Bumblebee Behaviour during Ecological Twilight

Abstract

In the early morning and late evening, diurnal terrestrial animals experience significant strongly directional changes in the light environment as the sun rises and sets. These are key transitional times in the initiation and cessation of behaviours, at the interface between diurnal and crepuscular or nocturnal habits. However, these times of day are both poorly defined and frequently neglected in descriptions of animal behaviour and sensory ecology, despite being regularly encountered and potentially challenging sensory environments. I first argue the importance of examining these understudied periods of the day in the context of diurnal insects, redefining periods of directional photic change in the diel cycle as ecological twilight. I then focus on the buff-tailed bumblebee, Bombus terrestris audax, a diurnal forager known to utilise these times. Using laboratory and field experiments, I determine elements of foraging activity, choice, and behaviour under the natural changing light conditions of ecological twilight. I combine behavioural observation, pollen microscopy, radio-frequency identification automated monitoring, and manipulation of the light environment with artificial light to explore (i) initiation of foraging at sunrise and sunset, (ii) floral choices and foraging success at sunset, (iii) individual and colony-level activity patterns through the diel cycle, and (iv) potential impacts of artificial illumination on sunset foraging. The findings in the following chapters indicate that a subset of available foragers initiate foraging bouts during ecological twilight after assessing the current light condition. Evening foragers maintained strong species preferences established in the daytime, but suffered a pollen acquisition cost. Individual identity and colony identity predicted when foragers initiated the earliest and latest bouts of the day, although bees retained plasticity in their timings in response to daily fluctuations in environmental factors. Finally, bees exposed to a point-source of artificial light did not expand their foraging day later than bees in ambient conditions, nor did they show increases in evening activity. Together, the results of this thesis expand the understanding of foraging behaviours of bees across the entire day, including under naturally dimmer and changing light conditions rather than only under brighter conditions. For diurnal insects more generally, bumblebees can model the potential behavioural changes we may expect from those taxa which access ecological twilight. Additionally, it is vital to improve our knowledge of how animals utilise ecological twilight as increasing anthropogenic pressures such as rising temperatures and sensory pollution push more species’ activity patterns into these times, while simultaneously altering the sensory environment of the niche.