External or Internal Fertilization

The world is an uncertain and changing place, to which humans and animals respond by considering the potential reward and cost of different options and estimating the odds of success before committing to a choice. 

Some animals fertilize their gametes externally. In this case, both eggs and sperm are released into the surroundings, and fertilization occurs when sperm encounter eggs. Because external fertilization requires an environment where the young can develop without heat stress or desiccation, it occurs almost exclusively in moist places. Externally fertilized eggs that also develop 

Species with internal fertilization usually produce fewer zygotes, but provide more parental protection than species with external fertilization. There is a greater resource investment in each offspring with internal fertilization, so far fewer can be produced. With fewer numbers, and more expensive offspring, greater protection is required to assure an adequate survival rate. 

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