Tuesday, March 29, 2011
Making Scents
In chapter 8, Neil Shubin discusses the differences between "water-based receptors in [a fish's] nasal neurons" and the "air-based ones" (Shubin 145) in mammals and reptiles. Elaborate more on the specific differences between these two receptors. Is there some sort of advantage between these two different apparatuses? Is one able to be more accurate or identify more odors than the other? What would be the possible evolutionary advantage of each type of receptor? Kathy Li, kathy2132@gmail.com
Subscribe to:
Post Comments (Atom)
Smell, or the ability of neuron receptors to send a signal to the brain based on the molecule it receives is an important ability to organisms whether they are predator or prey. As Neil Shubin said, reptiles and mammals do have “air-based” receptors, but even those differ. Mammals have three types of glands that can pick up scent, sweat glands, scent glands and mammary glands; however, scent and mammary glands are just modified sweat glands. Mammals can have scent glands in different locations, such as male deer that have them on the lower legs or elephants that have them behind their eyes (http://www.earthlife.net/mammals/smelly.html). The main way that reptiles can smell is using Jacobson’s organ. This organ is a chemoreception organ found in amphibians, reptiles and some mammals and is vestigial in humans. Snakes and other reptiles flick substances into Jacobson’s organ using their tongue(http://chemistry.about.com/cs/medical/a/aa051601a.htm). In this way, the reptile smelling capability is less air based than the mammalian one.
ReplyDeleteFish, on the other hand, use a different system for smelling. They have holes, which look like nostrils, but are called Nares. Fish do not breathe through these holes and they don’t lead to the throat like in mammals, they do lead to sensory organs that pick up the chemicals and send the signal to the brain (http://www.seaworld.org/aquademics/tetra/all_about_fish2.htm). Since the nose in a fish is only used for smelling, it is very good at it as seen with sharks who can pick up one part fish scents per billion parts (http://animals.howstuffworks.com/fish/shark-senses1.htm).
In effect, fish can smell much farther and more accurately than land animals. The purpose for smelling, is however the same in both. Finding prey or staying away from predators, or simply for aromatherapy (http://www.thewholedog.org/id36.html), it seems that smell is sort of a universal language for animals.
(Alex Sapozhnikov marijio@gmail.com)
As Alex stated, the ability to smell and recognize different scents and odors is critical to the lives of all animals. As stated by the Campbell textbook, many animals such as bees, ants, and moths, rely on these chemical receptors and substances to mate, navigate during migration, or trace territory(Campbell). As explained by Encyclopedia Britannica, "Proteins capable of binding odor molecules that play a central role in the sense of smell", are called olfactory receptors, and are found in their active forms in the nasal area of animals(Encyclopedia Brittanica online). These receptors interact with odor molecules that are inhaled to send signals to the brain in a lock and key model, where the lock is the odor molecule, and "the key is the receptor on the nerve cell"(Shubin 140). When the receptor attaches to the odor molecule, it sends chemical signals to the brain which we observe as a certain smell. Due to the fact that water and air have different chemical make-ups, terrestrial animals have air-based receptors, and aquatic creatures possess water-based receptors. Because "the chemical reaction between odor molecule and receptor is different in water and air", terrestrial species have different receptors in their nasal neurons than from aquatic animals. Thus, it is not very fair to give the advantage to either type; both variations of the receptors have specific purposes. The water-based receptors allow for fish to smell odors in the water, while the air-based receptors allow humans and other land animals to recognize particular smells. In addition, there is a correlation between the fact that aquatic animals that have a very small number of receptors; amphibians have more, and yet humans and mammals possess thousands of genes for olfactory receptors. Thus, we can trace back evolution to see that at some point, there was a split and one group of animals developed water-based receptors while another group developed air-based receptors.
ReplyDeleteAlthough neither one is more effective or efficient, there is a connection between the number of olfactory receptor genes that an animal has, and how acute their sense of smell is. Neil Shubin states that the humans and other mammals exhibit thousands of these genes, while reptiles exhibit less, and fish even less. This can be linked to the theme of evolution that over time, animals that had the air-based receptors developed higher numbers of genes in their respective genomes, which allowed for them to have a selective advantage over other organisms: a stronger sense of smell. Consequently, all other animals that did not develop large numbers of genes that create olfactory receptors either died, or were unable to transition from water to land. However, water receptors could be advantageous if life on land became impossible, and would thus allow for animals with water-based olfactory receptors to survive better than humans and mammals with air-based receptors.
In addition, Shubin states that mutation to some of these genes beyond repair, and that explains why some mammals such as dogs, have better senses of smell than humans. This is also the reason that dolphins and whales no longer us their nasal passages to smell, but have actually adapted them into blowholes for breathing. Again relating to the theme of evolution, mutation modifies and crops out the genome generation after generation. If a mutation knocks out a useless gene, these genes continued to be passed down generation after generation, and thus explains by even after all of evolutionary time, some mammals have less developed senses of smell, even though they may have more of the specific genes for smell.
cont. in next post
Matt Kim(matthewkim0803@gmail.com)
Finally, although neither air-based nor water-based olfactory receptors are more accurate, I believe that it would be more advantageous in terms of evolution to have air-based olfactory receptors. This is because over evolutionary history, the animals that developed stronger senses of smell by duplicating olfactory receptor genes have been able to transition from an aquatic environment, to full adaptation to terrestrial life. Since fish such as lampreys and hagfish have less genes and thus, a worse sense of smell, they would have difficulty surviving on land. The division of receptors from a combination of air and water to exclusively water and air receptors allowed for certain species to transition to land, which has provided for much further evolution from ancestors than seen in aquatic species.
ReplyDeleteMatt Kim (matthewkim0803@gmail.com)