By Regina Bailey, About.com Guide
Individual organisms come and go, but, to a certain extent, organisms transcend time through producing offspring. Reproduction in animals occurs in two primary ways: through sexual reproduction and through asexual reproduction.
Now let's take a look at sexual reproduction.
In sexual reproduction, two individuals produce offspring that have genetic characteristics from both parents. Sexual reproduction introduces new gene combinations in a population.
In animals, sexual reproduction encompasses the fusion of two distinct gametes to form a zygote. Gametes are produced by a type of cell division called meiosis.
The gametes are haploid (containing only one set of chromosomes) while the zygote is diploid (containing two sets of chromosomes).
In most cases, the male gamete, called the spermatozoan, is relatively motile and usually has a flagellum. On the other hand, the female gamete, called the ovum, is nonmotile and relatively large in comparison to the male gamete.
Types of Fertilization
There are two mechanisms by which fertilization can take place.
The first is external (the eggs are fertilized outside of the body); the second is internal (the eggs are fertilized within the female reproductive tract).
Patterns and Cycles
Reproduction is not a continuous activity and is subject to certain patterns and cycles. Oftentimes these patterns and cycles may be linked to environmental conditions which allow organisms to reproduce effectively.
For example, many animals have estrous cycles that occur during certain parts of the year so that offspring can typically be born under favorable conditions. Humans however, do not undergo estrous cycles but menstrual cycles.
Likewise, these cycles and patterns are controlled by hormonal cues. Estrous can also be controlled by other seasonal cues such as rainfall.
All of these cycles and patterns allow organisms to manage the relative expenditure of energy for reproduction and maximize the chances of survival for the resulting offspring.