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1:1 Pollen:Cellulose

Pure Pollen

Fig. 3.4. Relationship between pollen quality and dance rate of honeybees from two genotypic strains. (After Waddington et al. 1998, Fig. 1A. Reprinted by permission of Academic Press.)

1:1 Pollen:Cellulose

Pure Pollen

Pollen Type

Fig. 3.4. Relationship between pollen quality and dance rate of honeybees from two genotypic strains. (After Waddington et al. 1998, Fig. 1A. Reprinted by permission of Academic Press.)

foraged for pure pollen than after they foraged for the lower-quality pollen-cellulose mixture (Fig. 3.4). These results suggest that the dance can indeed be used in the future to make more detailed studies of honeybees' evaluation of pollen quality and perhaps quantity.

Genotypic variation has been found in the dance rate, and therefore in subjective evaluation, in honeybees. We studied dance behavior of bees that had been subjected to colony-level selection for variation in collecting and storing pollen. Colonies of the high-pollen strain filled the comb with pollen, while bees from the low-pollen strain stored little pollen. High-strain bees preferentially foraged for pollen and bees from the low-pollen strain preferentially foraged for nectar (Page et al. 1995). Bees from the two genotypic strains were given dishes of pure pollen and a pollen and alpha-cellulose mixture (as described above). Pollen foragers from the high-pollen strain had higher dance rates (Waddington 1982) for both types of pollen than did bees from the low-pollen strain; this suggests a genotypic basis for the subjective evaluation of pollen quality (Fig. 3.4) (Waddington et al. 1998).

In summary, the honeybee's dance may give insight into the perceptual processes of foraging honeybees and, by extension, such processes in other taxa of pollen and nectar foragers.

Subjective evaluations by honeybees and genotypic variation - proboscis extension response

The proboscis extension response (PER) is routinely used to study classical conditioning in honeybees (Bitterman et al. 1983). Page et al. (1998) suggested that the PER test could also be used as a window into honeybees' perceptions of sugar concentration. They collected bees and harnessed each bee in a metal tube for the PER test. The antennae, which have sugar receptors, were touched with a droplet of sugar solution; a record was made of whether the bee responded by extending its proboscis. Page et al. found that the probability of proboscis extension increased with increasing sugar concentration. This is interpreted as follows. There is a threshold sugar concentration that will elicit proboscis extension and imbibition of the solution. Concentrations below that threshold do not elicit a response; those above threshold do elicit a response. A honeybee's threshold can be a measure of its subjective evaluation of the solution.

As judged by the PER test, the perception of sucrose is not constant, but is affected by previous experience. T. Pankiw, KD Waddington & RE Page (unpublished data) demonstrated that the evaluation depends on the concentration of sugar previously imbibed and on the bee's nutritional status. Bees recently fed lower concentrations of sugar or starved were more responsive thanbees that had fed on high concentrations or were well fed. The former bees made a higher subjective evaluation of sucrose than those fed a higher concentration. The response also was affected by the amount of fluid, water, or nectar in the crop at the time of testing. Empty bees evaluated sucrose concentration more highly than filled bees.

The studies of the PER by Page et al. (1998) and by Pankiw et al. (unpublished data) also suggest genotypic variance for evaluation of nectar concentration. Bees from the high-pollen strain were more likely than bees from the low-pollen strain (selection described above) to respond to each concentration touched to the antennae.

Choice behavior: expected rewards

Some models and concerns about currency

Models of food choice based on energetics (e.g., Charnov 1976) initially focused on responses made in relation to the long-term expected rate of net energy gain [(gain - cost)/time] of alternative foods. Because of a presumed positive relationship between long-term rate of intake and fitness, animals are expected to prefer alternatives with the highest intake rate. Some experiments suggest that the long-term rate of net energy gain provides a reasonable currency and time-scale for studying choice behavior based on expected payoffs (Stephens & Krebs 1986). In some situations other than individual food choice behavior, the ratio [(gain - cost)/cost] seems to better predict the behavior of honeybees (Schmid-Hempel et al. 1985; Seeley 1986; Wolf & Schmid-Hempel 1990).

Other researchers, however, have suggested that bees maximize short-term rather than long-term rates of energy gain. Waddington & Holden (1979) developed an optimality model that assumed bees choose the best flower (highest gain/time) among all nearby flowers on each move. The model successfully predicted honeybees' choice behavior under the limited set of conditions tested. Bumble bees also appear to maximize short-term rate of net energy gain (Real et al. 1982; Harder & Real 1987). Real et al. (1990) point out that bumble bees may be forced to base foraging decisions on a few recent visits, because they are constrained by the amount of information they can remember from previous visits. Although some evidence suggests that bees maximize in the short-term, this important problem deserves further investigation.

A problem with this general modeling approach used to predict choice behavior is that the model can fail at different places in the modeling process. If predicted and observed choice behavior differ, it may be because an incorrect maximized currency is assumed, or it may be that one of the operational assumptions is incorrect. These possibilities are very difficult to tease apart. How, for example, does one, outside of the choice model itself, independently test the validity of the assumed currency.?

Choice - subjective evaluation of objective information

If the goal is simply to predict bees' choice behavior between two or more different kinds of flowers with different expected rewards (e.g., nectar concentration), it should be possible to make the predictions based on the functional relationships between subjective evaluations and objective information (Fig. 3.1). Since animals are presumed to make decisions based on their evaluations, it is these relationships that should be most revealing in predicting choices.

Nectar concentration The strength of preference for the more rewarding of two types of flowers that differ in associated expected concentration of nectar is expected to vary depending on the magnitude of the concentration difference. The more similar the concentrations, the more likely it is that the difference

Fig. 3.5. Honeybee's dance rate in relation to concentration, taken from Fig. 3.2. The perceived values of concentration are shown by projecting concentration onto the perceptual scale (y axis). On the perceptual scale the difference between 10% and 20% is greater than the difference between 40% and 50%; thus, the preference for 20% over 10% is predicted to be stronger than the preference for 50% over 40%.

Sucrose concentration (%)

Fig. 3.5. Honeybee's dance rate in relation to concentration, taken from Fig. 3.2. The perceived values of concentration are shown by projecting concentration onto the perceptual scale (y axis). On the perceptual scale the difference between 10% and 20% is greater than the difference between 40% and 50%; thus, the preference for 20% over 10% is predicted to be stronger than the preference for 50% over 40%.

will not be perceived, or that the perceived difference will not elicit a preference. Thus, a bee might prefer a 40% solution to 30%, but might be indifferent to, or prefer less strongly, the 5% difference of 40% over 35%. Now, because the relationship is non-linear between dance rate (subjective evaluation) and sucrose concentration, the strength of preference for any given difference in concentration should vary along the concentration scale. For example, it is expected that the preference for 50% over 40% will be less than the preference for 20% over 10%, because the perceived difference between 20% and 10% is greater than the perceived difference between 50% and 40% (Fig. 3.5).

We examined this prediction (Fig. 3.6). Bumble bees (Bombus impatiens) were given a choice between two types (colors) of artificial flowers that differed in sugar concentration (K.D. Waddington, S. Lamenta & M. Jordan, unpublished data). Two sets of bees were tested. One set always obtained 1 ^l of 10% sucrose from one type of flower (e.g., blue) and 20% from the other type (e.g., yellow). The other set of bees obtained 40% from one type

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