It has been observed that sympatric bumblebees species assemblages often exhibit similarities in their colour patterns (Plowright & Owen, 1980; Williams, 2007). An attractive suggestion is that this inter-specific similarity in coloration could result from learnt avoidance by avian predators: if a bird learns from adverse experience that a certain colour pattern is associated with a painful sting, it might subsequently generalize this experience to avoiding similar patterns displayed by other bee species, potentially leading aposematic insects to form local mimicry rings (Plowright & Owen, 1980; Williams, 2007). We recently tested this idea with reciprocal transplant experiments using bumblebees in which we compared the worker loss rates of foreign and local colour pattern variants. If local mimicry rings protect the local colour variant, we would expect foreign individuals, with unusual colour patterns, to be exposed to higher predation than local bee populations whose colour patterns should be familiar to local predators (Stelzer et al., 2010). The hypothesis that foragers that differ in coloration from the local native population would suffer higher predation risk was not upheld by our experiments. However, Owen (2014) now disputes the conclusions we draw from our empirical study. Having undertaken a re-analysis of our published data, he proposes a number of additional, alternative hypotheses, as yet unsupported by empirical evidence, to explain the perceived discrepancy between our data and the mimicry ring hypothesis. Owen suggests that if all the additional conditions he outlines apply, our data might be consistent with the mimicry ring hypothesis. Here, we carefully explore Owen’s hypotheses and provide evidence to reject them. Other explanations are therefore needed to explain the apparent convergence of bumblebee colour patterns in certain locations and its impact on predation rates. Insectivorous birds do not initially avoid aposematic insects but swiftly learn to associate visual patterns with aversive reinforcement (Mostler, 1935; Skelhorn & Rowe, 2006; Chittka & Osorio, 2007). Such learnt avoidance has been suggested to explain the observation that bumblebees from different species sometimes appear to display similar colour patterns, even though there is a remarkable diversity in coat colour both within and between bumblebee species (Williams, 2007). Support for this hypothesis requires more than just quantification of bumblebee colour patterns in various locations and an assessment of their apparent similarity from an avian predator’s perspective; it also requires an empirical demonstration that bumblebees with different colour patterns experience differential predation, depending on how similar their appearance is to the predominant local aposematic colour pattern(s). We explored whether diversity in bumblebee colour patterns, found in several locations in which different bee colorations naturally predominate, affects the loss rate of workers during foraging – some of which must be because of predation (Stelzer et al., 2010). However, although we found that there were pronounced differences in loss rates between bumblebees with different colour patterns in all locations tested, the native, local colour patterns did not provide consistently higher safety from being lost during foraging than non-native coloration (Stelzer et al., 2010). Owen (2014) observes that during summer (after fledging), the number of young, inexperienced birds can outnumber older, experienced individuals (King, Farner & Mewaldt, 1965). During this period, one might not expect local aposematic insects to be better protected because naive birds will initially sample all colour patterns equally. Our data allow us to test this hypothesis as two of the experiments were conducted during summer months (July and August in the UK and Germany) and compared loss rates of bees presenting native-like colour patterns with foreign ones (Stelzer et al., 2010). In the UK summer, the loss rate for the native-like colour pattern (Bombus terrestris dalmatinus) was consistently lower than for the non-native pattern (Bombus terrestris canariensis) in both years, though this difference was not statistically significant in our analysis (Stelzer et al., 2010). In Owen’s re-analysis of the same data using a different statistical test, he found a significant advantage for the native-like colour pattern (Owen, 2014) – refuting his own suggestion that there bs_bs_banner Journal of Zoology
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May 27, 2024
Tadgh Hegarty + 1
Open Access