Hypotheses from biology propose that females that are in better condition are better able to provide resources to their offspring through maternal investment. The investment can be either direct nutritional or greater behavioral investment. The objectives of these studies were to determine the influence of several maternal factors on differential maternal investment. In the first study, data sets were taken from the beef records from the Wye Angus cattle at the University of Maryland Wye Research and Education Center, Queenstown, Maryland and Grazinglands Research Laboratory west of El Reno, Oklahoma. The relationship between age of dam and gestation traits were analyzed based on calf sex for each data set. Two of six maternal traits were significant predictors of calf sex.

For the second study, data from the Animal Improvement Programs Laboratory (AIPL) of the USDA, which conducts a national evaluation of birthing difficulty (calving ease), were used. Over 12.4 million dairy records were included in an analysis of sex ratio as a function of parity. Cow productivity influenced future sex ratios. The data support the contention that females in better condition produce male offspring and also indicate that domestic animals, in this case cattle, still exhibit genetic influences that result from evolutionary influences - even though artificial selection influences have occurred. The determination of what mechanisms actually underlie the differential sex ratios that result from females of different production capabilities is an important question that needs further consideration.

In the third study, feed competitions between a mother, daughter, and non-related cow were conducted to determine the level of maternal investment after a prolonged separation of mother and daughter. Mothers ate for a significantly greater percentage of the total eating time than either daughter or non-related cows. On the second day of the trials there was a non significant trend for the occurrence of mother-daughter alliances. These results indicate that post weaning associations between mothers and daughters may be disrupted by management practices. The Trivers-Willard hypothesis was supported in dairy cattle, but the beef cattle data only provided limited support for this hypothesis.