Why do dolphins have nostrils on the tops of their heads?

Dolphins have nostrils on the tops of their heads, called blowholes, for a very practical reason: to breathe while swimming.

Here's why:

* Streamlined body: Dolphins have streamlined bodies for efficient movement through water. Having nostrils on top of their heads keeps them out of the way of the water flow, minimizing resistance.

* Surface breathing: Dolphins are mammals and need to breathe air. They can't breathe underwater like fish. Having blowholes on top allows them to surface quickly and easily to take a breath without having to turn their whole body.

* Sight while breathing: With their blowholes on top, dolphins can keep their eyes above water while breathing, allowing them to stay alert for predators and prey.

Essentially, the placement of their blowholes is a perfect adaptation for a life spent swimming and surfacing for air.