The word animal comes from a Latin word, animalis, that means “being shaped like an animal.” Animals are multicellular, eukaryotic organisms which form the basic biological kingdom Animalia. In contrast to the eukaryotic (single cell) organisms such as plants and fungi, animals are diploid (double-cell) and metamorphic (change into a different form). In both cases, a cell possesses both a nucleus and a tail. The nucleus is a structure for making connections with other cells while the tail is the part of an organism that allows movement in various directions and is often used to measure an organism’s girth. All animals have teeth and, in some cases, eyes; however, birds are the only creatures with vocal cords.
Most animals reproduce by means of a process called sexual reproduction. In most cases, animals reproduce asexually by way of a direct physical action (or sexual) on another living thing. Sexually reproduced organisms must reproduce asexually, in order to survive. Sexually reproduced organisms can also reproduce asexually by means of a gene transfer from an animal to a human or an animal to a plant, but this type of reproduction usually occurs through an egg cell.
Embryology is the study of animals’ embryos and reproduction. Embryology is the science of creating an embryo from a living person or an animal. There are three broad categories of animal research: Comparative genomics, comparative anatomy, and embryo selection. Comparative genomics compares genetic differences between species. Comparative anatomy identifies and studies similarities among living beings and the differences that differentiate between species. Finally, embryo selection uses living donors to select embryos that are capable of conceiving a child.
Hormones and parasites have been a major focus of animal research. To understand why and how hormones and parasites affect animals, scientists must understand how hormones affect humans. Hormones dictate many aspects of animal and human reproduction; when males produce testosterone, females produce estrogen. When males produce a hormone called androgen, females produce another hormone called estrogen.
Many factors have been implicated in why some animals do not reproduce and why other animals can easily reproduce. One major factor cited in the case of humans and animals is the level of stress the animal undergoes to keep it reproducing well. Researchers studying chimpanzees have determined that when animals are stressed, it becomes harder for them to remember things and perform actions related to moving toward adulthood. Some researchers believe these same researchers think aging, disease and stress may also be linked to infertility in humans. In chimps this theory is being tested as well.
Other animal reproductive systems include ovulation, fertilization and pregnancy. Ovulation is the process wherein an egg is released from an ovary. Fertilization occurs when the sperm is deposited on an egg. Pregnancy lasts between the time an egg is released from an ovary and implantation into the uterus. Sexual reproduction, as its name suggests, occurs through mating. An asexual reproduction process occurs when no sexual contact occurs between animals.