Learningrelated individuality

Pollinators learn about diverse aspects of their environment (see other chapters of this volume). Because each pollinator's experience is unique, its behavior may also be unique. Much of this experience, however, is beyond the control of the observer. Moreover, each bee's experience (e.g., which flower species it experiences as rewarding) may in part be an epi-phenomenon of its decision where to forage (see above) or may simply reflect stochastic processes.

The efficiency and accuracy with which pollinators handle flowers depends substantially on their experience with the respective flower type (Laverty 1994; Chittka & Thomson 1997). Some complex handling skills, such as nectar robbing in Corydalis cava (Fumariaceae) by bumble bee queens can take several days to develop (Olesen 1996). But handling efficiency on a given flower type can also be influenced by pollinators'

experience on other flower types. Depending on the similarity of motor patterns involved (and depending on the timing of visits to the two flower types), transfer or interference may occur (Chittka & Thomson 1997; Gegear & Laverty, this volume).

In the following paragraphs, we are concerned with the possibility of more formative types of learning, i.e., the possibility that early experience may substantially influence how a pollinator later reacts to flowers. Memory-through-metamorphosis has been invoked as a possible mechanism to determine foraging preferences in specialist bees (Dobson 1994), i.e., the possibility that bees become imprinted on particular scents (such as pollen odor) as larvae, and as adults show a preference for flowers with the same scent. The nervous system is entirely reorganized during metamorphosis; therefore a memory that persists through fundamental rewiring of neuronal circuitry is not trivial. However, Lindauer (1985) earlier found evidence for memory-through-metamorphosis in honeybees. The phenomenon, however, was also shown in grain beetles (Alloway 1972) and fruitflies (Tully et al. 1994), so it is not restricted to pollinators.

Does early experience shape the brain, as some studies on humans suggest (Elbert et al. 1995).? The mushroom bodies, a prominent structure in the insect brain, are essential in memory formation (Menzel, this volume). Interestingly, the size of the mushroom bodies in honeybees is correlated not only with age, but also with type of activity. Durst et al. (1994) showed that foragers have larger mushroom-body volumes than nurse bees of the same age, concluding that mushroom-body size is experience-dependent. The rationale was that more information storage requires more neural substrate (e.g more neurons or dendritic proliferations). However, it was not clear whether the mushroom bodies increase in size as a result of experience, or whether the increased mushroom-body volume is a prerequisite in honeybees to switch from nursing to foraging activities.

To resolve this problem, Fahrbach et al. (1998) reared honeybees in an extremely deprived environment (social isolation and complete darkness). Mushroom-body volume increased even when bees collected no foraging experience, suggesting that the observed changes in brain structure served to prepare the animal for handling complex information in the context of foraging. But the correlation between brain-region size and storage capacity (or behavioral/cognitive ability) remains to be shown empirically.

Early learning may influence later learning without fundamental changes in brain structure, however. We found that when bees were trained extensively on only a single artificial flower type, they had more difficulty in learning to switch between flower types than did bees that learned to switch without the prior phase of visiting only one flower type (Chittka & Thomson 1997). The effect extended to only a few hundred visits (or a few hours), so it may be marginal during a bee's several-weeks-long foraging career. On the other hand, the training phase also involved only a few hundred flower visits, and therefore was much shorter than what bees may really experience in nature. Some bees may spend the first several days of their life foraging in low-diversity situations - such as flowering trees - visiting tens of thousands of flowers of exactly the same type in rapid succession. Might such bees later have more difficulties in learning new flower types, or in learning to minimize interference when switching between flower types.? Or do bees maintain complete flexibility, even if their foraging history includes phases where no flexibility is required?

The skill with which bees solve a particular foraging task depends substantially on their earlier experience with related tasks (Zhang & Srinivasan 1994). If bees are exposed to several flower types, some of which are rewarding and others not, bees are able to extract categories and concepts to predict the profitability of novel flowers (Dukas & Waser 1994, Giurfa et al. 1996). Whether or not bees acquire such complex skills depends substantially on the sequence with which different flower types are encountered (Zhang & Srinivasan 1994, Chittka & Thomson 1997). An entirely unresolved question is whether, in nature, this sequence is predominantly determined by the spatial arrangement of different flower types, or whether young bees actually choose to forage in diverse floral patches in order to gain the experience necessary for complex cognitive abilities.

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