The terms in their most basic forms literally mean

ability to impregnate the most dominant/biologically fit female or get pregnant by the most dominant/biologically fit male, multiplied by frequency of pregnancy

In nature it can relate to frequency of pregnancies to the most fit partner, which can imply an polygamous like ordeal, where the male has multiple female mates, the reverse isn't possible for females as they can only get pregnant once at a time to the alpha male.

However in looking at this in more detail, the increased variation of female partners actually contradicts the alpha male' success, simply because there aren't supposed to be many alpha females. To assert their respective definitions an alpha male and alpha female are conjoined as pairs, multiplied by successful pregnancies.

In simpler terms 'alpha male' by assumption might seem to imply multiple mates in nature, however an alpha male isn't one if per say he has 20 children with 5 different average mates, than if he has 10 children with an alpha female, as even 1 of the 10 children between an alpha male and alpha female dynamic is likely to dominate each of the 4 children from the other 5 average mates.

We could also theorise that the alpha male is more of alpha male if they obtain the alpha female plus other females, however that contradicts the alpha female status, since an alpha female's territorial integrity dictates that she seizes the alpha male for herself unless she permits him to other relations with other females.

By 'dominate' I mean a collective of being physically fit, social dominance, intelligence, fertility, attractiveness and biodiversity.

So at the end of day an alpha male and alpha female is a monogamous dynamic because an alpha male can't truly be an alpha male unless they're with an alpha female and at the same time an alpha female isn't truly an alpha female unless they're with the alpha male × pregnancies.