With a song composed in honor of a yerima named Mohammadu, J.R. Patterson suggested it dated to the reign of Ali b. Umar. A major mai of 17th century Borno reigning from c.1639-1677, various sources attest to conflict with the Tuareg of Air at this time. For instance, the enslaved French doctor in Tripoli, who wrote extensively about Borno, mentioned conflict between the powerful kingdom and Air. Additional sources, including works translated as the Chronicles of Agadez by Yves Urvoy allude to this conflict. Written sources from Borno, including an Account of Gazargamo dated to 1658 by Palmer in his Bornu Sahara and Sudan, similarly mention one of the conflicts with the Tuareg of Air. Sadly, none of these sources agree on the name of the Air sultan and of the names suggested (Ali, in this praise song, Ada Hamma in the Account of Gazargamo), none are supported by Air internal sources.
In fact, the Account attributes the conversion of the Air ruler to Islam to his capture by the forces of Borno, something contradicted by a plethora of sources referencing the Islamic "Istambulawa" dynasty ruling at Agades since the earliest time. For the time period during Ali b. Umar of Borno's reign, Air was ruled by the following sultans, according to Djibo Hamani: Muhammad al-Tafrij, succeeded by his brother, Muhammad al-Mubarek. Thus, we believe it likely that some of the names attributed to Air "sultans" in the Borno sources may refer to Tuareg leaders of clans who autonomously raided Borno's northwestern frontier in the 1600s (and continued to do so into the 1700s and 1800s). This seems plausible given the paltry number of horsemen who endeavored to raid the cattle of Fulan in the land of Digima, under the care of the yerima, Mohammadu. It was essentially a small raiding party, but the praise singer turns it into an epic instance of the military prowess of Mohammadu who fought the raiding Tuareg with only 7 horsemen. In other words, he heroically fulfilled his duties in protecting Borno's border.
Ultimately, we suspect this Mohammadu probably was a yerima sometime during the second half of the reign of Ali b. Umar, and was his grandson through a daughter of his. Using evidence from a mahram dated by Palmer to 1647 in Bornu Sahara and Sudan, which named the yerima as Kaday, we suspect this Mohammadu must have served in perhaps the 1660s or 1670s. The praise song states, "But good fortune belongs to the man twenty years old." This suggests the recipient of the praise song was a young man and, if he was not yerima until after 1647, we suspect this was a youthful yerima who may have been in office after the more alarming actions of the Tuareg in the 1660s (one of which led to the capture and enslavement of a nephew of Ali b. Umar in the Mediterranean after the Air Tuareg were invited by princes fighting for the throne of Borno to help their cause against the mai). It seems unfathomable that the Sayfawa mai would have appointed a young man in his 20s as the yerima during such troubled times.
Either way, he would have been well-connected in Gazargamo, as his mother was the daughter of Ali b. Umar. According to Maikorema Zakari, the yerima usually lived in Gazargamo himself, while Nachtigal emphasized he was (always?) the son of a princess. Furthermore, Ali Eisami, whose testimony for pre-jihad Borno requires attention, described the yerima as capable of fielding 1000 cavalry forces. Based in the capital, a grandson of the mai and perhaps among the more loyal relatives of Ali b. Umar than his the sultan's brothers, Mohammadu may have been a large supporter to the state in the waning days of Ali b. Umar's reign.
Here is the "Song to Yerima Mohammadu" translated by Patterson in Kanuri Songs:
Son of Doguma Lafiya
Who was like a mudu of kanya fruit which was better than a sa of dates
Lafiya, precious as choicest food, (you Yerima) are the son of Sultan Ali’s daughter
Your land Air is fruitful, full of jujube trees, has rivers full of fish in which to bathe
Guzun is your town in which “darra” is played, it is mosquito-ridden
Yerima is our chief
The holder of the office of Yerima
(compared with) a he-goat is bright red,
(Compared with) a she-goat is light brown as men of the Kingena tribe
Three (sections of the) N’gal tribe and thirty (sections of the) Ngalaga are yours
The Yerima is like a prancing horse whose legs are not hobbled
Like a camel which wanders where it will without a halter.
He is but the grandson of a Sultan, but he is greater than a prince,
Yerima Mohammadu, son of Doguma:
Let me sing his praises and do you give heed.
My song (will fit him) as a gown made with a neck-band,
As trousers made with a slot for a string round the waist.
If you doubt it, ask the Ngijima, Babuma and Zakkama
who are the three people whose news is trustworthy.
White hairs are dignified,
But good fortune belongs to the man twenty years old.
The fair women of the capital are Asma, daughter of Rugaya,
Kagudi, daughter of Jamall,
Zahra, daughter of Ali the leather worker:
(The women said) ‘Kelima, playful fellow, we do not see the truth of what you say’
He said, ‘Is what I said a piece of reading?’
Playfulness it is that makes the world forgetful of trouble
To rouse the men we said it
There is a prince of Kanem, a son of Fanna
And a prince son of Mune, and whose town is Chungul
And one whose land is Gajiga, who is a son of Gombo
And the prince who is greater than all others
And the prince whose town is Titiswa, and whose father was Gwali
And the fair-skinned prince whose land is Ngamma
And the prince whose land is Danna
You are grandson of a Sultan but greater than a son of a Sultan
Prince Yerima son of Doguma
Kelima Mele son of Hawa (says to you),
Prince Yerima, did you hear what took place last night?
The Angel of Death with his followers,
Seventy horsemen all men of Sultan Ali whose country is Air
Came to take away the cattle of the Fellata of the land of Digima who are your special care
“What is your plan my six horsemen?” (the Yerima said)
There is Kagel, Mele’s son
Mohammadu son of Askura
Adam whose “furra” (?) consists of weapons
Bukr who never fights a fight in vain
Mohammadu whose pleasure is to hear of deaths in battle
With Mohammadu son of Forto there are six, I myself make seven
We shall conquer them
(The Yerima’s) singer is the Kelima
A prince with a Sultan in his pocket
You are a greater man than Iyrima Daudu or Maidalla Hawami
Or than Gadamanga
We have received what we sought
May God preserve you.