A heifer and a ram

I saw something interesting in Genesis 15:9.

Abram asked a question.
(Genesis 15:8)

But he said, "O Lord GOD, how am I to know that I shall possess it?"

Then God told him to bring him some specific animals.

What is a heifer?
My English-Japanese dictionary says a female cow under 3 years old which has not produced a calf. One commentary said that 3 yeas old is ceremonially mature (for sacrifice). The dictionary.com said 'a young cow over one year old that has not produced a calf.' Then it means a mature female cow which can have a baby but hasn't had one yet.

Then a female goat.

Then a ram. What is a ram? It is a male sheep old or mature enough to be a father.

Two female animals, one mature male animal, then two birds. It almost looks like the heifer is Sarai and the ram is Avram!