Properties of Animals
Major Classifications of Animals
Issues of Survivorship
...... You may ask, what do we have in common with the earthworm? The sparrow at your backyard feeder? Buzzing bees? The smallmouth bass you caught yesterday? Interestingly, we human organisms belong to the Animal Kingdom, and as such we display most structural and functional features common to all of these other animals. These features include: a high degree of structural organization and complexity; the ability to transduce chemical energy into usable forms; ability to reproduce at the cellular as well as the species level; we contain tissues which are stimulable; we are mobile; we continue to grow from the time of conception, and as a consequence develop new capabilities; and we humans are an integral, predominant part of the evolutionary tree initiated millions of years ago. However, what basically sets the human organism apart from all of the other animals is our brain, more specifically, the neocortex; it is this part of the brain which gives us the tremendous power to assemble and interpret sensory input, to think in abstract terms rather than simply react to environmental cues, as well as the power to deduct, rationalize and problem-solve. These and other characteristics of the human organism will be reviewed in the notes to Week 7.
Like the Plants, the Animal Kingdom contains a wide-range of organisms which share common properties which go to define animals as a distinct group of life-forms:
Complex Eukaryotic Cells - Made up of highly-organized eukaryotic cells; from the zygote stage, cells gradually differentiate into various tissues which ultimately provide specialized structure and function to the adult animal.
Tissue Specializations - Animals are multi-cellular; various tissue and organ systems are constructed to provide the specializations needed for day-to-day survival as well as survival of the species.
Animals are Aerobic - Aerobic metabolism is the means upon which animals breakdown food-stuffs and acquire ATP energy needed to drive all of their functions. In effect, oxygen is the final hydrogen acceptor molecule (forming water) in a step-wise breakdown of sugar and fat for energy (viz., metabolic processes called glycolysis, kreb's cycle, and electron transfer system).
Sexual reproduction - The survival of animal species is accomplished through sexual reproduction, where specialized organs called gonads (female ovary/male testis) through meiosis produce genetically-diverse gametes (female eggs/male sperm).
Mobility - Animals for the most part are capable of movement; this property has allowed animals to be distributed throughout the land as well as marine habitats around the world. Moreover, this property of motion is highly dependent upon the development of a stimulable skeletal-muscular system.
Land and Sea Habitats - Most of the early, evolutionary forms of animals were aquatic; however over evolutionary time, several animals groups adapted to and colonized the land masses.
...... Within the Animal Kingdom there are approximately 35 sub-divisions called Phyla; each phylum sub-division shares particular structural and functional properties which together separate it from other phyla. Research is underway today to apply genomic analyses to various animal phyla and develop an animal `family tree' based DNA sequences, rather than the conventional morphological/functional criteria used up to now. Until the day comes when all animals will be classified according to their DNA sequences, below are ten of the most common animal phyla classified according to traditional biological methodology:
Porifera These are the salt-water sponges; there are approximately 8,000 separate species existing today.
Cnidaria- This group is composed of jellyfish, and other lower aquatic animals; approx 15,000 species exist today
Platyhelminthes These are the flatworms which inhabit both marine and freshwater habitats; over 15,000 species exist today.
Nematodes This phylum consists mainly of about 80,000 known parasitic worms.
Rotifers This group consists of about 1,800 highly-mobile freshwater invertebrate animals.
Mollusca This major group consists of snails, clams, squid, and octopus; there are over 110,000 known species.
Annelida About 15,000 individual segmented worms comprise this phylum; the common earthworm is an example.
Arthropoda This very large group consists of insects; it is estimated that there are over 1 million species of insects existing today.
Echinodermata These are the marine starfish; about 6,000 species exist today.
Chordata This is a group of animals which are classified on the basis of possessing 3 common embryological features dorsal nerve cord, supportive structure called the notocord, and pharyngeal gill pouches. Within this phylum is a highly-advanced group called the `vertebrates' which include fish, amphibians, reptiles, birds, and mammals; it is this phylum to which the human organism belongs.
For more specific information on animals and their classifications and properties see:
www.ultranet.com/~jkimball/BiologyPages/I/Invertebrates.html , and
Issues of Animal Survivorship
...... There are a number of issues which have contributed to the survival and evolutionary success of the animal kingdom; this is particularly true for the mammals to which the human organism belongs. Some key survivorship issues:
...... Many Animals are Prolific As Survival Mechanism - As a species survival mechanism some animals reproduce at a remarkable rate. For example, representatives of the mollusca phylum produce 1000s if not millions of young with the hopes that a small percentage will survive predation, and the species in effect will survive. If one were to plot a survivorship curve (i.e., % alive versus relative age), the mollusks would demonstrate severe early mortality where most of the young would not survive the early going.
...... Parasitism as Survival Mechanism - Some groups of animals (e.g., nematodes) have adapted to a parasitic life style where nutrition is drawn from another free-living host organism.
...... Co-Evolution of Species (You Help Me, I'll Help You) - The evolutionary success of some groups of animals were in large part due to the success of other organisms. A good example of this is found in the co-evolution of the insects and the flowering plants; as the angiosperms prospered so did the insects which obtained food through nectar and pollen. In a synergistic manner, the angiosperms benefited from the pollinating actions of the insects. It was a win-win situation for both groups.
...... Homeostasis - The ability of an animal to control its own environment plays a large factor in whether or not an animal will survive. By far, most animals are at the mercy of changing environmental conditions. For example when temperatures get too hot or too cold, most animals die (or in some cases are metabolically shut for a period). On the other hand a number of higher animals like birds and mammals have evolved homeostatic mechanisms upon which the 19th century physiologist Claude Bernard stated A constant internal milieu allows for a free life! The abilities to maintain a relatively constant internal environment in the face of changing environmental pressures (e.g., temperature) have provided birds and mammals the unparalleled power to maintain activities year-round.
...... In Utero Protection - Compared to other animals, mammals produce few offspring at a time; however the offspring that are produced are well provided for. For example, mammalian embryos develop in utero which provides protection, insulation, and constant nutrition. After birth, the young are further protected through the nurturing power of the limbic part of the parental brain, as well as post-natal nutrients through the maternal mammary glands.
...... Sexual Reproduction Ensures Diversity in Offspring - Reproduction of animal species (with few exceptions) is accomplished through sexual reproductive means; through meiotic division of 2N stem cells to yield genetically-diverse gamete, the next generation is assured to be genetically different from the parental generation. Hence, individual diversity is assured within the animal kingdom.
Case Study Question
...... Today because of the in-roads of DNA technology and molecular biology, the lines between species (and even between Kingdoms) are often blurred. For example, bacterial and plant cells have been genetically-mutated to produced ...... medically-important human proteins. Are these bacterial and plant cells new species?
...... While the life-forms of the world can still be conveniently grouped into 5 kingdoms (6 if you include the ancestral bacterial group Archaea), today's molecular genetics has the potential of redefining biological classifications. What do you think?