Fluctuating resources and the evolution of litter size in the arctic fox

Fluctuations in essential resources cause a strong selection pressure on th e ability to adjust parental investment accordingly. In the dog family, Can idae, variance in female prebirth investment is adjusted by litter size. Th e arctic fox, Alopex lagopus, is a small canid living on the northern tundr as of the world. It has the largest known litter size in the order Carnivor a. up to 18 young, and litter size is highly variable. We have analysed dat a From arctic fox populations throughout the species circumpolar range. In some areas, arctic foxes feed on strongly fluctuating populations of small rodents. In contrast, they have more stable food resources at bird cliffs a nd along coast lines. Food availability determines arctic fos litter and po pulation sizes. A comparison between fluctuating and stable arctic fox popu lations showed that fluctuations are associated with large litter sizes. Th ere were significant differences in litter size means, maxima and variances , as well as in placental scar count means. We have discussed five hypothes es on the determination of variation in litter size: one energetic, one gen etic (based on density variation), one diet-determined, one based on reprod uctive allocation and one based on differences in reaction norms. Our findi ngs suggest that litter size in the arctic fox is determined by the combine d effect of immediate resource levels and the degree of resource predictabi lity. We describe reaction norms that suggest how litter sizes result from adaptive plasticity within each of two genetic strategies where, according to the jackpot hypothesis, populations with unpredictable food resources ge nerally have larger litter sizes. Within each genetic strategy, or reaction norm, litter sizes are adjusted through a number of plastic trails. These traits are influenced by nutritional limitations and include reduced ovulat ion rates, prenatal losses, and litter size reduction during the lactation period.