Do ewe make the best nutritional choice? A study on sheep food selection
The animal Article of the month for June is entitled ‘Expression of conditioned preference for low-quality food in sheep is modulated by foraging costs’
Past feeding experiences can change animals’ perception about foods. However, little is known about how animals use previously learned food information for decision-making during foraging. This is because previous research has simplified the complexity of the foraging scenario to a choice between foods that are offered in the same place and with similar levels of availability (i.e., free-choice tests traditionally used to evaluate animals’ willingness to eat a given food). In contrast, foods in nature have not only diverse spatial arrangement but also changing availability in time. The study by Catanese et al. (2015) was meant to improve our understanding about how a small herbivore (sheep) uses previously learned information about a particular low-quality food (oat hay) during a simulated “real-life” foraging situation.
In this study, previous experience of sheep with oat hay was manipulated in two ways; one group had feeding experience in a positive nutritional context (CS+), while a second group had a neutral feeding experience (CS-). Foraging behavior of all sheep was then evaluated in an experimental arena where sheep had basically to decide whether to search and exploit a preferred high-quality food (alfalfa hay), or to be more generalist and include also the oat hay. Alfalfa hay availability in the arena was restricted at different levels (i.e., from a very high level resembling traditional free-choice tests to an almost negligible level of availability) in different sessions, which gave a wide array of contextual frameworks for foraging decisions.
Overall, sheep used information about the low-quality food in the U-shaped corridor according to an optimality criterion. Regardless of the per
ceived value of the oat hay (i.e., previous experience) all sheep decided to eat almost exclusively the alfalfa hay when it was accessible enough. Inclusion of the low-quality food in the diet would depend on its own profitability only if foraging the high-quality food has associated high foraging costs. This was supported by our results because, when availability of alfalfa hay was increasingly restricted (i.e. the sheep had to work harder to get this food), CS+ sheep showed higher intake and selection of the oat hay than did CS− sheep. Moreover, these contrasting outcomes were associated to differences in foraging strategies; for instance, sheep in CS+ spent more time foraging oat hay and were more likely to switch to oat hay if they had previously been eating alfalfa hay than sheep in CS−.
As a final note, this study shed light on a likely interplay between learning models (what do animals learn about foods?) and optimal foraging models (how animals use nutritional and environmental information for decision-making?) of diet selection. Sheep probably ‘eat the best and leave the rest,’ but if the best is restricted, they ‘eat the best of the rest;’ and here is where past experiences with low-quality foods impact preferences.
Authors: F. Catanese, R. A. Distel and J. J. Villalba
The animal Article of the Month is selected by the Editor-in-Chief and is freely available for one month