What are Mekusharos and Atufim?
Rashi: Mekusharos are those that become pregnant early 1 . Atufim are those that become pregnant late.
Moshav Zekenim (37): Mekusharos are males tied [so they cannot mate]. Atufim are sheep totally wrapped [to protect their wool].
Tosfos ha'Shalem (40:4): Atufim are fat animals that will give birth by themselves. Mekusharos are barren animals.
Hadar Zekenim (38): Mekusharos are born in summer, and Atufim in winter (when the sheep is covered with wool. In summer, it is shorn.)
Rashi, citing Menachem: Ha'Atufim are totally covered in fleece, and are not interested in being heated through mating.
Ramban (37) #1: Mekusharos are males that constantly follow the females, and are inextricably bound to them
Just like people tie a string around Bechoros (Da'as Zekenim 41) or Bikurim (Rosh 41), Yakov tied around the forelegs of a Bechor.
