What are Mekusharos and Atufim?
Moshav Zekenim (to 30:37): Mekusharos are males tied [so they cannot mate]. Atufim are sheep totally wrapped [to protect their wool].
Tosfos ha'Shalem (to 30:40 (4)): Atufim are fat animals that will give birth by themselves. Mekusharos are barren animals.
Hadar Zekenim (to 30:38): 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 (to 30:37) #1: Mekusharos are males that constantly follow the females, and are inextricably bound to them - like "v'Nafsho Keshurah b'Nafsho" (44:30). The resulting babies are called "Mekusharos" after their fathers.
Ramban (to 30:37) #2: Some say that Mekusharos are strong; their limbs are tightly-knit. Atufim are weaker, and less lustful.
Ohr Zaru'a (1:769): Atufim are covered (with one color, white or black); they are of Lavan. Mekusharos are of Yaakov.
Just like people tie a string around Bechoros (Da'as Zekenim to 30:41) or Bikurim (Rosh to 30:41), Yaakov tied around the forelegs of a Bechor.
Sheep multiply rapidly (Chulin 84b), bearing offspring even within their first year. By taking the earlier yield for six years consecutively, Yaakov's portion would grow exponentially in relation to Lavan's. (CS)