listen to part of a lecture in a biology class. So let\'s go back to our discussion of how animals defend against predators. Here\'s an interesting phenomenon that caught the attention of some researchers. Female mule deer aggressively defend their young against predators. Not exactly shocking news, but there\'s more. They defend the young of other deer too, even ones of a different species. And not only that, they will even leave their own young alone to do so. This is certainly not common behavior among deer in other species, like the white tailed deer, if a mother senses a threat, she will only defend her own young. So why is the mule deer\'s behavior so different? To find out, the research team conducted an experiment at a cattle ranch where both these species of deer often gather, and to study this unusual mule deer behavior, they decided to simulate a threat from a predator, a coyote.So they recorded distress cries of different baby deer, different fawns. Then they placed loudspeakers in places where the adult deer could hear those recorded cries and respond to them, and they looked at how well different hypotheses could explain the mule deer\'s behavior. The first hypothesis was maybe the mule deer were defending the fawns of other deer simply because of confusion, what we call a recognition error. Maybe they couldn\'t tell the difference between the distress cries of their own fawns and those of others. And in fact, mule deer did not seem to distinguish between the cries of their own fawns and cries of other mule deer, or even white tailed deer, including adults and a mother. Mule Deer always responded the same way, even with her own fawns standing right in front of her. But in such cases, how could there be any confusion? Obviously the cry was not from her own fawn even so she approached the loudspeaker just as aggressively, not what you\'d see a whitetail doing, but in fact, all female mule deer responded aggressively, even ones that were not mothers of young fawns. So recognition errors may well occur, but I think we\'d better look elsewhere for our answer. Okay, the second hypothesis, let\'s call this the bedded fawn hypothesis, that because the younger fawns stay in a nest or a burrow, and so they\'re called bedded fawns, as opposed to active fawns, the researchers reasoned that a mule deer with a bedded fawn might respond more aggressively to any distressed cry because her own fawn couldn\'t run away easily. She\'d want to keep the area around her offspring secure by standing and fighting. But mothers with bedded fawns did not respond more aggressively. So the third hypothesis, what\'s called reciprocal altruism. Now, altruism has a very specific meaning in the world of animal behavior. Simply put, it means that the behavior of one animal benefits others at potential cost to itself, which clearly describes what\'s happening here and reciprocal altruism, that\'s when animals do favors for each other. One animal will do something to help another, like share food, expecting that it will someday be on the receiving end a mother mule deer then might endanger herself to help others, so that those others might later on protect her, or, in this case, her fawn. But theoretically, it only makes sense for her to help individuals that can be relied on to reciprocate, to help her out in a similar situation. So the rule about reciprocal altruism is that an animal has to be able to recognize who is helping Remember, though, that a mule deer cannot seem to identify the cry of its own species, much less of its own offspring. So why do mule deer choose to protect the young of others? Or is this the wrong question to ask? Maybe they\'re just sort of programmed to react mindlessly to any predatory threat without stopping to consider whether it\'s their own offspring or others that they\'re protecting. In any case, we\'ll need more evidence to figure out just how altruism is involved here, and that will require a good deal more research.