Man and woman

In Ch. 7 of Bareishis there is a curious a switch in terminology. First it describes animals that went into the Teiva as "man and wife (ish v'ishto).

ב  מִכֹּל הַבְּהֵמָה הַטְּהוֹרָה, תִּקַּח-לְךָ שִׁבְעָה שִׁבְעָה--אִישׁ וְאִשְׁתּוֹ; וּמִן-הַבְּהֵמָה אֲשֶׁר לֹא טְהֹרָה הִוא, שְׁנַיִם--אִישׁ וְאִשְׁתּוֹ (It also described "birds of heaven" but we will leave that for later, see Chizkuni for one answer).

Then it describes the sons and daughters-in-law of Noach as "man and wife":

וַיָּבֹא נֹחַ, וּבָנָיו וְאִשְׁתּוֹ וּנְשֵׁי-בָנָיו אִתּוֹ--אֶל-הַתֵּבָה:  מִפְּנֵי, מֵי הַמַּבּוּל

Then is sudenly calls animals and birds as "male and female".

מִן-הַבְּהֵמָה, הַטְּהוֹרָה, וּמִן-הַבְּהֵמָה, אֲשֶׁר אֵינֶנָּה טְהֹרָה; וּמִן-הָעוֹף--וְכֹל אֲשֶׁר-רֹמֵשׂ, עַל-הָאֲדָמָה.
ט  שְׁנַיִם שְׁנַיִם בָּאוּ אֶל-נֹחַ, אֶל-הַתֵּבָה--זָכָר וּנְקֵבָה:  כַּאֲשֶׁר צִוָּה אֱלֹהִים, אֶת-נֹחַ

Why the switch form "man and wife' to "male and female"?

I suggest that one can begin by talking of animal pairs as "man and wife". However, once Noah's family is introduced and we see what marriage is for Man, what an elevated partnership it can be, far byond its "male and female" aspect, it is no longer possible to refer to animal pairs as "man and wife", but only as "zakhar v'nikeiva", male and female.

Comments

[yitz..]

i posit something more technical:

when HaShem instructs, He says: "man and wife" meaning mates. When they fulfill HaShem's commandment, a male and female pair arrived but Noah had no knowledge of whether they were a matched/mated pair or just a male and a female.

In other words, the first description speaks from HaShem's all-knowing perfect perspective, and the second relates how it appeared to Noah and his family.

alternately:

When HaShem commanded the commandment they were still permitted to reproduce, hence "man and wife," once they boarded the ark, they were forebidden to reproduce, and so they were referred to as "male and female."