Goats work for food in a contrafreeloading task

Abstract

Contrafreeloading (CFL) is the phenomenon when animals work for a resource although an identical resource is available for free. Possible explanations for CFL are that animals seek context for species-specific behaviours or to control their environments. We investigated whether goats show CFL and whether breeding for productivity traits has altered its occurrence. In a manipulation task, we compared two selection lines: 27 Nigerian dwarf goats, not bred for productivity traits, and 30 dairy goats, bred for high milk yield. Over 10 trials, each goat could perform one of three behaviours: not participating in the trial, feeding for free from an open door, or opening a sliding door for a feed of similar value. The results were analysed using an Item Response Tree (IRTree) generalized linear mixed model (GLMM). The fitted probabilities to participate were > 0.87 over all trials in both selection lines. For dwarf goats, the probability of choosing the closed door, and thereby demonstrating CFL, increased from 0.30 in Trial 1 to 0.53 in Trial 10. For dairy goats, this probability was constant at approximately 0.43. Unlike dwarf goats, dairy goats were faster to approach the closed compared to the open door. Overall, our results suggest that both selection lines were similarly interested in CFL.

Figure 1

(a) Schematic drawing of the test arena with left door closed and right door open including the binary response tree with four nodes representing the sequential choices leading to one of five behavioural categories (= five square symbols). A number of observations with non-missing information at respective nodes are given in the tree. Positions of the experimenters are indicated with E1 and E2, and the position of the video camera is marked with a camera symbol. (b) The five behavioural categories with their symbols corresponding to the tree in (a) as well as the encoding of the node for the IRTree model.

Figure 2

Individual distribution of choices regarding door type and approach time from trials of dwarf and dairy goats. Names of individuals are composed of a letter for the pen affiliation (dwarf: A, B, C; dairy: X, Y, Z) and a number for the individual within each pen (0–9).

Figure 3

Fitted probabilities (lines) of the IRTree GLMM at the four nodes and observed proportions (bars). The shaded areas represent 95% confidence bands for the fitted values considering the fixed effect uncertainty.

Figure 4

Fitted probabilities of the IRTree GLMM for the five behavioural categories. These probabilities were calculated by multiplying the probabilities of the corresponding sequential choices (Fig. 1a,b). For example, the probability for the behavioural category ‘open door, long approach time’ (Figs. 1a,b, 3) was calculated as the probability to participate (Node 1), times the probability to choose the open door (Node 2), times the probability to show a long approach time (Node 4).

Figure 5

Fitted approach times of the linear mixed model (back-transformed from log2 to linear scale). The shaded areas represent 95% confidence bands for the fitted values considering the fixed effect uncertainty. Dots represent observed approach times. For each trial number, the dots were horizontally jittered for visual clarity. Data points > 8 s are not shown (numbers given in panels).