Sentence:  Sometimes, as we have seen, one member of a litter is a <WSD>runt</WSD>, much smaller than the rest. He is unable to fight for food as strongly as the rest, and <WSD>runts</WSD> often die. We have considered the conditions under which it would actually pay a mother to let a <WSD>runt</WSD> die. 
Meaning of the word marked with <WSD> tags:  The smallest animal of a litter.