| Mule deer populations have been declining in the western United States for several decades, but until now most studies have focused on either predation or over-winter survival without considering the importance of summer and autumn nutrition. A decrease in the quantity and/or quality of forage in the months proceeding the breeding season has been linked to decreased fertility in several ungulate species and domestic livestock. We simulated the decline in the digestible energy content of forages during summer and fall under a range of habitat conditions, and measured intake, nursing behavior, milk quality, body condition, blood hormones, estrus and pregnancy in captive mule deer and examined how these factors influenced fawn production, growth, and survival. Both lactating does and their fawns increased their dry matter intake (DMI) to try and compensate for a decrease in digestible energy (DE) content of the diet, but did not consume as much DE per day as those feeding on higher quality feed. Probability of pregnancy and twinning increased when does ingested more DE and had more body fat. Measures of the blood hormones insulin growth factor 1 and leptin taken at the beginning of November provided a weak index of pregnancy, twinning and DEI intake. Fawns on the lower DE diet nursed more often, which corresponded with a decline in body condition, and their nursing attempts were rejected more often by their mothers. They also had poorer survival until weaning. Mule deer may fail to become pregnant when forage quality in summer and autumn is especially poor, but our data suggest that food quality has a more drastic effect on fawn growth and survival, therefore, potentially reducing recruitment into the adult population. |
