In Chapter Four of "Call of The Wild" in the last paragraph, the dogs knew what had happened behind the river trees. But what happened? It...

The Scotch half-breed waited until the train had "passed out of sight behind a belt of river timber".  He then "slowly retraced his steps to the camp they had left", and, with "a single revolver shot", euthanized his faithful dog Dave. 

The last two pages of the chapter chronicle Dave's deterioration and his heart-rending determination to remain in the traces until the end.  "Something had gone wrong with (Dave)"; he became weaker and weaker, "falling repeatedly in the traces", and crying out from the pain of a an unidentified hurt deep within him.  The half-breed cut him loose, hoping that rest would allow the dog to get better, but Dave bit through the harness of the dog that replaced him, demanding to be allowed to pull in his usual spot.  The driver, knowing that "a dog could break its heart through being denied the work that killed it", allowed him to pull until he coould literally pull no more.  At the end, Dave did not even have strength to drag himself to the traces, and "the last his mates saw of him he lay gasping in the snow and yearning toward them".  It was then that the Scotch half-breed drove his team around the bend, leaving them while he went back to put Dave out of his misery.  Even though they could not see what was happening, the dogs heard the gunshot and knew that Dave has been killed (Chapter 4).