Title: Horton Hears a Who! (TV special)
Content:  prove their existence, but even with the Whos' shouting and loud instruments, the other animals still hear nothing. Dr. Hoovey searches through town and finds a small Who named JoJo, who is bouncing a yo-yo instead of making noise. He takes JoJo to the top of his tower, where JoJo utters a "Yopp", which breaks through the dust speck and allows the animals to finally hear the Whos; they release Horton and allow him to keep it. The special ends with Dr. Hoovey relaxing in his chair when he sees a small dust speck and hears a call for help coming from it, much to his dismay.
Title: Horton Hears a Who! (film)
Content:  of beezlenut oil. The Mayor enlists all of his people to make noise, so that all the animals will find out they're really there, assisted by JoJo's "Symphonophone", an invention which creates a huge musical contribution and reveals that JoJo's "true" passion is music, but still fails to penetrate the surface of the speck. The Kangaroo snatches the flower from the captured Horton and prepares to drop it into the pot. Meanwhile, JoJo grabs the horn used to project Horton's voice, runs up the highest tower, and screams his first word, breaking through the sound barrier just seconds before the speck hits the oil. Rudy grabs the flower and proclaims he hears it, and 
Title: Horton Hears a Who! (film)
Content:  his story, but still helps him on his mission. ; Dan Fogler as The Chairman, the lead Whoville councilman who is dedicated to traditions and mistrustful of Mayor McDodd. ; Fogler also voices Yummo Wickersham, the leader and overall largest of the Wickersham apes. ; Isla Fisher as Dr. Mary Lou LaRue, a professor at Who U. ; Jonah Hill as Tommy, a orange bear and one of Horton's students. ; Amy Poehler as Sally O'Malley-McDodd, Mayor McDodd's wife and mother to Jojo and her 96 daughters. ; Jaime Pressly as Mrs. Quilligan, a Russian Palooski and Jessica's mother. ; Jesse McCartney as JoJo McDodd, Mayor McDodd's quiet son (until the end) who doesn't want to be 
Title: Horton Hears a Who! (film)
Content:  In the Jungle of Nool, Horton the Elephant, the jungle's eccentric nature teacher, hears a tiny yelp coming from a dust speck. Believing that an entire society of microscopic creatures are living on that speck, he gives chase to it before placing it on top of a flower. Horton finds out the speck harbors the city of Whoville and its inhabitants, the Whos, led by Mayor Ned McDodd, whose family includes his wife Sally, 96 daughters whose names all begin with the letter H, and one teenage son named JoJo. Despite being the oldest child and next in line for the mayoral position, JoJo does not want to be the next mayor, and he does 
Title: Horton Hears a Who!
Content:  To save Whoville, Horton implores the little people to make as much noise as they can, to prove their existence. So almost everyone in Whoville shouts, sings, and plays instruments, but still no one but Horton can hear them. So the Mayor searches Whoville until he finds a very small shirker named JoJo, who is playing with a yo-yo instead of making noise. The Mayor carries him to the top of Eiffelberg Tower, where JoJo shouts out a loud "Yopp!", which finally makes the kangaroo and the monkeys hear the Whos. Now convinced of the Whos' existence, the other jungle animals vow to help Horton protect the tiny community.
Title: Horton Hears a Who! (film)
Content:  next mayor. ; Selena Gomez as Helga McDodd, one of the mayor's daughters. ; She also voices Angela, a teenager glummox and one of Horton's students. ; Josh Flitter as Rudy Kangaroo, Jane Kangaroo's doubtful son who is much kinder than his mother. ; Niecy Nash as Miss Yelp, Mayor McDodd's secretary. ; Laura Ortiz as Jessica Quilligan, a Russian Palooski, Mrs. Quilligan's daughter and one of Horton's students. ; Joey King as Katie, a cute and eccentric baby yak. ; Bill Farmer as Willie, an orange bear and Tommy's dad. ; Marshall Efron and Tim Nordquist as the Wickersham Guards, Yummo's younger brothers. ; Heather Goldenhersh as Who Girl ; Charles Osgood as the Narrator 
Title: Horton Hears a Who! (TV special)
Content: Hans Conried as Horton the Elephant / Narrator / Dr. H. Hoovey ; Chuck Jones as Junior Kangaroo / Vlad Vladikoff (renamed "Whizzer McKwoff") / JoJo ; June Foray as Jane Kangaroo / Birds / Mother Who / Baby Who ; The Mellomen as the Wickersham Brothers ; The MGM Chorus as the Citizens of Whoville 
Title: Horton Hears a Who!
Content:  Horton Hears a Who! was adapted into a half-hour animated TV special by MGM Animation/Visual Arts in 1970. It was directed by Chuck Jones, produced by Theodor Geisel (Dr. Seuss), and narrated by Hans Conried, who also voiced Horton. The Sour Kangaroo's name is Jane Kangaroo while her son is named Junior. Horton's contact in Whoville was not the Mayor, but a scientist named Dr. Hoovie who was also voiced by Hans Conried. Jane Kangaroo was voiced by June Foray. Dr. Seuss was awarded a Peabody Award for the animated special Horton Hears a Who!
Title: Horton Hears a Who! (film)
Content: Jim Carrey as Horton the Elephant, a big-hearted, eccentric, outgoing, sweet and thoughtful elephant and teacher in the Jungle of Nool who possesses acute hearing abilities. ; Steve Carell as Mayor Ned McDodd, the eccentric and fun-loving mayor of Whoville who has 96 daughters, 1 son named JoJo and a wife named Sally. ; Carol Burnett as the Sour Kangaroo, an egomaniacal kangaroo who mistrusts Horton's inquisitive nature as a threat to her legal authority over Nool. ; Will Arnett as Vlad Vladikoff, a savage and scary Russian-accented vulture hired by the Sour Kangaroo to steal Horton's flower. ; Seth Rogen as Morton, a mouse and Horton's best friend in the Jungle of Nool who seemingly doesn't 
Title: Horton the Elephant
Content:  film. ; Vlad Vladikoff: A black eagle that takes the flower that Horton protects and drops it in a huge patch of identical flowers. In the animated special of Horton Hears a Who!, his name is changed to Whizzer McWoff and he appears more like a vulture than an eagle. Voiced by Will Arnett in the film adaptation, speaks in a Russian accent, and truly is a vulture yet made to resemble a condor. ; JoJo: One of the Whos: a small boy who, when exhorted by the Mayor, announces Whoville's existence to the larger world by shouting "YOPP". In Seussical, and the subsequent film, he is the Mayor's son; in the latter, voiced by Jesse McCartney. 

Question: who is jojo in horton hears a who?
Answer: Jesse McCartney.