Biology concepts – circadian rhythm, vision sense, adaptation, parasitism, form follows function
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The sun and the moon are symbols of different activity cycles. As with everything else, we have to give them human characteristics (anthropomorphism). |
Many animals are active in the day or the night, but not both. So what are humans, diurnal (active in the daytime), nocturnal (active in the nighttime), or something else?
Maybe humans are two species, because I know folks who can’t accomplish anything before noon, and do their best work after 11:00 pm, whereas I get up around 5:00 am and am pretty much useless after 8:00 pm.
Whether diurnal or nocturnal, organisms are physically and behaviorally adapted to their activity pattern. This includes the way they sense their environments. Diurnal animals are more likely to have color vision, while nocturnal animals may only see in black and white. The upside for nocturnal animals is greater visual sensitivity, so they can see better than diurnal animals in low light conditions.
The reasons for these different visual talents lies in the types of light receptors on the retina. Rods sense light, but only its presence or absence (white/black). Different receptors, called cones, detect various wavelengths of light (colors). Diurnal animals have about 5-10 times more cones than nocturnal animals (3 types, one for yellow, one for green to violet, and one for red to orange), but they only function in higher levels of light. Therefore, the greater number of rods in nocturnal animals allow for more sensitive night vision, a good thing to have if you are active after sundown.
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Rods (yellowish) and cones (blue) are different light receptors located on the retina. Rods are more numerous and detect low levels of light. Cones are less numerous and sense colors of light, but require more light. As shown in the middle image, the tapetum is located beneath the retina in some animals, and can bounce light back to the retina. This bouncing around is responsible for animals glowing eyes at night. |
Many nocturnal species have an additional adaptation to improve their night vision. Their retina has an iridescent layer called the tapetum lucidum that bounces the available light around so it may hit more rods. This improves sensitivity, but at a cost to acuity (the image gets a little fuzzier). When you shine a flashlight in the woods at night, the little pairs of reflections you see are the tapetum lucida of the animals looking back at you. The light bounces around inside the eye and some escapes back out through their pupils and that is what you see. Some look at your flashlight to see if you are a predator, others look to see if you are worth eating.
But not every animal with a tapetum lucidem is necessarily nocturnal. An interesting new study has looked at the visual system of the Peter’s elephant nose fish (Gnathonemus petersii). This weakly electric fish has a long nose-like appendage that was thought to mediate location and communication through electrical pulses. But scientists at the University of Cambridge have found that this fish has surprisingly good vision to go along with electrical impulse usage.
Humans don’t have a tapetum lucidum, so when reflected light bounces off our retinas and back out the pupils, they appear red like the retinal blood vessels and tissues. This is the eerie red eye effect on some flash photography. I always thought it was a sign of vampirism!
Other nocturnal animals, like many owls, rely on hearing and smell more than vision. They are adapted to maximize these senses. We have discussed previously the changes in owl anatomy (Do You Have To Be Ugly To Hear Well) as examples of form following function to improve hearing. Other animals, like raccoons, have a heightened sense of touch. Their paws have elongated sensor pads, and thousands of touch receptors. With these, raccoons can differentiate textures well enough to tell if a fruit is ripe or not, even in the darkest night.
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Raccoons have a strong sense of touch for moving around in the dark. Their elongated paws have thousands of touch receptors to increase the sensitivity of this sense. On the dorsal (back) side of the raccoon’s paw, whiskers (vibrissae) on the ends of their digits heighten the sense of touch. |
Raccoons don’t even have to touch something to sense it; they have vibrissae (whiskers) on the ends of their digits, above their claws. Whiskers in general are a potent aid to nocturnal animals, whether located on faces, paws, or bodies (remember the naked mole rat’s whiskers on its torso in Take Off Your Coat And Stay A While).
Even plants can be adapted for nocturnal activity. Moonflowers, night-blooming philodendrons, and other flowers that rely on nocturnal pollinators tend to be white (since their pollinators most likely can’t sense color), and strong smelling. Indeed, the increased temperature of the P. selloum spadix (Is It Hot In Here Or Is It Just My Philodendron) is an adaptation to nocturnality.
So why be nocturnal? Anyone who has tried to negotiate an unfamiliar room in the dark knows that being active in the dark brings certain obstacles that must be overcome. There must be distinct advantages to it or needs for it, or else nature wouldn’t go to the trouble of adapting. Some scientists believe that nocturnality arose from originally diurnal organisms taking advantage of an underused ecological niche. Being active at night can be a form of crypsis (hiding), either to make them better hunters, or to avoid being hunted.
Nocturnality can also reduce the amount of water lost to the environment, and can lower the thermal stress on certain species of animals. For example, many frogs lose water through their skin, so daylight and higher temperatures can dehydrate them quickly.
That doesn’t mean that certain species won’t be exceptions. Moths are all nocturnal, except for the polka-dotted wasp moth, that is. There are four species of wasp moths, all diurnal, but the polka-dot is the prettiest, so we will fall into that old trap and give the pretty one all the attention. Diurnally active, this moth has abandoned many of the nocturnal adaptations of its brethren.
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The polka dot moth has color and patterns that might be useful for mating or for warding off other animals, but they would be wasted if the animal was nocturnal. |
For instance, it is beautifully colorful, a no-no for nocturnal moths. Since color doesn’t show up at night, moths are generally white, tan, or grey. Second, the coloration, especially the bright rump, mimics a wasp (hence the name) and warns of a toxic mouthful if consumed. This defense is called aposematism (apo = away from, and soma = body, basically, keep away from me). Many brightly colored insects will make predators sick, purely a diurnal method of survival, as the warning colors would be of no use at night.
Just as this moth species is diurnal when its close relatives are nocturnal, there is a single genus of primate that has chosen to be nocturnal when all others, including humans, are diurnal. Owl monkeys (8 species) live in Central and South America, and leave their sleeping sites about 15 minutes after sunset each day. They forage for fruits and the odd flower or insect until just before sunrise, then retreat to a hollow tree or within dense foliage to sleep away the day.
Owl monkeys adopted a nocturnal pattern after millions of years being diurnal, so it must have afforded them some advantage or was an answer to some overwhelming stressor. They have adapted by developing larger eyes, with more rods and fewer cones. They still see color, but less so than other monkeys.
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The owl monkey is nocturnal, so it needs to have more sensitive vision. For this reason, it eyes (and eye sockets) are huge! Compare the eye size and skull morphology in the diurnal capuchian monkey. Form of the skull follows the functional capacity of the eye. |
Owl monkeys are interesting to science for being the source of another exception, as they are the only primates susceptible to the human form of malaria. In The Perils of Plant Monogamy, we used malaria in chimps and humans as an example of divergent evolution; malaria developed into species-specific forms. But the owl monkey is susceptible to both the primate and human species, so they can substitute for humans in malaria research.
Malaria is caused by a parasite, and as such, depends on its host organism for nutrition. The rule is that parasites are active when their host is active (feeding). A good example is the intestinal parasite of the surgeonfish, E. fishelsoni (Of Fish Guts And Cancer).
As I am sure you have committed to memory and made a part of your life, E. fishelsoni grows to an amazing size and replicates its DNA thousands of times before it divides into two or three progeny organisms. It takes tremendous energy for a bacterium to grow 80 fold and produce 85,000 copies of its DNA in one day, so it must occur when nutrients and carbohydrates are plentiful - during the day when the fish is feeding. Although it is a stretch, I guess you could call E. fishelsoni a diurnal parasite.
The malaria parasite, Plasmodium falciparum, has chosen a different path. P. falciparum’s host is man, and man is diurnal (teenagers and third shift workers excepted), but the parasite works to produce many progeny (gametophytes) and have them mature in the nighttime. The reason is simple; malaria has two hosts.
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Plasmodium falciparum needs two hosts to complete its life cycle. One immature form (sporozoite from oocyst) grows only in the mosquito, while another (gametocyte) forms only from mature sporozoites in the human red blood cells. |
While one stage of the organism grows in the human, another needs to be inside a mosquito in order to complete its life cycle. After finishing its development, it is ready to be injected into another human when the mosquito feeds again. The key is that the mosquito is nocturnal and the gametophyte is short-lived. The gametophyte must be produced and mature just in time to be sucked and deposited into the mosquito gut. P. falciparum has pressured to conform to the activity of one host while it is inside a host with the opposite activity pattern.
It is common that most species within a group will have similar activity patterns, since they are derived from common ancestors and therefore many characteristics are similar, including those that determine fitness for day life or nightlife. But there are exceptions. For instance, most rodents are nocturnal, but we see squirrels all day long - they are diurnal. Also, we mentioned above that most primates are diurnal, but the owl monkeys are nocturnal.
But there are bigger exceptions, organisms that aren’t diurnal or nocturnal. Ants, primates, and cats have species that are all over the place; some are nocturnal, some are diurnal and some are neither. It is the in-betweeners and the neithers that we will talk about next time.
Kreysing, M., Pusch, R., Haverkate, D., Landsberger, M., Engelmann, J., Ruiter, J., Mora-Ferrer, C., Ulbricht, E., Grosche, J., Franze, K., Streif, S., Schumacher, S., Makarov, F., Kacza, J., Guck, J., Wolburg, H., Bowmaker, J., von der Emde, G., Schuster, S., Wagner, H., Reichenbach, A., & Francke, M. (2012). Photonic Crystal Light Collectors in Fish Retina Improve Vision in Turbid Water Science, 336 (6089), 1700-1703 DOI: 10.1126/science.1218072
For more information or classroom activities on activity cycles, night vision or adaptation, see:
diurnal/nocturnal –
night vision –
adaptation –
http://www.nationalgeographic.com/xpeditions/lessons/17/g35/smcreatecreature.html
