Reproductive Energetics in Giraffes

Motherhood has a high metabolic cost. Mammalian mothers provide energy for growth and development of their offspring during both pregnancy and lactation. To compensate for these energetic demands, females may increase food intake by spending more time feeding, reduce metabolic rate by spending more time resting, and/or alter their diet by consuming more energy-rich foods.

Giraffes have a challenge because they become pregnant while still nursing their previous offspring, produce high-quality milk that is rich in protein and fat, and have calves that grow rapidly. How giraffes manage their metabolic costs depends upon determining a giraffe's reproductive state, but pregnancy is not visually detectable until late in gestation. To understand how giraffes manage their maternal costs, it is important to determine how their daily activity budget might change with a different reproductive state. Dr. Meredith Bashaw and Dr. Fred Bercovitch of the Behavioral Biology Division of the San Diego Zoo's CRES observed giraffe behavior at the San Diego Zoo's Wild Animal Park and collected daily fecal samples to examine how hormone changes during the reproductive cycle might also modify social and feeding behavior.

The amount of time calves spent feeding on solid foods increased dramatically in their third to fourth month of life, although they were generally still nursing at this age. Female giraffes do not appear to compensate for raising two young simultaneously by increasing food intake, but daily time budget does change with reproductive state. Females spent less time involved in social activity when pregnant than when acyclic or cycling, with cycling females spending the most amount of time engaged in social behavior. In addition, females tended to locomote less when pregnant than when acyclic. Pregnant females could be adjusting metabolic expenditure by reducing activity level rather than increasing dietary intake, and cycling females could be spending more time involved in social behavior due to male sexual interest. Female giraffes have probably evolved a reproductive strategy enabling them to channel resources into both a growing fetus and a calf. The mother produces high-quality milk for the calf during the period of slow fetal growth, and fetal growth is accelerated during weaning.