Importance of Reptiles
The chief impact of reptiles, such as lizards, on humans is positive as they are significant predators of pest species. Snakes are also very useful rat exterminators, for example, in the Irula villages of India.
Reptiles can be important as food sources: green iguanas are eaten in Central America, the tribals of “Irulas” from Andhra Pradesh and Tamil Nadu in India are known to eat some of the snakes they catch, Cantonese snake soup is consumed by local people in the fall to prevent colds, cooked rattlesnake meat is commonly consumed in parts of the Midwestern United States, and turtle soup is widely consumed.
Reptiles also make good pets. Numerous lizard species are prominent in the pet trade. In the Western world, some snakes, especially docile species such as the ball python and corn snake, are kept as pets. Turtles, particularly small terrestrial and freshwater turtles, are also commonly kept as pets. Among the most popular are the Russian tortoises, Greek spur-thighed tortoises and red-ear sliders (or terrapin).
For medical and scientific research, snake venom collected by the “Irulas” is used for producing life-saving antivenin and for other medicinal products. Observations about turtle longevity (the liver, lungs and kidneys of a centenarian turtle are virtually indistinguishable from those of its immature counterpart) have inspired genetic researchers to begin examining the turtle genome for longevity genes.
Finally, reptiles play a significant role in folklore, religion and popular culture. Lizard symbology plays important, though rarely predominant roles in some cultures (e.g. Tarrotarro in Australian mythology). The Moche people of ancient Peru worshipped animals and often depicted lizards in their art. Crocodilians have starred in several science fiction movies such as Lake Placid and DinoCroc. There are also many cultural depictions of turtles and tortoises.
Snakes or serpents (the latter usually referring to a mythic or symbolic snake) are associated with healing in the Bible (the account of the brass serpent of Moses) as well as with the devil (the Biblical account of Adam and Eve). The periodic renewal, as in the shedding of snake skin, has led to the snake's being a symbol of healing and medicine, as pictured in the Rod of Asclepius (Figure below). In Egyptian history, the snake occupies a primary role with the Nile cobra adorning the crown of the pharaoh in ancient times. It was worshipped as one of the gods and was also used for sinister purposes, such as murder of the adversary and ritual suicide by the Egyptian queen Cleopatra. Snakes also play a role in Greek mythology, in Indian tradition and religion, and in other religions and customs.
Figure 13.31
The Rod of Asclepius, where the snake is a symbol of healing and medicine.
Lesson Summary
Reptiles are air-breathing, cold-blooded vertebrates characterized by a scaly skin.
Reptiles have a variety of reproductive systems, with different strategies for providing nutrition to developing young.
Lizards and snakes are distinguished by a unique type of scaly skin and movable quadrate bones.
There is a tremendous variety in color, appearance and size of lizards, and they have some unique adaptations, including regeneration of lost limbs or tails and changing color.
Snakes are distinguished by lack of eyelids, limbs, external ears and vestiges of forelimbs.
Snakes have various adaptations for killing and eating their prey.
Crocodilia have a flexible semi-erect posture, thecodont dentition, replacement of teeth, and a secondary bony palate.
The sex of developing crocodilians is determined by the incubation temperature of the eggs.
Other crocodilian traits, such as salt glands, nictitating membranes, ear flaps and sensory pits, are adaptations for aquatic living.
Turtles are characterized by a special bony or cartilaginous shell; have specialized adaptations for aquatic living, such as eye placement and salt glands, and adaptations for terrestrial living as well (placement of eyes and protection of eggs).
Reptiles play important roles as predators of pest species, food sources, pets, in medical and scientific research, and in folklore, religion and popular culture.
Review Questions
Describe the general traits of reptiles.
Describe the different types of reproduction in reptiles.
How are snakes distinguished from legless lizards?
Pit vipers, pythons and some boas have infrared-sensitive receptors in deep grooves between the nostril and eye. What role might such receptors play?
Name two adaptations of a crocodilian stomach which help it in digestion.
The shape and structure of a turtle’s shell can give its inhabitant advantages for avoiding predators, aid in swimming and diving, and for walking on land. Given what you know about a turtle’s shell, explain how the structure and shape could help the turtle in the above situations.
Further Reading / Supplemental Links
Unabridged Dictionary, Second Edition. Random House, New York, 1998.
http://en.wikipedia.org
http://kids.nationalgeographic.com/Animals
http://www.amnh.org/exhibitions/lizards
http://teacher.scholastic.com/activities/explorations/lizards/index.htm
http://www.turtles.org
http://www.gma.org/turtles
http://www.kidskonnect.com/content/view/54/27
http://www.flmnh.ufl.edu/cnhc/cbd.html
http://www.nature.com/news/2010/100524/full/news.2010.262.html
Vocabulary
amniotes
Vertebrates whose embryos are surrounded by an amniotic membrane.
nictitating membrane
A third transparent eyelid.
oviparous
Reproduction involving the laying of eggs.
parthenogenesis
A form of asexual reproduction, where the egg develops without fertilization.
pheromones
Chemicals released by an animal that influence the behavior or physiology of other individuals of the same species.
poikilothermic
Cold-blooded; without the ability to independently warm the blood.
thecodont
Any animal whose teeth are set in bony sockets.
Points to Consider
Some lizards have a dewlap, a brightly colored patch of skin on the throat, which is used in displays. What colorful displays do you think are used for courtship in birds and mammals?
Lizards and snakes use smell to track their prey, using the Jacobson’s or vomeronasal organ in the mouth, as well as a forked tongue. How do you think this compares to the sense of smell in birds and mammals and the structures used for smelling in these groups?
Like the scales comprising the shell of a turtle, or the cross-section of a tree trunk, crocodile osteoderms (small plates of bone under the scales) have annual growth rings, and by counting them it is possible to tell their age. Can we determine age in the same way in either birds or mammals?
Chapter 14: Birds and Mammals
Lesson 14.1: Birds
Lesson Objectives
List and describe general traits of birds.
Explain how birds are adapted for flight.
List different breeding systems in birds and describe nesting, incubation and parental care.
Illustrate the diversity of birds with examples of some of the varied groups.
Explain how birds are important, both economically and ecologically.
Check Your Understanding
Birds and reptiles have some traits in common. For example, birds are egg-layers and most reptiles are also oviparous. What do the eggs of both groups have in common?
What traits do birds have as a result of their being warm-blooded?
Introduction
We all think we know what a bird is. It seems fairly obvious. But if you were to really stop and think about birds, you would be amazed at the diversity of these organisms. From hummingbirds to ostriches, and countless varieties in between, birds are amazing creatures.
> It is pretty easy to be aware of birds all around us. From pet birds in our houses to those seen flying and perching in the out-of-doors, birds constantly remind us of their diversity in both appearance and habits. Birds have special adaptations for flight, including feathers and a lightweight skeleton. They also have a wide variety of reproductive strategies. Let us examine some of their principle traits so we can get a better appreciation of what birds can do.
Characteristics of Birds
Birds (class Aves) are warm-blooded, vertebrate animals with two legs (bipedal), who lay eggs. They range in size from the tiny 2 in (5 cm) Bee Hummingbird to the 9 ft (2.7 m) ostrich (Figure below). With approximately 10,000 living species, birds are the most numerous vertebrates with four limbs (tetrapods). They occur in diverse habitats around the globe, ranging from the Arctic to the Antarctic.
Figure 14.1
The ostrich can reach a height of 9 feet! Pictured here are ostriches with young in Namibia, Africa.
Defining characteristics of modern birds include:
feathers
high metabolism
a four-chambered heart
a beak with no teeth
a lightweight but strong skeleton
production of hard-shelled eggs
The digestive system of birds is unique, with a crop for storage and a gizzard that contains swallowed stones for grinding food to compensate for the lack of teeth. Birds have forelimbs modified as wings and nearly all can fly. Which of the above traits do you think might be of importance to flight?
Adaptations for Flight
In comparing birds with other vertebrates, what do you think distinguishes them the most? Of course, in most birds flight is the most obvious difference (Figure below), and birds have adapted their body plan for this function. Their skeleton is especially lightweight, with large pneumatic (air-filled) cavities connecting to the respiratory system. Cervical, or neck, vertebrae are especially flexible and in birds that fly the sternum has a keel, or longitudinal ridge, for the attachment of two large flight muscles: the pectoralis, which encompasses 15% of the bird’s total mass, and the supracoracoideus, the primary upstroke muscle for flight.
What other traits do you think might be important for flight? Of course, feathers are lightweight too and a forelimb modified as a wing serves as an aerofoil. This surface is designed to aid in lifting or controlling by making use of the air currents through which it moves. A bird’s wing shape and size will determine how a species flies. For example, many birds have powered, flapping flight at certain times, while at other times they soar, using up less energy
Figure 14.2
One birds flight, as seen in a tern species.
About 60 living bird species are flightless, such as penguins, as were many extinct birds. Flightlessness often arises in birds on isolated islands, probably due to limited resources and the absence of land predators.
Reproduction in Birds
How do birds reproduce? We are all familiar with the classic chicken egg. So what is involved in the process of a bird laying an egg? It all starts with courtship. Courtship involves some type of courtship display, usually performed by the male, leading up to the breeding. Most displays involve a type of song and some displays are very elaborate and may include dancing, aerial flights, or wing or tail drumming.
One of the most distinguishing features of bird reproduction is internal fertilization and the laying of eggs. The hard-shelled eggs have a fluid-filled amnion, a thin membrane forming a closed sac around the embryo. Eggs are usually laid in a nest. How do you think where a bird lays an egg might influence the egg color? If an egg is hidden in a hole or burrow, away from predators, then the eggs are most often pale or white. Nests in the open have eggs that are camouflaged, thus giving protection against predation (Figure below). However, some species like the ground-nesting nightjars, have pale eggs, but the birds themselves provide the camouflage with their feathers.
Figure 14.3
Nest and eggs of the common moorhen (), showing camouflaged eggs.
The shape of birds’ nests varies quite a lot too. Most create somewhat elaborate nests, consisting of such structures as cups, domes, plates, mounds or burrows. The albatross, however, makes a nest that is simply a scrape on the ground. Still others, like the common guillemot, do not use nests, instead they lay their eggs on bare cliffs. The male emperor penguins are even more elaborate in the care of their eggs: they incubate the eggs between their body and feet.
How else might a bird help protect its young from predators? Most species locate their nests in areas that are hidden, in order to avoid predators. Other birds that are large or nest in colonies may build nests in the open, since they are more capable of defending their young.
Young Birds and Parental Care
Parent birds usually incubate their eggs after the last one has been laid. In the 95% of species which are monogamous, (the species pair for the duration of the breeding season or sometimes for a few years or until one mate dies) the parents take turns incubating. In polygamous species, where there is more than one mate, one parent does all the incubating.
Brood parasitism, in which an egg-layer leaves her eggs in another individual's nest, is more common among birds than any other type of animal. The host bird often accepts and raises the parasite’s eggs, at the expense of the host’s own offspring.
Some precocial chicks, like those of the Ancient Murrelet (Synthliboramphus antiquus), follow their parents out to sea the night after they hatch, in order to avoid land predators. In most species, however, the young leave the nest just before, or right after, they can fly, sometimes making it necessary for them to walk until they have mastered flying.
The length and type of parental care varies widely amongst different species of birds. At one extreme in a group of birds called the magapodes, parental care ends in hatching. In this case, the newly-hatched chick digs itself out of the nest mound without parental help and can take care of itself right away. At the other extreme, many seabirds care for their young for extended periods of time, the longest being that of the Great Frigatebird, whose chicks take up to six months to fledge (getting parental care until they are ready to fly) and then an additional 14 months of being fed by the parents (Figure below).
Figure 14.4
The Great Frigatebird () adults are known to care for their young for up to 20 months after hatching, the longest in a bird species. Here, a young bird is begging for food.
Although male parental care is rare among most groups of animals, in birds it is quite common, more so than in any other class of vertebrates. Often, the tasks of defense of territory and nest site, incubation, and feeding of chicks are shared between the parents; sometimes one parent undertakes all or most of a particular duty.
Given all the information so far about birds, what would you say is true about bird diversity?
Diversity of Birds
If you guessed that there is a lot of diversity in birds, you guessed correctly. About 10,000 bird species belong to 29 different orders, or groups, within the class Aves. They live and breed in most terrestrial habitats and on all seven continents. The greatest biodiversity of birds occurs in the tropics.
There is enormous diversity and a wide range of adaptations of various body parts, such as beaks and feet: Adaptations to the birds' specific habitats. There is also enormous diversity in the feeding habits of birds. The feeding habits of birds are related to the beak shape and size, as well as the shape of the feet. Birds can be carnivores, insectivores, or generalists, which feed on a variety of foods. Some, such as hummingbirds, feed on nectar. Can you think of some examples of beak shape and size that are adapted to the type of food a bird eats?
Beaks
For example, parrots and their allies have down-curved, hooked bills, which are well-adapted for cracking seeds and nuts, and eating the meat inside (Figure below). Hummingbirds, on the other hand, have long, thin and pointed bills, which are ideal for probing tubular flowers for n
ectar (Figure below). Can you also think of some different types of bird feet, which might be adapted to different types of habitats?
Figure 14.5
The down-curved, hooked bill of a scarlet macaw, a large colorful parrot ().
Figure 14.6
A long, thin and pointed bill of the Swallow-tailed Hummingbird ().
Feet
Webbed feet used for swimming or floating, as in waterfowl or gulls and terns, may come to mind (Figure below). Other birds, for example, herons, gallinules and rails have four long spreading toes, which are ideal for walking delicately in the wetland in which they live (Figure below). You can now see that you could come up with your own ideas for how a particular bird trait is adapted to a specific habitat, food, or other specialized requirement. That might even make going out for an outdoor hike more of an adventure!
Figure 14.7
The webbed feet of a great black-backed gull ().
Figure 14.8
The long spreading toes of an American purple gallinule ().
Why Birds are Important
Now that you have some general knowledge about birds, you may want to make a list yourself of how you think birds are important. Just think about your daily living and how birds play a role. Do you eat chicken or turkey at meals? Do you have pet birds? Do you enjoy going out in your backyard or for a walk and listen to the beauty of birdsong or see the iridescent plumage of a bird in the sun?
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