Characteristics of the acorns selected by free range Iberian pigs during the montanera season

Acorn weight, size, shape and composition vary a great deal among evergreen oaks (Quercus ilex rotundifolia); however these characteristics remain constant for a tree throughout the fruiting period. The number of consumed acorns under the canopy of a tree could be an indicator of preference for its acorn characteristics. This paper analyses the characteristics of the acorns selected by free-ranging Iberian pigs at the start (November) and end (February) of the montanera or acorn mast-feeding season, when these are sustained fattened. At these two stages, acorn samples were taken from 20 oak trees (Q. ilex rotundifolia) that had been rejected by the pigs (only eating between 2 and 9 acorns from the ground underneath the tree) and from 20 oak trees that were actively sought out by the pigs (eating at least 40 acorns). The differences observed between the sought out and rejected acorns at the start and end of the montanera season are too great to be only a matter of chance, suggesting that Iberian pigs must form associations between variables when choosing to eat or reject the acorns from a specific oak tree. The results show that pigs tend to select heavier acorns at the start of the montanera season, while at the end their selection is based more on the composition of the acorns. The greatest number of significant differences between the groups of acorns were found in the sought out acorns in November and February, owing to all the variables related with the composition of the acorns: crude fat (10.8 ± 0.3 vs. 7.4 ± 0.3), starch (58.3 ± 1.0 vs. 50.3 ± 0.4), sugars (6.8 ± 0.4 vs. 13.2 ±0.3); most of the fatty acids; dimensions: length (38.1 ± 0.8 vs. 31.9 ± 0.7) and diameter (16.6 ± 0.4 vs. 13.9 ± 0.1); acorn weights: fresh whole acorn (6.9 ± 0.3 vs. 3.4 ± 0.1) and kernel DM (2.5 ± 0.2 vs. 1.7 ± 0.1). Pigs prefer acorns with larger kernel. This selective consume goes along with the theory of Optimal Foraging and suggest that pigs learn to use visual stimuli when grazing. In conclusion, this study shows a constant selective acorn consumption and a variation in acorn preferences throughout the montanera period; consequently it is proven that acorn diet is not homogeneous and varies over the fattening period.