Why do four legged mammals have breasts on their stomach yet with two legs chest?

This is a fascinating observation! You're right, four-legged mammals tend to have their mammary glands on their abdomen (belly), while humans, with two legs, have them on their chest.

Here's the breakdown of why this is the case:

* Evolutionary Adaptation: Mammary glands are designed for nursing offspring. Their location evolved based on the needs of the species and how their young develop.

* Quadrupedal Mammals: In four-legged mammals, the abdomen is the most practical location for mammary glands. It's close to the ground, making it easy for young to access and nurse.

* Bipedal Humans: Humans evolved to walk upright on two legs. This change in posture meant the mammary glands ended up on the chest. This location is still close enough to the ground for nursing, but it also allows for greater mobility and ease of movement while caring for the infant.

Key factors influencing the location of mammary glands:

* Locomotion: How the animal moves (walking, running, climbing)

* Young development: The size and mobility of the offspring

* Body shape: The overall structure and anatomy of the species

It's important to note: While most four-legged mammals have their mammary glands on their abdomen, there are exceptions. Some species, like bats, have mammary glands on their chests. This is likely due to their unique flight adaptations.

So, the location of mammary glands is an interesting example of how evolution shapes the anatomy of different species based on their lifestyle and needs.