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. 

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. 

Lordosis 

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. 

University of California, Davis

Reproductive behavior has four main stages: 
- Sexual attraction
- Appetitive behavior
- Copulation
- Postcopulatory behavior

Reproductive Behavior

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

Behavioral Neuroscience 8th Edition

The first step in bringing two individuals together is sexual attraction. An individual who posses sexually selected traits can be seen as desirable by a  potential mate. Attraction is shaped by association and life experiences (an individual's definition of "attractive" can change and develop with experience). We can measure attractiveness by watching responses of potential mates, however, attractiveness is not necessarily objective. 

Sexual Attraction

Appetitive behaviors are behaviors that start and continue sexual interaction. When a female displays her readiness it is called being proceptive. Behaviors vary across species but they include staying close to a mate, sniffing, wiggling ears for rats, or engaging in song for birds. This is the second step of reproductive behavior, so once two individuals are attracted to each other, they display these species-specific behaviors in order to engage in sexual acts. 

Appetitive Behavior

Copulation

Postcopulatory behaviors vary across species but include grooming (rats), rolling (cats), and parental behaviors (mammals and birds). 

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