Color is necessary for face discrimination in the Northern paper wasp, Polistes fuscatus

Abstract

Visual individual recognition requires animals to distinguish among conspecifics based on appearance. Though visual individual recognition has been reported in a range of taxa including primates, birds, and insects, the features that animals require to discriminate between individuals are not well understood. Northern paper wasp females, Polistes fuscatus, possess individually distinctive color patterns on their faces, which mediate individual recognition. However, it is currently unclear what role color plays in the facial recognition system of this species. Thus, we sought to test two possible roles of color in wasp facial recognition. On one hand, color may be important simply because it creates a pattern. If this is the case, then wasps should perform similarly when discriminating color or grayscale images of the same faces. Alternatively, color itself may be important for recognition of an image as a "face", which would predict poorer performance on grayscale discrimination relative to color images. We found wasps performed significantly better when discriminating between color faces compared to grayscale versions of the same faces. In fact, wasps trained on grayscale faces did not perform better than chance, indicating that color is necessary for the recognition of an image as a face by the wasp visual system.

Keywords: Color; Face discrimination; Individual recognition; Operant conditioning; Polistes fuscatus; Vision.

Fig. 1

Stimuli and training paradigm used for this study. (a) Image set used in this study. A total of 6 unique wasp stimuli with identical antenna, mandible positions, and body backgrounds were used to test face discrimination in Polistes fuscatus. Each wasp stimulus also had a corresponding grayscale image used to train half of the animals. For details of stimuli spectral properties and modeled photoreceptor excitation in honey bee visual system see Figure S1. (b) Schematic of pseudorandomized side pairing of shock associated stimulus over trials, including side switching of shock associated stimulus and trial type. Animals experienced two trial types: training (t) and Testing (T). On training trials wasps were allowed to freely move about the arena for 120 seconds and were shocked in the central zone every 10 seconds and when entering the shock associated stimulus zone every 3 seconds. Every third trial was a Testing trial, in which wasps were allowed to freely move about the arena for 60 seconds and no shocks were delivered. To-scale schematic of rectangular training arena schematic with electrical grid flooring, acrylic sheet cover, and removable central zone barriers. Red highlighted sections are electrified. Two stimuli images were affixed to each wall of the choice zones denoted by dark rectangles and faces were denoted by F1 and F2 respectively, (−) denotes electrical shock pairing. (d) Diagram of animal tracking methods and data acquisition. Red dot denotes tracked position on the wasp. Distance of track, left and right side zone entries, and time spent in each zone were collected during each trial.

Fig. 2

Testing trial performance of wasps trained across stimulus treatments: color (orange) or grayscale (gray). (a) Total proportion of safe associated stimulus zone first choices for all testing trials for animals trained across treatments. (b) Latency to enter safe associated stimulus zone for animals trained across treatments. (c) Averaged mean velocity (cm/s) for animals trained across treatments. Treatments denoted by coloration. Symbols denote significant differences: ** p<0.01, *p<0.05, n.s. = non-significant. See methods and results for statistical test details.