Bumblebees locate goals in 3D with absolute height estimation from ventral optic flow

When foraging, flying insects such as bees are often required to change their flight altitude from close to the ground to above the height of vegetation to reach their nest or a food source. Although the mechanisms of navigating towards a goal in two dimensions are well investigated, the explicit use of height as a source for navigation in three dimensions remains mostly unknown. Our study aimed to unravel which strategies bumblebees use for height estimation and whether they rely on global or local cues. We expanded a 2D goal localization paradigm, where a goal location is indicated by cylindrical landmarks, to the third dimension using spherical landmarks to indicate a feeder's position in 3D and examined the search pattern of bumblebees. Additionally, we assessed the ability of bees to estimate the height of a feeder based on local landmarks and global references such as the ground. The search distribution for a feeder's position in 3D was less spatially concentrated compared with 2D. Assessing the height estimation ability of the bees, we found they could estimate a feeder's height using the ground as a reference. However, the feeder needed to be sufficiently close to the ground for the bees to choose correctly. When bumblebees are faced with the challenge of foraging in a 3D environment where the height of a food source and landmark cues are important, they demonstrate the ability to learn and return to a specific flower height. This suggests bumblebees rely on ventral optic flow for goal height estimation.