Abram asked a question.
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!