Reproductive Effort in the Red Fox, Vulpes vulpes, and Future Supply of a Fluctuating Prey

The red fox Vulpes vulpes L. in boreal Sweden is for several reasons likely to exhibit adaptive adjustment of reproductive effort to future abundance of prey. Mean litter size at birth in late April is adjusted to ambient vole (food) supply through variations in ovulation rate two months earlier. Yearly ovulation rate correlates better with vole supply at the birth of young than with average vole supply during winter, amount of voles ingested during January-March or condition of females during the same period. Hence, foxes seem to be able to anticipate future food supply. I discuss three explanations for this trait: 1) an inherited knowledge of the normal vole cycle; 2) a capacity to detect the direction of change in the availability of voles irrespective of the absolute level; and 3) a reaction to the presence of reproducing voles in winter during increase years. For different reasons I find the third explanation the most likely; either as an adaptation to smell and taste of the voles or as a physiological by-product of the gonadotropic hormones provided by the ingested voles.