"Issachar is a strong donkey, lying down between the sheepfolds". 
From the Midrash, we learn about the agreement between Zevulun and Issachar. Zevulun, who lived by the coast, engaged in trade and used his earnings to support the tribe of Issachar, who dedicated themselves to Torah study. 
Similarly, in Midrash Bamidbar Rabbah, it is mentioned that Zevulun and Issachar shared the reward of Torah study and supported each other. It is also established that Zevulun was greater than Issachar, as "the one who does is greater than the one who studies."