On the surface, Mawdo marries again because of pressure from his mother, Aunty Nabou. Mawdo's first wife is Aissatou, a woman Aunty Nabou disdains for her low birth. To reclaim what she believes is Mawdo's right to a wife of rank, Aunty Nabou sets a manipulative plan into action. She asks her brother, Farba Diouf, for permission to raise one of his daughters. Diouf agrees and sends young Nabou home with his mother. Interestingly, the girl and her grandmother share the same name.
When young Nabou is of marriageable age, Aunty Nabou tells Mawdo that Farba Diouf has offered young Nabou to him for a wife and that he (Mawdo) must not turn down the generous offer. Stunned by his mother's announcement, Mawdo agrees. He later tells Aissatou that he has only married Nabou to placate his mother. When young Nabou eventually begins bearing Mawdo children, however, Aissatou realizes that her husband has been less than truthful with her. So, she leaves him.
In Mawdo's case, he takes a second wife because of maternal pressure and the traditions of his culture. Mawdo's capitulation to his mother's wishes reflects his culture's respect for its elders. He knows that members of his community would have considered him unfilial for going against his mother's wishes. Yet, on another level, Mawdo is not disinclined to take a younger, more nubile wife. One can argue that his mother's insistence on him marrying young Nabou provides him the necessary cover for his own desires.
In this, Mawdo is similar to Modou, who marries the seventeen-year-old Binetou because of his lust. Both men are not averse to indulging their sexual appetites for younger wives. After all, it is expected that they will do so. So, even before Modou marries Binetou, he positions himself as an indispensable resource in the young girl's life. He acts as her "sugar daddy," buying her expensive presents and indulging her whims. After he marries Binetou, Modou leaves Ramatoulaye to her own devices; now, she must raise their twelve children by herself. It is clear that Modou takes a second wife because he is no longer enamored with his first one, Ramatoulaye.
So, to recap, although Mawdo and Modou both take second wives because of their lust for youthful beauty, Mawdo is the only one who manages to hide his true intentions under the guise of filial piety.
Mawdo was pressured by tradition and his mother to take a second wife. He still loved his first wife, but his mother had never accepted Aissatou and insisted her son marry someone of noble blood. His mother shames him by saying she'll never get over it if he doesn't take the wife his mother has groomed for him. Mawdo wants to continue his life with Aissatou and just visit his second wife when necessary, but Aissatou refuses to accept his polygamy.
Modou fell in love with his daughter's friend, Binetou, and sneaked about planning his marriage to her. Binetou is basically sold by her family to be Modou's second wife. Modou doesn't even have the decency to tell his wife that he's married Binetou. Instead, he sends his brother and his best friend to do his dirty work after he has already married Binetou. Modou will no longer provide for his first wife and their twelve children or to honor his vows to her.
Both men take a second wife, but one does so because of tradition and an overbearing mother. Modou does so because as a man in his society, he can do as he pleases with the women in his life. The two men reflect very differing attitudes toward a long-standing tradition.