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.
7. Navigating by the stars
The constellation of cues animals use to orient and navigate is truly stunning. It can be hard enough to orient and navigate in daylight, but the task becomes even more complicated at night. Even so, organisms have found a variety of ways to accomplish the task. For example, it has been known since the 1960s that migratory songbirds can use star rotation to navigate at night (papers here and here), but what about other nocturnal creatures whose vision isn’t nearly as good as our feathered friends? Marie Dacke and colleagues discovered that a species of nocturnal Dung Beetle (Scarabaeus satyrus) in South Africa can actually use the Milky Way to roll their stinky treat in a straight line! They use the Milky Way because it’s the only reliable signal that is regularly in the night sky, even on mostly cloudy nights. These beetles are the only organism known to use this cue for navigation. A constellation of cues indeed! Read the article here.
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!