Goffin's cockatoos, which are members of the parrot family known for their problem-solving abilities. It's a semicircular array of 20 clear plastic boxes containing puzzles requiring different solutions to release rewards like cashews or corn kernels. Hauling the seven-foot-wide arena to the Falklands was not an option. So Harrington designed a 16-inch-wide innovation box" attached to a wooden board, with eight compartments and puzzles adapted from the cockatoo studies.

The birds loved it. We were having caracaras run full speed to participate," Harrington says. The challenge was keeping other birds away while one worked the box. The birds were able to solve the puzzles, which involved things like rattling a plank to knock down a bit of mutton or pulling a twig out from under a platform with mutton on it. They were even able to solve a tricky one that required them to punch a hole in a piece of tissue that obscured the treat-a task that eluded some cockatoos.

In fact, 10 of 15 falcons solved all the puzzles, most of them within two sessions with the box. So Harrington designed eight new, harder tasks, but soon learned that some required unnatural movements for caracaras. She plans to keep trying to find tasks that reveal what they're physically and mentally capable of. They're totally willing to show us," she says, as long as we can design things that are good enough to allow them to show us."

The raccoon smart box

Why are raccoons so good at city living? One theory is that it's because they're flexible thinkers. To test this idea, UC Berkeley cognitive ecologist Lauren Stanton adapted a classic laboratory experiment, called the reversal learning task. For this test, an animal is rewarded for learning to consistently choose one of two options, but then the correct answer is reversed so that the other option brings the reward. Flexible thinkers are better at reacting to the reversals. They're going to be more able to switch their choices, and over time, they should be faster," Stanton says.

To test the learning skills of wild urban raccoons in Laramie, Wyoming, Stanton and her team built a set of smart boxes" to deploy on the outskirts of the city, each with an antenna to identify raccoons that had previously been captured and microchipped. Inside the box, raccoons found two large buttons-sourced from an arcade supplier-that they could push, one of which delivered a reward. Hidden in a separate compartment, an inexpensive Raspberry Pi computer board, powered by a motorcycle battery, recorded which buttons the raccoons pushed and switched the reward button as soon as they made nine out of 10 correct choices. A motor turned a disc with holes in it below a funnel to dispense the reward of dog kibble.

Many raccoons-and some skunks-were surprisingly eager to participate, which made getting clean data a challenge. We had multiple raccoons just shove inside the device at the same time, like, three, four animals all trying to compete to get into it," Stanton says. She also had to employ stronger adhesive to hold the buttons on after a few particularly enthusiastic raccoons ripped them off. (She had placed some kibble inside the transparent buttons to encourage the animals to push them.)

Surprisingly, the smart boxes revealed that the shyer, more docile raccoons were the best learners.

The jumping spider eye tracker

The thing about jumping spiders that intrigues behavioral ecologist Elizabeth Jakob is their demeanor. They look so curious all the time," she says. Unlike other arachnids, which spend most of their time motionless in their web, jumping spiders are out and about, hunting prey and courting mates. Jakob is interested in what goes on inside their sesame-seed-size brains. What matters to these tiny spiders?

BARRETT KLEIN

For clues, Jakob watches their eyes, particularly their two principal ones, which have high-acuity color vision at the center of their boomerang-shaped retinas. She uses a tool evolved from an ophthalmoscope that was specially modified to study the eyes of jumping spiders more than a half-century ago. Generations of scientists, including Jakob and her students at UMass Amherst, have built on this design, slowly morphing it into a mini movie theater that tracks the retinal tubes moving and twisting behind the spiders' principal eyes as they watch.

A spider is tethered in front of the tracker while a video of, say, a cricket silhouette is projected through the tracker's lenses into the spider's eyes. A beam of infrared light is simultaneously reflected off the spider's retinas, back through the lenses, and recorded by a camera. The recording of those reflections is then superimposed on the video, showing exactly what the spider was looking at. Jakob found that just about the only thing more interesting to a jumping spider than a potential cricket dinner is a black spot that is growing larger. Could it be an approaching predator? The spider's lower-resolution secondary eyes catch a glimpse of the looming spot in the corner of the video screen and prompt the primary eyes to shift away from the cricket to get a better look.

Jakob's eye tracker has also inspired other scientists' creative experiments. Visual ecologist Nate Morehouse of the University of Cincinnati used the tracker to reveal that females of one jumping spider species aren't all that interested in male suitors' flashy red masks and brilliant green legs-it's the males' orange knees that they focus on during courtship displays. To get this insight into what they actually care about is really cool," Jakob says.

The hermit crab escape room

Hermit crabs won't just settle for the best empty snail shell they can find-they also remodel their homes. Hermit crab shells get better with time as each subsequent inhabitant makes home improvements, like widening the entranceway or carving out a more open, spacious interior.

Dartmouth's Mark Laidre has been studying crabs and their shell preferences for more than a decade. So when he realized he could use a micro-CT x-ray machine to create a three-dimensional digital scan of a shell, he immediately began envisioning the experimental possibilities. To better understand the choices crabs make, he scanned shells that crabs clearly favored and then made alterations before 3D-printing them in plastic. We could add little elements onto those that changed the external or the internal architecture," Laidre says.

Next, he presented crabs with a dilemma. They were placed alone inside a box with a small exit (as shown below) and given a choice between two shells: a really nice, spacious model but with spikes added to the outside so that the crabs would not fit through the exit, and a shell that they would fit through but with uncomfortable spiny protrusions added to the inside. Could they figure out how to get out? It's effectively an escape room," Laidre says.

When not trapped, crabs preferred the comfy shell with protrusions on the outside, claws down. But hermit crabs are social animals that prefer to be with other crabs, giving them motivation to escape solitary confinement. By the end of the day, more than a third of the trapped crabs had sized up their situation, moved from the crummy shell, and escaped.

Solving a completely novel problem takes a certain amount of mental wherewithal that crabs don't often get credit for. And Laidre suspects that cognitive capability may be what separated the successful escapees from the crabs that didn't make it out of the escape room.

The bee insominator

Sleepy people tend to be poor communicators. Entomologist Barrett Klein of the University of Wisconsin-La Crosse wanted to know if the same was true for drowsy honeybees. These social insects have a sophisticated communication system, known as the waggle dance, to convey to other bees where to find nectar. Are tired bees worse wagglers? To find out, Klein needed a way to keep bees up all night.

A metal disc attached to the back of a bee, seen on the right side of the photo, is painted yellow to hide whether it is made of steel that will be jostled by the magnets to keep her awake, or copper that won't react to the magnets.BARRETT KLEIN

He thought of shaking the hive, but this would just send all the bees angrily flying out. He wanted to keep some bees from sleeping while the rest slumbered peacefully, so that their dances could be compared the next day. Klein considered putting individual bees in vials that would be periodically shaken, but he couldn't be sure if changes in their dance were due to sleepiness or isolation. He also thought of poking bees, aiming streams of air at individual bees, or even shining focused infrared beams at their faces. Try to do that on all these bees facing all different directions," Klein said. It would be insane."

Eventually he landed on using neodymium rare earth magnets to jostle bees that had metal wafers glued between their wings with pine resin. I had to make a hive that was narrow, with only two-millimeter-thick glass on either side, and have the magnets very close but not touching or scraping the glass," Klein says. The biggest catch with this contraption-dubbed the Insominator-was that Klein had to stay up all night rolling the banks of magnets back and forth alongside the hive three times a minute, depriving himself of sleep along with the bees.

But it paid off: He found that sleepy bees are indeed sloppy dancers. They did shorter dances that were less accurate with direction-a miscommunication that could send hivemates on a flowerless search. In a follow-up study, Klein showed that other bees were not impressed with the drowsy displays and would promptly leave to find better wagglers.

Happily, he has since upgraded the Insominator to automatically roll the magnets.

Betsy Mason is a freelance science journalist and editor based in the San Francisco Bay Area.