Dissociating the effects of alternative early-life feeding schedules on the development of adult depression-like phenotypes

Experimental biology can inform our understanding of food insecurity

Food insecurity is a major public health issue. Millions of households worldwide have intermittent and unpredictable access to food and this experience is associated with greater risk for a host of negative health outcomes. While food insecurity is a contemporary concern, we can understand its effects better if we acknowledge that there are ancient biological programs that evolved to respond to the experience of food scarcity and uncertainty, and they may be particularly sensitive to food insecurity during development. Support for this conjecture comes from common findings in several recent animal studies that have modeled insecurity by manipulating predictability of food access in various ways. Using different experimental paradigms in different species, these studies have shown that experience of insecure access to food can lead to changes in weight, motivation and cognition. Some of these studies account for changes in weight through changes in metabolism, while others observe increases in feeding and motivation to work for food. It has been proposed that weight gain is an adaptive response to the experience of food insecurity as 'insurance' in an uncertain future, while changes in motivation and cognition may reflect strategic adjustments in foraging behavior. Animal studies also offer the opportunity to make in-depth controlled studies of mechanisms and behavior. So far, there is evidence that the experience of food insecurity can impact metabolic efficiency, reproductive capacity and dopamine neuron synapses. Further work on behavior, the central and peripheral nervous system, the gut and liver, along with variation in age of exposure, will be needed to better understand the full body impacts of food insecurity at different stages of development.

Time perception and patience: individual differences in interval timing precision predict choice impulsivity in European starlings, Sturnus vulgaris

Impulsivity, in the sense of the extent rewards are devalued as the time until their realization increases, is linked to various negative outcomes in humans, yet understanding of the cognitive mechanisms underlying it is limited. Variation in the imprecision of interval timing is a possible contributor to variation in impulsivity. We use a numerical model to generate predictions concerning the effect of timing imprecision on impulsivity. We distinguish between fixed imprecision (the imprecision that applies even when timing the very shortest time intervals) and proportional imprecision (the rate at which imprecision increases as the interval becomes longer). The model predicts that impulsivity should increase with increasing fixed imprecision, but decrease with increasing proportional imprecision. We present data from a cohort of European starlings (Sturnus vulgaris, n = 28) in which impulsivity had previously been measured through an intertemporal choice paradigm. We tested interval timing imprecision in the same individuals using a tri-peak temporal reproduction procedure. We found repeatable individual differences in both fixed and proportional imprecision. As predicted, birds with greater proportional imprecision in interval timing made fewer impulsive choices, whilst those with greater fixed imprecision tended to make more. Contradictory observations in the literature regarding the direction of association between timing imprecision and impulsivity might be clarified by distinguishing between fixed and proportional components of imprecision.

Negative mood affects the expression of negative but not positive emotions in mice

Whether and to what extent animals experience emotions is crucial for understanding their decisions and behaviour, and underpins a range of scientific fields, including animal behaviour, neuroscience, evolutionary biology and animal welfare science. However, research has predominantly focused on alleviating negative emotions in animals, with the expression of positive emotions left largely unexplored. Therefore, little is known about positive emotions in animals and how their expression is mediated. We used tail handling to induce a negative mood in laboratory mice and found that while being more anxious and depressed increased their expression of a discrete negative emotion (disappointment), meaning that they were less resilient to negative events, their capacity to express a discrete positive emotion (elation) was unaffected relative to control mice. Therefore, we show not only that mice have discrete positive emotions, but that they do so regardless of their current mood state. Our findings are the first to suggest that the expression of discrete positive and negative emotions in animals is not equally affected by long-term mood state. Our results also demonstrate that repeated negative events can have a cumulative effect to reduce resilience in laboratory animals, which has significant implications for animal welfare.

Reward and punisher experience alter rodent decision-making in a judgement bias task

The influence of affective states on decision-making is likely to be complex. Negative states resulting from experience of punishing events have been hypothesised to generate enhanced expectations of future punishment and ‘pessimistic’/risk-averse decisions. However, they may also influence how decision-outcomes are valued. Such influences may further depend on whether decisions at hand are germane to the rewards or punishers that induced the affective state in the first place. Here we attempt to dissect these influences by presenting either many or few rewards or punishers of different types (sucrose vs air-puff; 50 kHz vs 22 kHz ultrasonic vocalisations) to rats, and investigating their subsequent decisions in a judgement bias task that employed sucrose and air-puff as decision outcomes. Rats that received many sucrose pellets prior to testing were more risk-averse than those receiving many air-puffs. Ultrasonic vocalisations did not alter decision-making. Computational analysis revealed a higher weighting of punishers relative to rewards (in agreement with findings from a separate behavioural task) and a bias towards the risk-averse response following pre-test sucrose compared to pre-test air-puff. Thus, in this study reward and punisher manipulation of affective state appeared to alter decision-making by influencing both expectation and valuation of decision-outcomes in a domain-specific way.

Developmental history, energetic state and choice impulsivity in European starlings, Sturnus vulgaris

Impulsivity—the extent to which a reward is devalued by the amount of time until it is realized—can be affected by an individual’s current energetic state and long-term developmental history. In European starlings (Sturnus vulgaris), a previous study found that birds that were lighter for their skeletal size, and birds that had undergone greater shortening of erythrocyte telomeres over the course of development, were more impulsive as adults. Here, we studied the impulsivity of a separate cohort of 29 starlings hand-reared under different combinations of food amount and begging effort. The task involved repeated choice between a key yielding one pellet after 3 s and another key yielding two pellets after 8 s. Impulsivity was operationalised as the proportion of choices for the short-delay option. We found striking variation in impulsivity. We did not replicate the results of the previous study concerning developmental telomere attrition, though combining all the evidence to date in a meta-analysis did support that robustness of that association. We also found that early-life conditions and mass for skeletal size interacted in predicting impulsivity. Specifically, birds that had experienced the combination of high begging effort and low food amount were less impulsive than other groups, and the usual negative relationship between impulsivity and body mass was abolished in birds that had experienced high begging effort. We discuss methodological differences between our study and studies that measure impulsivity using an adjusting-delay procedure.

Developmental history and stress responsiveness are related to response inhibition, but not judgement bias, in a cohort of European starlings (Sturnus vulgaris)

Judgement bias tasks are designed to provide markers of affective states. A recent study of European starlings (Sturnus vulgaris) demonstrated modest familial effects on judgement bias performance, and found that adverse early experience and developmental telomere attrition (an integrative marker of biological age) both affected judgement bias. Other research has shown that corticosterone levels affect judgement bias. Here, we investigated judgement bias using a modified Go/No Go task in a new cohort of starlings (n = 31) hand-reared under different early-life conditions. We also measured baseline corticosterone and the corticosterone response to acute stress in the same individuals. We found evidence for familial effects on judgement bias, of a similar magnitude to the previous study. We found no evidence that developmental treatments or developmental telomere attrition were related to judgement bias per se. We did, however, find that birds that experienced the most benign developmental conditions, and birds with the greatest developmental telomere attrition, were significantly faster to probe the learned unrewarded stimulus. We also found that the birds whose corticosterone levels were faster to return towards baseline after an acute stressor were slower to probe the learned unrewarded stimulus. Our results illustrate the potential complexities of relationships between early-life experience, stress and affectively mediated decision making. For judgement bias tasks, they demonstrate the importance of clearly distinguishing factors that affect patterns of responding to the learned stimuli (i.e. response inhibition in the case of the Go/No Go design) from factors that influence judgements under ambiguity.

Early-life begging effort reduces adult body mass but strengthens behavioural defence of the rate of energy intake in European starlings

Animals require strategies for coping with periods when food is scarce. Such strategies include storing fat as a buffer, and defending the rate of energy intake by changing foraging behaviour when food becomes difficult to obtain. Storage and behavioural defence may constitute alternative strategies for solving the same problem. We would thus expect any developmental influences that limit fat storage in adulthood to also induce a compensatory alteration in adult foraging behaviour, specifically when food is hard to obtain. In a cohort of hand-reared European starlings, we found that higher manipulated early-life begging effort caused individuals to maintain consistently lower adult body mass over a period of two years. Using an operant foraging task in which we systematically varied the costs of obtaining food, we show that higher early-life begging effort also caused stronger behavioural defence of the rate of energy intake when food was more costly to obtain. Among individuals with the same developmental history, however, those individuals who defended their rate of energy intake most strongly were also the heaviest. Our results are relevant to understanding why there are marked differences in body weight and foraging behaviour even among individuals inhabiting the same environment.

A marker of biological ageing predicts adult risk preference in European starlings, Sturnus vulgaris

Why are some individuals more prone to gamble than others? Animals often show preferences between 2 foraging options with the same mean reward but different degrees of variability in the reward, and such risk preferences vary between individuals. Previous attempts to explain variation in risk preference have focused on energy budgets, but with limited empirical support. Here, we consider whether biological ageing, which affects mortality and residual reproductive value, predicts risk preference. We studied a cohort of European starlings (Sturnus vulgaris) in which we had previously measured developmental erythrocyte telomere attrition, an established integrative biomarker of biological ageing. We measured the adult birds' preferences when choosing between a fixed amount of food and a variable amount with an equal mean. After controlling for change in body weight during the experiment (a proxy for energy budget), we found that birds that had undergone greater developmental telomere attrition were more risk averse as adults than were those whose telomeres had shortened less as nestlings. Developmental telomere attrition was a better predictor of adult risk preference than either juvenile telomere length or early-life food supply and begging effort. Our longitudinal study thus demonstrates that biological ageing, as measured via developmental telomere attrition, is an important source of lasting differences in adult risk preferences.