"All that year the animals worked like slaves. But they were happy in their work; they grudged no effort or sacrifice, well aware that everything that they did was for the benefit of themselves and those of their kind who would come after them, and not for a pack of idle, thieving human beings." What does the underlined sentence above mean? Many thanks.
It meanst that they did not resent making effort or sacrifice. They did it willingly.
Look up "to grudge/begrudge", if you didn't already.