R. W. Chambers is of the opinion that the Grettissaga and the Beowulf descended from a common source. One important variation between the two, however, is that the female monster in the Grettissaga is the one which roams about and the male monster remains in the cave. In Beowulf the reverse is true; Grendel ravages Heorot and his mother only ventures into the hall of men after her son is killed. On this variation Chambers surprisingly commented in 1931:
In this the Grettissaga probably represents a corrupt tradition: for, that the female should remain at home whilst the male searches for his prey, is a rule which holds good for devils as well as for men. (Beowulf: An introduction to the poem with a discussion of the stories of Offa and Finn, 49)
Overlooking the more interesting historical elements of Chambers' work, the reversal of genders in the Beowulf and Grettir monsters explains much in Beowulf's story. It is unlikely that merely ripping off an arm would kill a monster like Grendel; the act of diving into the underwater cave in the Grettissaga is in pursuit of the male monster residing in his home. The story from which the two descended probably went like this:
This hypothetical story would also account for the reason why Beowulf brings back Grendel's head instead of his mother's. If everyone believes that Grendel is dead, why not bring back the female monster's head as proof of her death? Again, if there was only a single male monster to begin with, only his head would appear in Beowulf
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