Fusion of gametes (sperm and ovum) inside of the females body to produce a zygote. This type of fertilization is found in all mammals, birds, and reptiles, and is a possible result of copulation. Only one or a few zygotes can grow. 

University of California, Davis

If both individuals display appetitive behaviors, copulation, which is the sexual act, will occur. In mammals, copulation includes one or more intromissions, which leads to stimulation, and finishes with ejaculation. The four stages of human copulation observed in males and females are increasing excitement, plateau, orgasm, and resolution. 

Copulation

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Internal Fertilization

The climax of a sexual experience, which involves pleasurable sensations. Females and males experience different sexual responses and patterns. 

Orgasm 

The process where eggs are fertilized outside of the body. This is seen in fish and amphibians. The gametes are released into the outside world at a very high rate but have a high chance of loss or failure. 

External Fertilization 

The production and release of an egg/ovum. In rats, ovulation is spontaneous every 4-5 days. For the few hours around ovulation, female rats will seek out a male and display certain behaviors. Both individuals will produce ultrasonic vibrations, which will prompt copulation. 

Ovulation

Lordosis is a posture assumed by female rats during copulation to show that she is receptive. This includes raised hindquarters and tail turned to one side to allow for intromission by the male. 

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