Reptilia

~Annalise Ritter



Green Iguana
Green Iguana

Diagnostic characteristics

Reptiles are a class of the Kingdom Animalia. They are characterized by their skin covered in scales that contain the protein keratin. These scales make reptiles' skin waterproof. The dry scaly skin of reptiles makes gas exchange impossible, so repiles rely on their lungs for oxigen. (6RM) In addition, they are characterized by their skulls with a single occipital condyle, lungs and a 3 or 4 chambered heart. (EK) Reptiles are ectotherms, meaning they manage their body heat with outside sources rather than internal sources (to be explained later). There are about 6,500 (?) species of reptiles, which fall into four orders: Testudines (turtles and tortoises); Sphenodontia (tuataras); Squamata (snakes and lizards); and Crocodilia (crocodiles and alligators). Also, the majority of reptiles are oviparous, egg-laying. (9-SC)
Typical anatomy of a lizard (DO)
Typical anatomy of a lizard (DO)


The video above discusses reptilian locomotion, temperature regulation, and self-protection (1 MB).


Acquiring and Digesting Food
Reptiles take in food through their mouths, whether they chew it or swallow it whole. Some reptiles are carnivorous, mainly Crocodilia and snakes, though most lizards and turtles eat insects also; some eat vegetation as well. Carnivorous diets require hunting for food, whether it is insects, small mammals, or even large animals in the case of crocodiles. Most reptiles use camouflage and lie in wait for prey rather than chasing it down.

Snakes, because they cannot tear up their food, have the incredible ability of expanding their jaws to swallow their prey whole (SW 17). After a snake eats, it becomes dormant because digestion is an extremely intense activity (SW 17). If it is disturbed during digestion, a snake will often regurgitate its prey in order to escape from a perceived danger (SW 17). Some snakes are equipped with poisonous toxins delivered through their sharp teeth, which kills or stuns their prey before they swallow it. Snakes have developed changes in oral glands and venom glands that aid in the immobilizing prey and swallowing prey. The salivary glands found in snakes include the palatine, lingual, sublingual and labial gland. These glands help moisten the prey for swallowing. In venomous snakes, poison glands are modifications of the labial glands, which immobilize the prey before consuming it. (AW 10)

Food is digested through a digestive system similar to most animals, with a stomach, intestines, etc.


Sensing the Environment
external image lr001728.gif Smell is likely one of the most important senses to the majority of reptiles. Snakes, for example, flick their tongues to direct odors to olfactory receptors on the roof of the mouth. Snakes are also very sensitive to ground vibrations, rather than having acute hearing. An additional characteristic of snakes is the ability to sense heat in the surroundings, meaning that snakes can detect warm-blooded animals. This is made possible by heat sensative "pits" that are extra sensative to infrared radiation or heat. (EK23) Crocodiles, lizards and turtles also rely largely on smell. Vision is relatively important as well; water-dwelling reptiles have adapted eyes that see underwater with special coverings for protection.


Locomotion
The majority of reptiles are tetrapods, organisms that possess two pairs of limbs. Most lizards are strictly land creatures, so they use their legs merely to walk; however, water-dwelling animals like crocodiles and turtles have limbs adapted to propelling the body through water. In an extreme case, sea turtles have acquired flippers in place of legs, as these creatures spend almost all of their lives underwater.
Snakes are unique in that they have no legs, and move by flexing muscles along their long, thin bodies. Smooth scales facilitate motion of this sort. Despite the difference in structure and movement of snakes versus other reptiles, there is evidence in vestigial pelvic and limb bones in primitive snakes that snakes evolved from reptiles with legs—probably lizards.


Respiration
Since reptiles cannot breathe through their keratinized dry skin, they must use lungs to acquire all of their oxygen. Many species of turtles supplement gas exchange across the wet surface of the cloaca, a cavity at the end of the digestive system for excretion of waste and present in most birds, reptiles, and amphibians; and the inside of the mouth.
The majority of reptiles simply have a common respiratory system with lungs and alveoli to gather air and blood vessels to transport the oxygen to each cell by use of red blood cells. Most reptiles have an unusual breathing pattern in which long pauses follow a series of inspirations (inhaling), and expirations (exhaling); since constant lung ventilation in not needed, reptiles have between one-fifth and one-tenth the metabolic rate of birds and mammals (12 J Stein).


Metabolic Waste Removal
Reptiles’ waste removal system consists of filtration and selective reabsorption involving kidneys and liver. The kidneys are made up of cortical nephrons, which are relatively short compared to some of the nephrons found in mammals, so reptiles produce urine that is mostly isoosmotic to (have the same concentration as) body fluids. The epithelium of the cloaca helps conserve water by absorbing extra from feces and urine. Most terrestrial reptiles excrete nitrogenous wastes in the form of uric acid.


Circulation
Reptiles possess, like other vertebrates, a closed circulatory system otherwise known as the cardiovascular system. This includes a three-chambered heart (four-chambered in Crocodilia) with right and left atria and one partially divided ventricle. Blood is pumped out of the right atrium and circulated to the lungs to absorb oxygen, and then enters the left atrium and is pumped out to the rest of the body to deliver the oxygen. The oxygen-depleted blood is then returned to the right to begin the cycle again. Mixing of oxygen-poor and oxygen-rich blood in the ventricle is limited, allowing for relatively efficient circulation.
reptile_heart_a.jpg
Three-Chambered Heart (CS 11)



Self-Protection
There are many methods of self-protection among reptiles. These include thick scales; hard outer shells on turtles; claws, teeth and spines; and sometimes venom in certain snakes, though it is mostly used for hunting. Many species use camouflage to hide themselves among foliage, rocks, etc.; one of the most well known examples of this is the chameleon, in the order Squamata, which changes its color to match the hues of its surroundings.
gilalizard.pnga camouflaged gila lizard-the world's largest lizard (13 AN)


Osmotic Balance
Reptiles are osmoregulators, meaning that they must internally control their osmolarity to avoid excess water loss to the environment. Thick skin and scales help decrease the energy expended on maintaining osmotic balance, but terrestrial vertebrates must use active transport to control concentration levels and keep from dehydrating. Marine reptiles drink seawater, excrete isotonic urine, and eliminate excess salt through their salt glands (5 JSun).


Temperature Balance

basking_turtle.jpg
A basking snapping turtle (DPOD 17)

Being ectotherms, reptiles rely on sufficient environmental conditions to regulate body temperature—they absorb external heat rather than generating it internally through the breakdown of food. This reduces the number of calories reptiles need to intake, but it also limits the amount of suitable habitat temperature-wise compared to that of mammals. Behavioral adaptations, such as basking in the sun to increase body heat and seeking shade to reduce it, help them stay at optimal body temperature. Certain species have adapted other means of regulation; for example, the marine iguana of the Galapagos Islands, while swimming in the ocean, conserves body heat through vasoconstriction of superficial blood vessels, which directs more blood to the center of its body.


The Reptilia/Aves Debate (EG 11)
It is common knowledge among biologists that mammals and reptiles all shared a common ancestor. The debate, however, is whether birds are part of the reptilia clade (and should or could be consitered an order of reptilia) or whether they are part of the mammal clade (see image below).
25-16-AnalogyVsHomology-Leditblank.GIF
The biggest difference between birds and reptiles is their hearts (birds have 4-chambered hearts, like mammals, whereas reptiles have a 3-chambered heart); however, there may be more similarities between birds and reptiles than there are differences (or than there are similarities between birds and mammals). For example, both birds and reptiles have scales (for birds it's on their feet, and is called scutes), lay eggs, and have skeletal similarities. Birds also have feathers, from which scales may have formed. According to research, aves' DNA is less similar than reptilia's DNA to that of placental mammals. The argument that birds evolved from reptiles is partially based on the belief that the four-chambered heart was formed in both birds and mammals due to convergent evolution (rather than a trait from a direct common ancestor). This adaptation is not out of the question, as 3-chambered reptile hearts (with the exception of crocodiles) have an incomplete wall inbetween the ventricles (the wall is complete in mammals and birds, who require the division of oxygenated and deoxygenated blood to support their active metabolism), so even that difference, although important, is not that drastic.
external image blood.jpg external image Reptileheart.gif
A Bird Heart (left).....................................................A Reptile Heart
The recent discovery of the Archaeopteryx fossil has triggered fresh debate about the relationship between reptiles and birds because while the Archaeopteryx has feathers, wings, and a beak like a bird, it also has teeth in the bill, claws on the wings, no keel on the breast bone, an unfused backbone, and a long, bony tail as reptiles tend to have. (AS 20) Some scientists counter this by stating that the lungs of the Archaeopteryx are incapable of sustaining endothermic gas exchange requirements, thus could not have given rise to the lungs of modern birds. (13 AL)

For more information on the other side of the debate, click here: Chordata-Aves

Review Questions
1) What do reptiles use to acquire their oxygen? (10 DC)
2) What methods of self-protection are used by reptiles? (1 VK)
3) Explain how reptiles regulate their temperature compared to mammals. (HL 11)
4) What makes the reptiles' skin so unique? (CH)
5) Explain two ways a snake can sense the environment. (KA)
6) What pieces of evidence show that birds are descended from reptiles (14t2)?
7) Explain how reptiles bodies regulate what climates they can successfully live in. (JE)

Sources:
1) Campbell, Neil A., and Jane B. Reece. Biology. Sixth Edition. San Fransisco: Pearson Education, Inc, 2002.
2) Parker, Gary. "Vertebrates: Animals with Backbones." Answers in Genesis, 2009. Web. 1 Nov. 2009. <http://www.answersingenesis.org/home/area/cfol/ch3-vertebrates.asp>.
Image 1: http://animals.nationalgeographic.com
Image 2:
1) "Reptilian Characteristics." 24 Oct. 2009 http://www.youtube.com/watch?v=oBz00-Go3qg
2) http://www.economicexpert.com/a/Reptilia.htm
3) http://occawlonline.pearsoned.com/bookbind/pubbooks/campbell6e_awl/chapter25/deluxe.html
4) http://www.dinosauria.com/jdp/archie/scutes.htm
5. http://www.discoverseaz.com/Wildlife/GilaMons.html
5) http://vivekmittal.wordpress.com/2008/10/23/are-birds-close-to-reptiles/
6) http://www.springerlink.com/content/q016n88302g81x22/
7) http://library.thinkquest.org/C003758/Development/reptile.htm
8) http://www.earthlife.net/birds/images/anatomy/blood.jpg
9.http://animaldiversity.ummz.umich.edu/site/accounts/information/Reptilia.html
10. "Digestive System of Snakes." 2 Nov. 2009 <http://campus.murraystate.edu/academic/faculty/terry.derting/cva_atlases/Stephsnake/snakepage3.htm>.
11. "Snake." Wikipedia.org. Wikipedia, the free encyclopedia. Web. 3 Nov. 2009. <http://en.wikipedia.org/wiki/Snake>.
12. "Reptile Respiration." The Encyclopedia Brittanica Online Encyclopedia. 4 Nov. 2009 <http://www.britannica.com/EBchecked/topic/499513/respiration/66212/Reptiles>.
13. http://mathphysicschemistrybiology.com/images/reptile_heart_a.JPG
14. http://media.photobucket.com/image/turtles%20basking/joshsnakeman/2007%20pictures/2007-05-18%20ispe/P1040332-Copy.jpg (DPOD 17)