Parental environment: effects on survival, growth and aggressive behaviors of 2 rodent species

The influence of the parental environment on the development of aggressive behaviors was studied in 2 muroid rodent species. Litters of southern grasshopper mice and northern white-footed mice were reared by the natural parents or were reciprocally cross-fostered soon after birth to parents of the opposite species. After weaning at 24–26 days, mice of both species were isolated and observed at 10-day intervals from 30 to 100 days of age in one of the following tests: (1) predation on house crickets; (2) interspecific aggression toward Swiss-Webster laboratory mice; and (3) intraspecific aggression toward opponents of the same gender and approximate age. Naturally reared grasshopper mice males and females displayed extremely high levels of aggressive and predatory behaviors whereas white-footed mice controls were passive and defensive in all tests. Rearing by white-footed mice foster parents resulted in a significant decrease in the aggression of grasshopper mice males and females toward laboratory mice. During intraspecific encounters, fostered grasshopper mice initiated fewer social interactions than naturally reared controls. The predatory behavior of grasshopper mice was not altered by the fostering procedure. The behavioral measures of fostered white-footed mice showed no systematic changes when compared to controls. These results indicate that the postnatal parental environment contributes to the naturally high levels of aggressive behaviors of grasshopper mice. In contrast, the limited aggressive behaviors of a naturally passive species, white-footed mice, were not increased by fostering at birth to parents of a highly aggressive species, grasshopper mice.