A reference to the sometimes violent natural world, in which predatory animals unsentimentally cover their teeth and claws with the blood of their prey as they kill and devour them.
This has the sound of a proverbial phrase which might come from the Bible or from Shakespeare. Search the Bible for 'tooth' and you'll find little other than 'an eye for an eye, a tooth for a tooth'. Shakespeare comes a little closer and refers to 'an adders' tooth', 'a serpent's tooth' and even to an animal with claws - 'a mad dog's tooth'. The line is in fact much more recent than either of those sources and comes from Alfred Lord Tennyson's In Memoriam A. H. H., 1850. The quotation comes in Canto 56 (it is a very long poem) and refers to man:
Who trusted God was love indeed
And love Creation's final law
Tho' Nature, red in tooth and claw
With ravine, shriek'd against his creed
'Tooth and claw' was already in use as a phrase denoting wild nature by Tennyson's day. For example, this piece from The Hagerstown Mail, March 1837:
"Hereupon, the beasts, enraged at the humbug, fell upon him tooth and claw."
A.H.H. was Tennyson's friend Arthur Henry Hallam and the poet used the elegy to pose questions about the apparent conflict between love as the basis of the Christian religion and the callousness of nature. If nature is purposeless and heartless, how can we believe in creation's final law? But, as a Christian, how could he not?
The wide-ranging poem didn't attempt to provide an answer, but did become part of the debate over the major scientific and theological concern of Victorian thinkers - Charles Darwin's theories on natural selection, as expressed in The Origin of Species, 1859. On into the 20th century, the enthusiastic Darwinist Richard Dawkins used 'red in tooth and claw' in The Selfish Gene, to summarize the behaviour of all living things which arises out of the survival of the fittest doctrine.