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When primates make decisions about sensory signals, their choices are biased by the costs and benefits associated with different possible outcomes. However, much remains unknown about the neuronal mechanisms by which reward information is integrated into the perceptual decision-making process. I used electrophysiological, psychophysical and imaging techniques to explore the effect of reward on sensory representations, whilst humans and monkeys made perceptual judgements about structure-from-motion (SFM) stimuli. Electrical microstimulation of visual area V5/MT in the macaque monkey was used to bias per- ceptual judgements, under different available reward sizes for correct choices. The behavioural effect of microstimulation interacted with available reward, and, in the context of a drift diffu- sion model of decision-making, the results demonstrated that reward must influence sensory processing before visual signals and micro stimulation signals are accumulated in sensorimotor areas. In a parallel human psychophysics experiment, viewers made decisions about SFM stim- uti whilst their choices were biased towards one outcome or the other by unequal pay-offs. A full drift -diffusion model was fitted to human choice and reaction time data. There was a signif- icant effect of reward on model drift rate, a parameter known to be dependent upon evidence represented in sensory brain regions. A second set of experiments used magneto encephalography (MEG) to explore activity in visual , areas in human cortex and the effects of reward. Brain responses to retinotopic visual stimuli were localized with three different MEG source analysis methods, and localization accuracy was evaluated by comparison with fMRI maps obtained in the same individuals. The results demon- strated that the beamformer and minimum norm estimate (MNE) methods were most suitable for investigating early visual activity with MEG. Human brain activity was then recorded with MEG whilst viewers made perceptual judgements about SFM stimuli, under unequal pay-offs. The results revealed an effect of reward size on early MEG responses in the region of the occipi- tal cortex and visual precuneus. Taken together, the experiments presented in this thesis provide consistent evidence that in- formation about reward can influence the processing of sensory information during perceptual decisions.

One basic requisite for rationality is that choices are consistent across situations. Animals commonly violate rationality premises as evidenced, for example, by context-dependent choices, and such apparent irrationalities stand as paradoxes that instigate re-examination of some assumptions in behaviour ecological modelling. The goal of the present thesis was to study the psychological mechanisms underlying apparent irrationalities in order to assess the functional implications of general processes of valuation and choice. A common thread through the different studies is the hypothesis that most animal 'irrationalities' are due to misinterpretation of what the optimum would be in natural circumstances, and hence of the maximised currency in the theoretical predictions. I believe that the trait that may have been of paramount influence in many organisms' selective history was the ability to learn about the predictability of events and their biological value, and that this is implemented in an overriding force of associative learning mechanisms. In chapters 2 and 3, I present evidence of context-dependent foraging choices in European starlings, Sturnus vulgaris, in the laboratory, and I implement a version of the Rescorla- Wagner learning model to account for both present data and apparent irrationalities reported by other authors. In chapter 4, I test the notion that context dependence may in fact be adaptive when animals face sequential choices, namely when they have to decide whether to take a prey item or to skip it in order to search for better alternatives. In chapter 5, I explore the functional implications of starlings' relative responding to incentives during an unexpected shortfall in reinforcement, and I also examine the extent to which information about the new environmental status helps them avoid energetic and time costs commonly seen in uninformed individuals. Last, in chapter 6, I present a brief summary of the main discussions considered and conclusions reached along this thesis.

From cells to societies, cooperation occurs at all levels of biological organization. In vertebrates, the most complex societies occur in cooperative breeders where some group members (helpers) forego reproduction, sacrificing their immediate direct fitness to assist in raising the offspring of others (breeders). Individuals in cooperative breeding societies can gain indirect fitness benefits from passing on shared genes when they help the offspring of close genetic relatives (kin selection), such that cooperation is expected to correlate with genetic relatedness. However, some cooperatively breeding societies include cooperation between nonrelatives. Cooperatively breeding societies range in complexity, from singular (one breeding pair) to plural (two or more breeding pairs). In the majority of singular breeding societies, helpers are relatives of breeders. Thus, kin selection is thought to underlie helping behavior in singular breeding societies. Plural breeding societies, such as in superb starlings (Lamprotornis superbus) inhabiting the East African savanna in central Kenya, involve multiple territory-sharing families raising offspring with helpers who can assist more than one family simultaneously. The superb starling’s complex and dynamic social system, mixed kin structure, relatively long lives, and stable social groups make them an ideal study species for investigating how patterns of individual decision-making have shaped and maintained cooperative societies. My dissertation research focuses on using long-term data on cooperatively breeding superb starlings to explore how temporally variable environments, such as the East African savanna, influence individual decisions across lifetimes, and subsequently how individual behavior shapes the structure and organization of the society. In Chapter 1, I apply a Bayesian approach to the animal model to estimate how genetic versus nongenetic factors influence among individual variation in the social roles: “breeder”, “helper”, and “non-breeder/non-helper”. Non-breeder/non-helper indicates that the individual maintained membership in the social group but did not breed or help during that season. I then estimated heritability and found, as predicted, overall low heritability of traits responsible for each role. This result is consistent with the findings of other studies on the heritability of social behavior, which tends to be low compared to non-social traits, primarily because the social behavior of an individual is highly influenced by interactions with other individuals. In Chapter 2, I show that superb starlings (i) are nepotistic, and (ii) switch between the social roles of “helper” and “breeder” across their lives. This role switching, which unexpectedly includes breeders going back to helping again, is linked to reciprocal helping between pairs of helpers and breeders, independent of genetic relatedness. Reciprocal helping was long thought to be irrelevant for cooperative breeders because most helping is assumed to be unidirectional, from subordinate helpers to dominant breeders, and reciprocal helping is often measured on short timescales. These long-term reciprocal helping relationships among kin and nonkin alike may be important for the persistence of this population because previous research has demonstrated that enhancing group size by immigration from outside groups, while reducing group kin structure, is necessary to prevent group extinction. Finally, the results of Chapter 3 reveal how social and ecological factors shape role switching across individual lifetimes. Overall, my dissertation highlights the remarkable flexibility of superb starling cooperative behavior and the crucial role of mutual direct fitness benefits from reciprocal helping, which may help promote the stability of cooperative group living among nonkin as well as kin group members, contributing to the resilience of this population within a harsh and unpredictable environment.

Decision making is a framework we impose on a vast universe of possible behaviors to better understand the advantages and disadvantages of acting in different ways. Decisions under uncertainty are of particular interest because stochasticity is a feature of environments both today and throughout evolutionary history. As a result, we might expect decision makers (DMs) to have evolved mechanisms to handle variability. In this dissertation, I examine common decision processes in several model species: European starlings (Sturnus vulgaris), Banded tetras (Astyanax fasciatus) and humans (Homo sapiens). The broad range of approaches discussed include currencies DMs are expected to maximize (Risk Sensitivity Theory, Expected Utility), the currencies DMs do maximize (e.g. long versus short term rate maximizing models), the representation of outcomes in memory (Scalar Expectancy Theory) as well as explicit choice mechanisms (Sequential Choice Model). The first section of this thesis discusses responses to risk, offering humans and starlings choices between options which deliver certain or variable outcomes. Starlings demonstrate sensitivity to changes in the probability of variable outcomes and strong support for local rate maximization. Humans appear similarly sensitive to their own accuracy when task difficulty is varied. When the DM’s affective state was manipulated, neither humans nor starlings exhibit changes in risk preferences, but the effectiveness of these manipulations used could not be confirmed. Another topic of inquiry is the effect of the DM’s state at the time of valuation learning. State dependent valuation learning is demonstrated for the first time in a fish species, but Within Trial Contrast is not observed in starlings. Lastly, two experiments find strong support for the Sequential Choice Model, a promising new model of the mechanism of choice. Taken together, these experiments offer a glimpse into shared decision processes, but leave open questions about the mechanisms through which value is acquired.

Cumulative culture is rare, if not altogether absent in non-human species. At the foundation of cumulative learning is the ability to flexibly modify, relinquish or build upon prior behaviours to make them more productive or efficient. Within the primate literature, a failure to optimise solutions in this way is often proposed to derive from low-fidelity copying of witnessed behaviours, sub-optimal social learning heuristics, or a lack of relevant socio-cognitive adaptations. However, humans can also be markedly inflexible in their behaviours, perseverating with, or becoming fixated on outdated or inappropriate responses. Humans show differential patterns of flexibility as a function of cognitive load, exhibiting difficulties with inhibiting suboptimal behaviours when there are high demands on working memory. Here I present a series of studies on captive chimpanzees which show that not only is inhibitory control compromised in chimpanzees, but indicate ape behavioural conservatism may be underlain by similar constraints as in humans; chimpanzees show relatively little conservatism when behavioural optimisation involves the inhibition of a well-established but simple solution, or the addition of a simple modification to a well-established but complex solution. In contrast, when behavioural optimisation involves the inhibition of a well-established but complex solution, and especially when the alternative solution is also complex, chimpanzees show evidence of behavioural conservatism. I propose that conservatism is linked to behavioural complexity, potentially mediated by cognitive resource availability, and may be an important factor in the evolution of cumulative culture.