Intraspecific variation in body size of bumblebee workers influences anti‐predator behaviour

Abstract

Flower‐dwelling predators make flowers dangerous foraging sites for pollinators, potentially affecting their anti‐predator behaviour. Moreover, predation vulnerability often varies among pollinators' body sizes with interspecific comparisons showing that smaller species are more vulnerable than larger ones. However, how intraspecific body size variation influences pollinator behaviour under predation risk is still unknown, especially under natural conditions. We hypothesized that bumblebee workers of different sizes will exhibit different foraging strategies under predation risk. We predict that (a) small workers should more often exhibit anti‐predator behaviours than larger workers. We also hypothesized that the anti‐predator behaviour should be influenced by predator size and reward availability; therefore, we expect (b) higher avoidance behaviour towards larger predator sizes and (c) more and longer visits to inflorescences with high nectar availability. Finally, we expect that (d) nectar availability should overcome the anti‐predator behaviour in less vulnerable, large, workers. We recorded flower visitation, time spent and rejection behaviours of different sizes of Bombus terrestris (Apidae) workers (large, medium and small) to inflorescences of Alstroemeria aurea (Alstroemeriaceae) with different treatments of artificial spiders (small and large) and nectar availability (with, without). Anti‐predator and foraging behaviour of bumblebees was affected by the size of the worker, the presence of artificial spiders and nectar availability. Large and medium size bumblebees strongly reduced flower visitation and time spent in the presence of artificial spiders, consistently avoiding flowers with spiders, regardless of spider size or nectar availability. Instead, small bumblebees seldom modified their behaviour when facing artificial spiders, only increasing their avoidance or decreasing their foraging time in nectarless flowers hosting large artificial spiders. This pattern of larger workers being more sensitive to predation risk than smaller ones at the intraspecific level in B. terrestris is contrary to the expected and acknowledged trend based on previous interspecific comparisons, but partially consistent with predictions of models of optimal foraging theory. Intraspecific behavioural variability was uncovered only when nectar was available, whereas artificial predator size rarely modified bumblebee anti‐predator and foraging behaviour. Therefore, our findings suggest that the trade‐off between maximizing resource intake and minimizing predation risk strongly varies across bumblebee worker body sizes.

The size dependency of foraging behaviour: an empirical test performed on aquatic amphipods

“…It is therefore possible that individuals who spent more time on resource patches reduced their energy requirements. Furthermore, small individuals may adopt strategies aimed at reducing activity and diverting energy towards growth (Killen et al 2007) or may differ in anti-predator behaviour (Gavini et al 2020), which could also explain the lower frequency and longer stays in the resource patches that we observed with increasing size. New experiments involving accurate measurements of the individual forager's metabolic rate are needed to better understand the relationship between body sizes, energy requirements, and patch exploitation behaviour.…”

Seasonal Variations of Pollinator Assemblages among Urban and Rural Habitats: A Comparative Approach Using a Standardized Plant Community

“…Analyses of ITD variations over time in the two habitats were carried out separately for bumble bees and other wild bee taxa. Indeed, bumble bees are eusocial species, with important body size differences among different categories of individuals (queens, drones, workers) [57], and whose abundances vary along the course of the active season. In addition, they have an endogenous thermoregulation capacity, which can play a role in the temperature-height relationship [28,29,58].…”

The context dependence of non‐consumptive predator effects

“…First, an individual's capacity to recognise danger may be state-dependent, as when prey acquires the capacity to detect and respond appropriately to cues via development/growth and learning (Kavaliers & Choleris 2001). For example large bumblebees (Bombus terrestris) are more sensitive to spider risk while visiting inflorescences, likely (at least in part) because they possess eyes with greater visual acuity than smaller conspecifics (Gavini et al 2019). Fathead minnows (Pimephales promelas) learned to recognise northern pike (Esox lucius) as predators from a paired exposure to conspecific alarm pheromones and pike odour (Ferrari et al 2006).…”