Male mice, like many of their rodent counterparts, tend to be absentee fathers. Moreover, when they do come in contact with newborns, they may even attack or kill their young. In the past, such hostile behavior has been attributed to high testosterone levels, although a direct link between that hormone and infant-specific aggression had not been found. Jon E. Levine of Northwestern University and his colleagues turned their attention to progesterone, a hormone typically associated with females, engineering a strain of mice lacking the gene that encodes progesterone receptors. The resulting animals were immune to progesterone's effects. "In male knockout mice we noticed something quite startling," Levine says. "They behaved differently, and the most obvious changes were a complete lack of aggression toward infants and the emergence of active paternal care. These animals are terrific dads." In fact, whereas 74 percent of the control mice committed infanticide after the birth of their first litter, none of the knockout mice killed their offspring. The team then administered progesterone inhibitors to normal mice and found that the animals behaved much as their knockout counterparts did.
Although the lack of progesterone diminished the male mice's aggression toward infants, their aggressive behavior toward other males did not change dramatically. The researchers thus posit that general aggression levels are testosterone-dependent and that progesterone mediates aggression toward progeny alone. "At least in the case of mice, this appears to be an important neurochemical switch that can increase paternal behavior and decrease aggressive behavior toward infants," Levine notes. "The same neuroendocrine mechanism may be important in other mammals, including humans, but further research is required."