Without further ado, here are the next five amazing things I learned while participating in Lund University's Sensory Ecology Course.
5. Through the eyes of a fly
Insect compound eyes are some of the most fascinating and beautiful structures in the animal kingdom, but what does the world look like to them? For all my life, I thought these animals saw the world as a honeycomb. However, Dan-Eric Nilsson shocked me when he told our class that is almost certainly not the way insects perceive the world. Insects are often quite near-sighted, but we have no reason to believe they see the world in honeycomb fashion. All existing evidence points to the fact that their eyes form just one image (albeit, a blurry one), as we do. Did that surprise anyone else, or just me?
4. Homeward bound – path integration in desert ants
A remarkable example of homing behavior comes from a group of unassuming animals, the Saharan Desert Ants (Cataglyphis sp.). These ants leave their subterranean burrows during the day and wander an enormous distance (relative to their size) around the scorching Sahara Desert in search of food scattered randomly on the landscape. As a result, their path while searching for food is lengthy and meandering relative to their return trip, which is essentially a beeline back to the cool safety of their burrow (see figure on right). How do these tiny animals do this in the open desert with no physical or chemical landmarks (i.e., they do not follow a scent trail like other ant species)? They need to integrate two types of information to make the direct journey home: relative angle and distance traveled. In a landmark study published in 1981, researchers found that these ants use celestial cues to determine what angle to travel back home. Check out the video below.
It wasn’t for another 25 years that scientists finally settled how these ants judge the approximate distance they need to travel. Researchers did this by first training Cataglyphis ants to walk from a burrow to a feeding station in an experimental arena. Then, the researchers caught the ants and experimentally manipulated the stride length in some ants to longer strides by attaching miniature stilts (pig bristles) to the ants' legs, made another group of ants’ strides shorter by cutting off the lower half of their legs, and left a third group unmanipulated as a control. Sure enough, the group with longer legs (and thus, longer stride-length) walked right past their burrow, while those with shorter legs walked only part of the way back before beginning a fruitless search for their burrow. The unmanipulated control ants made it back to their burrows perfectly. Finally, after a quarter century, the mystery had been solved; these ants use an onboard pedometer to judge distance, amazing! Read the article here and check out the NPR video summarizing the study below.
3. Pollution stinks
Air pollution has negative effects on humans and animals, no surprise there. What is surprising however, are some of the ways in which air pollution can adversely affect human health. Researchers in Mexico have found that chronic exposure to the air pollution of Mexico City reduces people’s olfactory abilities and trigeminal nerve sensitivity. Individuals tested in Mexico City had a higher detection threshold and more difficultly discriminating between everyday odorants (e.g. coffee, orange drink, horchata) than people living in the nearby, less-polluted state of Tlaxcala. Even more alarming is that people living in Mexico City also had a worse detection threshold and discrimination ability of contaminated food odors (e.g. spoiled milk), which could lead to food poisoning or other food-borne illnesses. Further experiments testing how chronic, non-occupational exposure (i.e., non-miners) to airborne manganese in the mining region of Molango, Mexico showed similar negative effects on olfactory performance and trigeminal nerve sensitivity. This indicates reduced trigeminal nerve function, which could be an early warning sign of neurological damage because unlike other toxic metals, manganese is transported transynaptically to structures deep within the brain. Global regulations need to be enacted on these air pollutants before overwhelming adverse effects on human health become commonplace.
2. Electric feel – bees' electric sense
Bees are remarkably resourceful little creatures. In 1973, Karl von Frisch won a Nobel Prize for decoding the honey bee’s waggle dance used by a returning forager to alert other bees in the hive to the relative angle (to the sun) and approximate distance to a food source, such as a patch of flowers. However, once a bee – alerted by the waggle dance of a hivemate – arrives at the flower patch, how does it choose which flower(s) to visit? Daniel Robert and colleagues at the University of Bristol recently discovered that they might select flowers to visit based on the flower’s electro-static charge. The theory goes like this: when bees fly through the air, they accumulate a positive charge, similar to what happens to a flying airplane. Since the flowers are grounded and have a slightly negative charge, when a positively-charged bee lands on a negatively-charged flower, some of the bees' positive charge is transferred to the flower. When other bees visit that same flower in the near future, they can detect the higher charge of the just-visited flower (relative to unvisited flowers) and choose to avoid it, since it would have less nectar than an unvisited flower. These are the first insects shown to have and potentially use their electric sense. This means that bees may integrate at least four different kinds of information (vision, olfaction, social, electrical) while foraging; multi-modal foraging at it’s best!
1. Good vibrations – seismic communication in vertebrates
Of all sensory modalities, I find the ones humans either don't possess (magnetoception, electroreception, etc.) or have very limited abilities in (e.g. olfaction) most fascinating. Last year, Robert Raguso gave a lecture at UC Davis where he described these signals as an invisible language just waiting to be decoded. Another example of this is substrate-transmitted seismic signals, which are imperceptible to us, but very important for animals specialized to detect them. Peter Narins has made a career investigating seismic communication. His work was the first to show that a vertebrate (male Gunther's White-lipped Frogs, Leptodactylus albilabris) incorporated ground-transmitted seismic signals in their display call, which unlike the auditory portion of their call, is used for male-male communication.
Probably the most adorable example of an organism using seismic signals is the Namib Desert Golden Mole (Eremitalpa granti namibensis). These small, mammals are functionally blind and when they emerge from their burrows at night to forage, they literally swim through sand (see video below). Until Narins' group investigated the problem, no one was knew for sure why these animals foraged this way. It was found that the moles can actually detect vibrational differences produced by wind passing through mounds of dune grass, which is where the moles find their termite prey. Additionally, the researchers looked into the strange inner ear morphology of these animals and discovered they have the largest malleus (relative to their body size) of any animal known to science, which they believe is used to detect subtle vibrational changes in the substrate. This amazing feat of bioengineering is now being used to develop even more advanced earthquake detection systems. So if you ever find yourself asking, “why are we funding basic science, like that on the seismic sense of the golden mole?” One reason is because basic science can become applied science in the blink of an eye. Another reason is because, as I’ve hopefully convinced you, animals (including humans!) are awesome, and we want to figure out how and why they do the bizarre and amazing things they do.
Phew, that was exhausting, but I hope you found it worthwhile! I learned so much about sensory ecology in this class and hopefully passed some of that on to you. If you want to learn more about sensory ecology, check out the Nevitt Lab's Sensory Ecology Resources Page.
Recently, I was lucky enough to participate in Lund University's Sensory Ecology Course. This international post-graduate course is held from late September through early October on even years only (i.e. next course is fall 2016). The course recruits experts from around the world, each specializing in a different sensory modality (e.g. vision, olfaction, audition, electro-sensing, etc.) to train eager young scientists in the field of sensory ecology. I think I can speak for others in the class by saying, we learned a great deal and made contacts that will last a lifetime, and were lucky enough to be hosted by the world-renowned and extremely hospitable Lund Vision Group. My labmate, Brian Hoover, actually gave me the idea for this post at the end of the course. So, in no particular order, here are the 10 most amazing things I learned. I will write about five this week, and the other five next week.
10. Why do giant squid have giant eyes?
Can you guess which animal has the largest eye? The title of this bullet point gives it away but, it’s not an elephant or even a whale, it’s actually a giant squid. Measuring nearly 30 centimeters across with a pupil 9 centimeters diameter, giant squid eyes are roughly three times larger than the next largest eye in the animal kingdom (that of the Swordfish). Considering eyes are one of the most metabolically expensive structures to produce and maintain, why have eyes so large? Is it to find their prey in the deep ocean? To find a mate in the dim light? Nope! Scientists believe that giant squid have such massive eyes to be able to see the faint bioluminescence of meso and bathypelagic plankton, which light up when they are disturbed by a passing Sperm Whale, the squid’s main predator. It appears the giant eyes evolved to try and help the squid avoid becoming a giant serving of calamari! Read the article here.
9. Bats “see” with sound
One thing impressed upon me by Annemarie Surlykke is that bats, which often get publicized as blood-drinking disease vectors, are finely-tuned auditory machines. To locate prey, they use echolocation, producing a series of loud (up to 140dB at the source; for reference, a jet airliner taking off 25 meters away is ~150dB) high-frequency (up to 20-80 kHz; humans can hear up to ~20kHZ) clicks and the returning echoes to locate prey or navigate in total darkness. Insectivorous bats’ hearing is so sensitive that they can use the Doppler shift generated by the returning echoes from beating insect wings to differentiate potential prey items from inanimate objects, such as buildings, trees, and leaves. Perhaps most amazingly, bats can alter the qualities of their echolocation clicks situationally. For example, by adjusting their mouth opening, a bat can alter the frequency and directionality of echolocation clicks, actively changing both while they hunt. Searching bats produce directional, evenly-spaced (temporally), high-intensity (dB) clicks. Once a bat has located a potential prey item, it clicks more frequently to increase the amount of detailed information about the target, then captures the item by widening its echolocation beam and reducing the intensity (dB) of it’s echolocation clicks to reduce far off echoes from distracting the bat from its target. Watch it in slow motion here. Unlike humans that have static vision (for example, we can’t widen our field of view or adjust how far away we can see), bats have dynamic auditory “vision” by being able to alter the temporal spacing, intensity, frequency, and beam-width of their echolocation clicks. So think twice the next time you utter the phrase “blind as a bat”. Check out Annemarie's Lab Website for more information.
8. A brief history of vision
According to vision expert, Dan-Eric Nilsson, organisms’ eyes are just about as good as it can get in terms of their physical properties. Advanced eyes with a camera-type lens (like ours) and a highly specialized region of the retina for detailed photon reception (in our case, the fovea) can produce images as sharp or sharper than the best cameras or machines ever produced. If vision is to improve over future evolutionary time, it is likely the advancements will be made in neurological image processing (i.e. transduction), rather than physical eye-design. What’s more, the evolution of vision was likely a very punctuated event. Meaning that the development of the first visual pigments and the ability for single-celled organisms to detect and respond to ambient light conditions (the simplest form of vision), all the way to complex, image forming, camera lens-type eyes occurred during a fairly discrete time period, beginning approximately 800 million years ago and concluding around the end of the Cambrian Explosion, roughly 500 million years ago. There has been limited development of vision, relatively speaking, in times before or since that ~300 million year period. Considering there has been life on earth for ~3.5 billion years, the fact that the majority of visual development happened over a < 10% period of that time is pretty astonishing.
6. Swimming with an internal compass
After hatching out of their eggs on beaches in the southeastern United States, young Loggerhead Sea Turtles (Caretta caretta) undertake a massive journey over the next 5-10 years of their lives, circling around the entire North Atlantic Gyre. How do they make it around this visually-featureless landscape back to the same area where they took their first strokes in the ocean nearly a decade earlier? Well, Ken Lohmann and colleagues discovered that one way these turtles navigate on such enormous spatial scales is via the earth’s magnetic field. These turtles can use the varying magnetic field strength and inclination angle to figure out which direction to swim. By systematically removing all other sensory cues, it has been shown experimentally that if young Loggerhead Turtles are magnetically displaced, they tend to swim in the direction they would need to swim to stay within the North Atlantic Gyre, thus keeping them on their proper migratory trajectory. The exact mechanism by which these turtles (or any animal known to use a magnetic compass) detect and orient to magnetic fields remains unknown.
Stay tuned next week for amazing facts #s 5-1!
My research on marine plastic debris is featured in the most-recent bulletin of the American Fisheries Society (Davis' subunit), put together by my good friend and superb sturgeon-surgeon, Emily Miller. Check out the bulletin describing the work (and other fish-related research going on at UC Davis) here. And if you're curious, stay tuned for more results from my summer's research coming up soon!
Matthew Savoca holds a PhD in Ecology from the University of California, Davis. His research interests include sensory behavioral ecology, marine conservation biology, and seabird ecology.