Simultaneous and sequential choice as a function of reward delay and magnitude: normative, descriptive and process-based models tested in the European starling (Sturnus vulgaris).

The authors analyze prevalent theoretical and empirical quantitative models of choice using data from European starlings (Sturnus vulgaris). The different models originate from diverse lines of thinking, including optimal foraging theory, descriptive behavioral analysis, and temporal discounting. The authors also investigate the Sequential Choice Model (SCM) that predicts choice between simultaneous alternatives as a function of behavior in sequential encounters. SCM assumes that simultaneous encounters are rare in nature, where animals often exploit or dismiss single opportunities. Mechanisms of choice adapted for sequential encounters may be evolutionarily stable even if they impose a cost in the rarer simultaneous cases. The best predictive power is achieved by the SCM and by hyperbolic discounting with value (or attractiveness) of each option proportional to reward amount and inversely proportional to the sum of delay to reward plus delay to respond. Choice probability matched the ratio of each option's value to the sum of values of all available options. The good fit of SCM supports the view that choice is driven by mechanisms adapted to sequential, rather than simultaneous encounters with feeding opportunities.

Playing a rigged game: Inequality's effect on physiological stress responses.

High income and wealth inequality corresponds with high rates of various health and social problems. One possible factor that could be contributing to this correlation is stress experienced by those being treated unfairly in an unequal society. The present experiment attempted to simulate aspects of income inequality in a lab setting while recording several measures of stress. Participants (n=96) were assigned to one of four groups and played a memory game against a confederate opponent to earn "money" to spend in a lab market. The four groups depended on the difficulty of the problems and the fairness of the game that they and their opponents experienced. Stress attitudes were assessed with the Short Stress State Questionnaire (SSSQ) and four physiological measures: salivary cortisol, medial frontalis and corrugator facial muscle EMG, heart rate, heart rate variability (HRV), and skin conductance levels (SCL). Cortisol levels and HRV scores were the highest in groups that competed in an unfair game regardless of the difficulty of the problem compared to the groups playing a fair game. The group playing an unfair game with hard problems (disadvantaged) also had elevated facial muscle activity indicating negative affect and reported higher distress on the stress questionnaire. The results of this experiment showed that experiencing inequality even for a short time elicited several stress responses even if the participant benefited from the inequality.

Decrease of memory retention in a parasitic wasp: an effect of host manipulation by Wolbachia?

Several factors, such as cold exposure, aging, the number of experiences and viral infection, have been shown to affect learning ability in different organisms. Wolbachia has been found worldwide as an arthropod parasite/mutualist symbiont in a wide range of species, including insects. Differing effects have been identified on physiology and behavior by Wolbachia. However, the effect of Wolbachia infection on the learning ability of their host had never previously been studied. The current study carried out to compare learning ability and memory duration in 2 strains of the parasitoid Trichogramma brassicae: 1 uninfected and 1 infected by Wolbachia. Both strains were able to associate the novel odors with the reward of an oviposition into a host egg. However, the percentage of females that responded to the experimental design and displayed an ability to learn in these conditions was higher in the uninfected strain. Memory duration was longer in uninfected wasps (23.8 and 21.4 h after conditioning with peppermint and lemon, respectively) than in infected wasps (18.9 and 16.2 h after conditioning with peppermint and lemon, respectively). Memory retention increased in response to the number of conditioning sessions in both strains, but memory retention was always shorter in the infected wasps than in the uninfected ones. Wolbachia infection may select for reduced memory retention because shorter memory induces infected wasps to disperse in new environments and avoid competition with uninfected wasps by forgetting cues related to previously visited environments, thus increasing transmission of Wolbachia in new environments.

Risk sensitivity for amounts of and delay to rewards: adaptation for uncertainty or by-product of reward rate maximising?

Observations that humans and other species are sensitive to variability in the outcome of their choices has led to the widespread assumption that this sensitivity reflects adaptations to cope with risk (stochasticity of action consequences). We question this assumption in experiments with starlings. We show that choices between outcomes that are risky in both amount and delay to food are predictable from preferences in the absence of risk. We find that the overarching best predictor of an option's value is the average of the ratios of amount to delay across its (frequency weighted) outcomes, an expression known as "Expectation of the Ratios", or EoR. Most tests of risk sensitivity focus on the predicted impact of energetic state on preference for risk. We show instead that under controlled state conditions subjects are variance- and risk-neutral with respect to EoR, and this implies variance neutrality for amounts and variance-proneness for delays. The weak risk aversion for amounts often reported requires a small modification of EoR. EoR is consistent with associative learning: acquisition of value for initially neutral stimuli is roughly proportional to the magnitude of their consequences and inversely proportional to the interval between the stimulus and its consequence's onset. If, as is likely, the effect of amount on acquisition is sublinear, the result is a deviation from EoR towards risk aversion for amount. In 3 experiments, we first establish individual birds' preferences between pairs of fixed options that differ in both amount and delay (small-sooner vs. large-later), and then examine choices between stochastic mixtures that include these options. Experiment 1 uses a titration to establish certainty equivalents, while experiments 2 and 3 measure degree of preference between options with static parameters. The mixtures differ in the coefficient of variation of amount, delay, or both, but EoR is sufficient to predict all results, with no additional explanatory role for riskiness.

Parameters of rewards on choice behavior in Siamese fighting fish (Betta splendens).

Five experiments were conducted with Siamese fighting fish (Betta splendens) to investigate how choices in a T-maze were affected by parameters of a social reward (aggression display to another male): presence or absence, amount, delay and distance traveled. Bettas showed a preference for the side associated with the presence of another male rather than the side associated with nothing (Exp 1), a greater length of time of the reward (Exp 2) and shorter delay (Exp 3). The animals were indifferent when one side offered a longer delay to a longer reward time compared with a shorter delay to a shorter reward time (Exp 4). What was most surprising, however, was that fish preferred to choose the side that was associated with swimming a greater distance to reach an opponent male (Exp 5). These experiments demonstrate that, while some parameters of a visual reward affect behavior in predictable ways (greater amount, shorter delay), the complex motivations underlying inter-male aggression can produce what appear to be paradoxical results.

Variable rewards and discrimination ability in an insect herbivore: what and how does a hungry locust learn?

With the exception of honeybees, there have been few good invertebrate models for associative learning. Grasshoppers and locusts (Orthoptera: Acrididae) possess a number of characteristics that make them excellent candidates for such studies, and in this paper we present a novel protocol, based on a Y-maze, that is specifically designed for studying their learning and choice behaviour. Three separate experiments were conducted using individual gregarious forms of the desert locust, Schistocerca gregaria. In our first experiment, coloured arms of a two-sided Y-maze provided a large or small amount of wheat for nine choice-trials. In the second experiment, locusts discriminated odours with wheat rewards for nine choice-trials. The odour-wheat reward combinations were then reversed for an additional nine choice-trials. For the third experiment, the locusts again discriminated odours, but here we used artificial foods and the rewards differed in their concentration of protein and digestible carbohydrate. The results indicate that, in addition to showing good acquisition of choice performance, the locusts also took less time to reach the larger-rewarded option. The data indicate that our protocol is highly sensitive for recording choice behaviour in acridids and reveals the potential they have for advancing our current understanding of associative learning and the field of learning in general.