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.
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.