Cannibalism amoung the Donner Party has been highly exaggerated, but it is something that did occur. However, assumptions have become a part of general knowledge.
The Donner Party possibly did not resort to cannibalism at least until February or early March, except the Forlorn Hope, but appearantly everyone was thinking of the subject at least before the Forlorn Hope had left Truckee Lake. This is based on the first mention of cannibalism ever written, in Patrick Breen's Diary from February 26th, 1846: "...Mrs.Murphy said here yesterday that [she] thought she would commence on Milt and eat him. I don't think she has done so yet, [but] it is distresing. The Donno[r]s told the California folks that they [would] commence to eat the dead people 4 days ago, if they did not succeed in finding their cattle then under ten or twelve feet of snow & did not know the spot or near it, I suppose they have [cannibalized] ...ere this time."
However one account involves cannibalism just after the Forlorn Hope left the lake, stating the Donners partook of the reamains of Sam. Shoemaker. However, some consider the accounts of Alder Creek shady, as many of the informants were very young children at the time, and few adults who stayed at Alder Creek survived.
We know the the Forlorn Hope, the Breens, Graves and Donners at Starved Camp, and those who were at the lake after the first relief, esp. the Murphys and Lewis Keseberg survived by cannibalism. It is possible those at the lake and perhaps Alder Creek who left with the First Relief never ate the dead. When William Eddy discovered that the bodies of his family had been mutilated by Keseberg, he threatened to kill Keseberg if they ever met again. The bodies were mutilated but identifiable, not bad considering people had survived on them for long over a month. Furthermore, most were careful to avoid eating the bodies of their family. Not nearly as morbid as many people believed the story of the Donner party to be.
To know who cannibalized is hard, as few were brave enough to admit the grotesque act, and our knowledge of cannibalism mostly comes of the accounts of others who "didn't" cannibalize and the discovery of terribly mutilated corpses by the rescuers. Some supposedly found it too hard to eat of the others, though this could be an exaggeration.
The only characters that have been known to have avoided cannibalism are those who died in or before December, as well as James Reed and William McCutchen. The two Indians, Luis and Salvadore, are also said to have avoided eating the bodies of the dead with the Forlorn Hope. A fair estimate said that at least half the Donner Party survived by cannibalism, however most of the survivors were in this half. (Only about half of the Donner Party survived.)
There were 91 in the Donner Party, including those who died before the winter, e.g. Sarah Keyes; and those who joined the party later, e.g., Luis & Salvadore. Of the 91, 44 died, and 47 survived. Ages taken from July 31, 1846.
Infants: Isabella Breen (S), Margaret Eddy (D), George Foster (D), Elizabeth Graves ([D] after reaching Sutter's Fort), Harriet McCutchen (D), Louis Keseberg, Jr. (D) Survival: 1/6 (-5)
Seniors (over 55) George Donner, age 60/62 (D) Jacob Donner, 56 (D) Franklin Ward Graves, 57 (D) Hardkoop, 60? (D) Sarah Keyes, 70 (D) Survival: 0/5
Total: 55 men/36 women, 91 persons.
Number of men who died, 33.
Number of women who died, 11.
Number of men who survived, 18/55.
Number of women who survived, 25/36.
With lack of food, the men became exhausted with the burden of physical labour. As they became skinnier and weaker, it became harder to fight the coldness of winter.