What are the five animals that fertilize internally?

It's impossible to give you five specific animals that *only* fertilize internally, as internal fertilization is the norm for a vast majority of animal groups. Here's why:

* Internal fertilization is very common: Many different animal groups, including mammals, birds, reptiles, amphibians, fish, and even some invertebrates, utilize internal fertilization.

* Evolutionary Advantage: Internal fertilization provides a more protected environment for the sperm and egg to meet, increasing the chances of fertilization. It also allows for the development of embryos within the female's body, providing further protection and nutrient supply.

Instead of giving you five specific examples, let's explore some groups where internal fertilization is dominant:

* Mammals: All mammals, including humans, fertilize internally.

* Birds: Birds also fertilize internally, though the development of the eggs takes place outside the female's body.

* Reptiles: Reptiles, like snakes and lizards, fertilize internally.

* Amphibians: Most amphibians, like frogs and salamanders, fertilize externally. However, some species have evolved internal fertilization.

* Insects: Some insects, like butterflies and moths, fertilize internally. Others, like dragonflies, fertilize externally.

Let me know if you'd like to explore a specific animal group or if you'd like to know about the differences between internal and external fertilization!