In the semi-reserves, in the Netherlands and Germany, lived a variety of game animals, such as mufflons, fallow deer, foxes, badgers and some wild boars. Although most animals kept their distance from the Przewalski horses, we once observed mutual investigatory behaviour of a young fox and a colt, which kept them busy for at least an hour. We had the strong impression that young Przewalski horses developed a much higher responsiveness to changes in their biological and physical environment in the semi-reserves through the variety of opportunities for perceptual and locomotion stimulation. These experiences helped them to facilitate survival after release into the wild.
VOLF (1967) studied the lower jaws of Przewalski horses caught wild comparing them with those from the first, second and third generation in captivity. The massively built lower jaw changed considerably in outline because of a different process of wear off the molars through zoo-food instead of the harsh food in the wild. The type as well as the quantity of food for the Przewalski horses in zoos is kept mostly the same during all seasons. Their condition is kept the same time in summer or winter. The change in life from confinement to free ranging has effected the Przewalski horses in many ways and has to be seen as a gradual process. It is our impression however that the rate of feralization is slower when the Przewalski horses have to rely on humans for food, like for instance the group at Noorderheide.
The main activity of the Przewalski horses in the semi-reserves was feeding, it took them many hours a day as well as night. During the seasons the nutrient value of the food varied greatly and in winter the protein content was much lower, they had to eat more to make up for reduced quality. It took the zoo-bred animals some years before they could maintain themselves on a high fiber, low protein diet in winter without extra food supply. The Przewalski horses were better able to build up sufficient fat in summer for the next winter.
The diets of the Przewalski horses varied greatly between the semi-reserves. As all horses they are opportunistic feeders, selecting the most palatable and accessible grasses and herbs available to them. In summer in Nature Park Lelystad they favoured eating reed and buds of thistles. In woodlands, they browsed in different seasons on a variety of plants, like leaves of birch, oak and brambles. They also did eat the harder buds of heather. In the fall Przewalski horses could be seen grubbing under oaks for fallen acorns. In late winter they sometimes consumed barks of different trees and nibbled on branches. On the Goudplaat the young bachelors even did eat the thorny Hippophaë rhamnoides and gorze. Social factors certainly influenced feeding patterns. When an older mare was introduced into Noorderheide, coming from the more experienced group at Lelystad, the two younger mares showed a different use of the territory and their menu was enlarged. It is for sure that young Przewalski horses, which learn to graze and browse whilst going taught by experienced mothers, might be better pre-adapted to the wild, than young horses, which are used to palleted food and hay only.