- "Oh-ho, that's incorrect; we totally abandoned you!"
- ―Marshall's indifference to his brother, Sid[src]
Marshall was responsible, along with the rest of his family, for abandoning Marshall's older brother Sid while the rest of the family migrated off.
Marshall, along with his family, had always been compelled to remain with Sid, who remained a nuisance to the others. Throughout the years, Marshall and his family continued to try to leave Sid behind. One year, the family succeeded in stopping Sid from following them by gagging him with a field mouse, tying his hands and feet, barricading their cave entrance and moving through water so that Sid would lose their scent. Sid found them regardless, and joined them for another migration until the year that his family, Marshall among them, woke up early for the migrations south and left Sid behind.
Marshall and his family, consisting of his mother Eunice, his father Milton, the elderly family matriarch Granny, and the family's uncle, Fungus, returned to find Sid, traveling down the snowy mountains in a hollowed-out log which they had tied Granny to. During the ride, when Uncle Fungus threw his arms up in celebration, this repulsed Marshall due to his stench. The log hit a Saber-Tooth Tiger named Diego during the ride, who Eunice hit in the face before the family of sloths were compelled to hold on tight as the log hit a rock, flinging Diego towards the back of the log, as Marshall and the others ducked. Moments after, the log stopped and they were all flung out of it, nearly colliding with a number of other animals. Moments later, Marshall's brother Sid arrived on the scene, and was overjoyed at seeing his family again and embraced his parents and brother, who pretended to also be happy to see him again, although Marshall openly and shamelessly admitted that they had indeed abandoned him.
At first, Marshall and the rest of the family acted as though they were happy to see Sid again. But once they had Sid lead Granny away and both were out of earshot, Marshall and the rest of Sid's family cheered aloud and rushed back to the log, pushing it towards another downhill slope. While Fungus and Milton were helping to push the log, Eunice explained to Manny, who called out that they couldn't simply abandon Sid again, that things were falling apart back home and that Granny was dead weight. The four sloths rushed off inland on their log, never to be seen again by Sid.
Personality and Traits
Like most of the rest of his family, Marshall was shown to be very shallow; always attempting very hard to get rid of Sid, and abandoning Granny on Sid and then abandoning Sid with her again as dead weight without any empathy. He was also unaware of and/or shameless about his shallowness, as Marshall openly admitted to Sid without any shame or remorse whatsoever that the family had indeed abandoned him.
Most likely Marshall is a dwarf, because despite he looks like a little boy, Sid mentioned his name when in the first movie, about 19-20 years before the fourth one, he woke up and found out that they abandoned him. It's not possible that Sid was referring to another Marshall, since in Ice Age: Continental Drift, he knows who Marshall is and, plus, this last said that they totally abandoned him, and it means that he was present during the abandonment of the first film. So, considering that Marshall is at least 20, the only explanation for his height is that he is a dwarf.