Evidence of object permanence, short-term spatial memory, causality, understanding of object properties and gravity across five different ungulate species

Abstract

In their natural environment, animals face a variety of ecological and social challenges, which might be linked to the emergence of different cognitive skills. To assess inter-specific variation in cognitive skills, we used ungulates as a study model, testing a total of 26 captive individuals across 5 different species (i.e., dwarf goats, Capra aegagrus hircus, llamas, Lama glama, guanacos, Lama guanicoe, zebras, Equus grevyi, and rhinos, Diceros bicornis michaeli). Across species, we used the same well-established experimental procedures to test individuals' performance in naïve physics tasks, i.e. object permanence, short-term spatial memory, causality, understanding of object properties, and gravity. Our results revealed that study subjects showed object permanence, were able to remember the position of hidden food after up to 60 s, and inferred the position of hidden food from the sound produced or not produced when shaking containers. Moreover, they showed an understanding of basic object properties, being able to locate objects hidden behind occluders based on their size and inclination, and could reliably follow the trajectory of falling objects across different conditions. Finally, inter-specific differences were limited to the understanding of object properties, and suggest that domesticated species as goats might perform better than non-domesticated ones in tasks requiring these skills. These results provide new information on the cognitive skills of a still understudied taxon and confirm ungulates as a promising taxon for the comparative study of cognitive evolution.

Keywords: Cognition; Gravity; Inference; Memory; Object properties; Permanence; Ungulates.

Figure 1

Setup for the object permanence, short-term spatial memory and causality tasks, consisting of two identical containers.

Figure 2

Setup for the object properties task, consisting of a board with attached panels, an occluder (left) and different panels (right).

Figure 3

Setup for the gravity task, as seen by the study animals, including the container condition (left), the vertical tubes condition (middle) and the crossed tubes condition (right).

Figure 4

Proportion of correct choices in each condition of the object permanence task. Light grey dots represent average values for each individual goat, light grey pluses for each guanaco, dark grey triangles for each llama, dark grey crosses for each rhino, and black circles for each zebra. Thick lines represent the median of the individual values for each condition, the horizontal ends of the box represent the 25% and 75% quartiles, and the ends of the whiskers represent the 2.5% and 97.5% quartiles. The grey dotted line represents chance level.

Figure 5

Proportion of correct choices for each species in the object properties task. Light grey dots represent average values for the flat condition, light grey pluses for the 4 × 16 condition, dark grey triangles for the 16 × 4 condition, and dark grey crosses for the control condition. Thick lines represent the median of the individual values for each condition, the horizontal ends of the box represent the 25% and 75% quartiles, and the ends of the whiskers represent the 2.5% and 97.5% quartiles. The grey dotted line represents chance level.

Figure 6

Proportion of correct choices in each condition of the object properties task. Light grey dots represent average values for each individual goat, light grey pluses for each guanaco, dark grey triangles for each llama, dark grey crosses for each rhino, and black circles for each zebra. Thick lines represent the median of the individual values for each condition, the horizontal ends of the box represent the 25% and 75% quartiles, and the ends of the whiskers represent the 2.5% and 97.5% quartiles. The grey dotted line represents chance level.

Figure 7

Proportion of correct choices in each condition of the object gravity task. Light grey dots represent average values for each individual goat, light grey pluses for each guanaco, dark grey triangles for each llama, dark grey crosses for each rhino, and black circles for each zebra. Thick lines represent the median of the individual values for each condition, the horizontal ends of the box represent the 25% and 75% quartiles, and the ends of the whiskers represent the 2.5% and 97.5% quartiles. The grey dotted line represents chance level.