Fitness Rewards Research Articles

Cyclic dynamics drive summer movement ecology of snowshoe hares (Lepus americanus)

Animals exhibit dynamic movement and activity in response to environmental variation including changes in reproductive opportunities, predation risk, or food availability. Yet, it remains unclear which factors are primary in affecting animal movement, and whether the relative importance of these factors are consistent through time. We tracked snowshoe hares (Lepus americanus) using GPS telemetry during eight summers spanning a hare population cycle (2015–2022) in southwestern Yukon, Canada, to determine associations between environmental variation and hare movement and home range size. Hare density varied 25-fold during the study and home range size increased markedly during low hare density, especially for males. Both sexes retained similar core space use and linearity of movements, but at low densities males had greater and more variable movement rates and time spent travelling. Trail cameras revealed that annual changes in hare movement were also correlated with relative abundance of lynx (Lynx canadensis) and coyotes (Canis latrans). However, hare detection rates within a season were not closely associated with seasonal variation in predator detection. Observed differences between male and female hares in some metrics highlighted that different life histories and reproductive behavior are likely the main drivers of hare movement dynamics. Therefore, fitness rewards associated with successful mate search and reproduction appear to outweigh risks associated with increased movement, even in highly variable environments where costs of prioritizing reproduction-related activities are notably high and variable.

Resolving the evolutionary paradox of consciousness

AbstractEvolutionary fitness threats and rewards are associated with subjectively unpleasant and pleasant sensations, respectively. Initially, these correlations appear explainable via adaptation by natural selection. But here I analyse the major metaphysical perspectives on consciousness – physicalism, dualism, and panpsychism – and conclude that none help to understand the adaptive-seeming correlations via adaptation. I also argue that a recently proposed explanation, the phenomenal powers view, has major problems that mean it cannot explain the adaptive-seeming correlations via adaptation either. So the mystery – call it the evolutionary paradox of consciousness – remains. Some have used this mystery to argue for non-naturalistic (e.g. theistic) explanations. But I propose a naturalistic, non-adaptive explanation of the adaptive-seeming correlations: namely, ‘sensational associative learning’ during development. In this perspective, pairing of particular sensations with unconditioned stimuli – fitness rewards or threats – cause the sensations themselves to come to be interpreted as good or bad, respectively. Sensations, like colours, that are not reliably paired with either fitness rewards or threats remain largely unvalenced. Sensational associative learning also provides explanations for adaptive-seeming structural aspects of sensations, such as the observation that sounds of different pitch are experienced as ordinal in correspondence to their wavelengths while the same is not true of colours of different hue. The sensational associative learning perspective appears compatible with physicalism, panpsychism, and dualism (though not epiphenomenalism).

Evolution of heterogeneous perceptual limits and indifference in competitive foraging.

The collective behaviour of animal and human groups emerges from the individual decisions and actions of their constituent members. Recent research has revealed many ways in which the behaviour of groups can be influenced by differences amongst their constituent individuals. The existence of individual differences that have implications for collective behaviour raises important questions. How are these differences generated and maintained? Are individual differences driven by exogenous factors, or are they a response to the social dilemmas these groups face? Here I consider the classic case of patch selection by foraging agents under conditions of social competition. I introduce a multilevel model wherein the perceptual sensitivities of agents evolve in response to their foraging success or failure over repeated patch selections. This model reveals a bifurcation in the population, creating a class of agents with no perceptual sensitivity. These agents exploit the social environment to avoid the costs of accurate perception, relying on other agents to make fitness rewards insensitive to the choice of foraging patch. This provides a individual-based evolutionary basis for models incorporating perceptual limits that have been proposed to explain observed deviations from the Ideal Free Distribution (IFD) in empirical studies, while showing that the common assumption in such models that agents share identical sensory limits is likely false. Further analysis of the model shows how agents develop perceptual strategic niches in response to environmental variability. The emergence of agents insensitive to reward differences also has implications for societal resource allocation problems, including the use of financial and prediction markets as mechanisms for aggregating collective wisdom.

Flexible habitat choice by aphids exposed to multiple cues reflecting present and future benefits

Abstract Mothers choose suitable habitats for laying offspring to maximize fitness. Because habitat quality varies in space and time, mothers gather information to choose among available habitats through multiple cues reflecting different aspects of habitat quality at present and in the future. However, it is unclear how females assess and integrate different cues associated with current rewards and future safety to optimize oviposition/larviposition decisions, especially across small spatial scales. Here, we tested the individual and interactive effects of leaf surface, leaf orientation, and leaf bending direction on larviposition site choice and fitness benefits of wheat aphids (Metopolophium dirhodum) within individual leaves. We found that females preferred upper over lower surfaces for gaining current food-related rewards, downward- over upward-facing surfaces for avoiding potential abiotic risks, and sunken over protruding surfaces for avoiding potential biotic risks. When facing conflicting cues during larviposition, females preferred downward-facing/sunken surfaces over upper surfaces, suggesting that females prioritize potential safety at the cost of current rewards during decision making. Most importantly, our combined-cue experiments showed females still assessed secondary cues (i.e., the upper surface) when first-ranked cues (i.e., the downward-facing/sunken surface) are available, even though females only gained relatively small fitness rewards through secondary cues, and females can integrate different cues associated with current rewards and potential safety in a multiplicative way to make flexible and complex larviposition decisions. Overall, our findings provide new insights into how animals collect and process multi-cue information associated with current rewards and potential safety to maximize fitness at small spatial scales.

Evidence of parthenogenetic populations from the Paratanytarsus laccophilus species group (Diptera: Chironomidae) in the Alaskan Arctic

Parthenogenesis, reproduction without fertilization, is not common in the Chironomidae (Diptera), a family of insects with more than 6,000 described species. Nonetheless, parthenogenetic species and strains have been documented in at least three subfamilies (the Chironominae, Orthocladiinae, and Telmatogoninae), spanning 17 genera and ~30 species. One such species, Paratanytarsus laccophilus Edwards 1929, is known to be parthenogenetic in a small portion of its range in Finland, with most other European populations of this species showing evidence of sexual reproduction. We present evidence of parthenogenetic populations from the Paratanytarsus laccophilus species group in the Nearctic, specifically a High Arctic site near Utqiaġvik (formerly Barrow), Alaska. During May-July of 2015 and 2016, we sampled emerging adult chironomids and pupal exuviae daily to document insect emergence phenologies. Across 15 local populations, all 623 pupal exuviae collected from the P. laccophilus species group were female. Larvae reared from two populations under controlled temperature treatments emerged as female adults (N=37). When isolated, these reared female adults oviposited, and eggs hatched successfully. These progeny were reared for another 12-13 days, reaching second instar larvae when they were preserved at the end of our field season. Taken together, this evidence strongly indicates parthenogenesis from the P. laccophilus species group at this location. This species was not previously documented at Utqiaġvik. Although parthenogenetic, their emergence at this location was highly synchronized. In the harsh environment of arctic Alaska, the fitness rewards of parthenogenesis are likely great. Indeed, chironomid parthenogenesis in the northern hemisphere is most commonly documented from far-northern extremes and in extreme habitats.

The evolution of infanticide by females in mammals.

In most mammalian species, females regularly interact with kin, which is expected to reduce aggressive competitive behaviour among females. It may thus be difficult to understand why infanticide by females has been reported in numerous species and is sometimes perpetrated by groupmates. Here, we investigate the evolutionary determinants of infanticide by females by combining a quantitative analysis of the taxonomic distribution of infanticide with a qualitative synthesis of the circumstances of infanticidal attacks in published reports. Our results show that female infanticide is widespread across mammals and varies in relation to social organization and life history, being more frequent where females breed in groups and have intense bouts of high reproductive output. Specifically, female infanticide occurs where the proximity of conspecific offspring directly threatens the killer's reproductive success by limiting access to critical resources for her dependent progeny, including food, shelters, care or a social position. By contrast, infanticide is not immediately modulated by the degree of kinship among females, and females occasionally sacrifice closely related infants. Our findings suggest that the potential direct fitness rewards of gaining access to reproductive resources have a stronger influence on the expression of female aggression than the indirect fitness costs of competing against kin. This article is part of the theme issue 'The evolution of female-biased kinship in humans and other mammals'.

Density‐Dependent Foraging and Interference Competition by Common Gartersnakes are Temperature Dependent

AbstractThe ideal free distribution (IFD) predicts that optimal foragers will select foraging patches to maximize food rewards and that groups of foragers should thus be distributed between food patches in proportion to the availability of food in those patches. Because many of the underlying mechanisms of foraging are temperature dependent in ectotherms, the distribution of ectothermic foragers between food patches may similarly depend on temperature because the difference in fitness rewards between these patches may change with temperature. We tested the hypothesis that the distribution of Common Gartersnakes (Thamnophis sirtalis) between food patches can be explained by an IFD, but that conformance to an IFD weakens as temperature departs from the optimal temperature because fitness rewards, interference competition and the number of individuals foraging are highest at the optimal temperature. First, we determined the optimal temperature for foraging. Second, we examined group foraging at three temperatures and three density treatments. Search time was optimized at 27°C, handling time at 29°C and digestion time at 32°C. Gartersnakes did not match an IFD at any temperature, but their distribution did change with temperature: snakes at 20°C and at 30°C selected both food patches equally, while snakes at 25°C selected the low food patch more at low density and the high food patch more at high density. Food consumption and competition increased with temperature, and handling time decreased with temperature. Temperature therefore had a strong impact on foraging, but did not affect the IFD. Future work should examine temperature‐dependent foraging in ectotherms that are known to match an IFD.

The effect of participation in an incentive-based wellness program on self-reported exercise

Employers are increasingly trying to promote healthy behaviors, including regular exercise, through wellness programs that offer financial incentives. However, there is limited evidence that these types of programs affect exercise habits within employee populations. In this study, we estimate the effect of participation in an incentive-based wellness program on self-reported exercise. Since 2008, the University of Minnesota's Fitness Rewards Program has offered a $20 monthly incentive to encourage fitness center utilization among its employees. Using 2006 to 2010 health risk assessments and university administrative files for 2972 employees, we conducted a retrospective cohort study utilizing propensity score methods to estimate the effect of participation in the Fitness Rewards Program on self-reported exercise days per week from 2008 to 2010. On average, participation in the program led to an increase of 0.59 vigorous exercise days per week (95% Confidence Interval: 0.42, 0.78) and 0.43 strength-building exercise days per week (95% Confidence Interval: 0.31, 0.58) in 2008 for participants relative to non-participants. Increases in exercise persisted through 2010. Employees reporting less frequent exercise prior to the program were least likely to participate in the program, but when they participated they had the largest increases in exercise compared to non-participants. Offering an incentive for fitness center utilization encourages higher levels of exercise. Future policies may want to concentrate on how to motivate participation among individuals who are less frequently physically active.

Spatial distribution and settlement strategies in willow ptarmigan

In heterogeneous landscapes individuals select among several habitat patches. The fitness rewards of these choices are assumed to play an important role in the distribution of individuals across landscapes. Individuals can either use environmental cues to directly assess the quality of breeding sites, or rely on social cues to guide the settlement decision. We estimated the density of adult birds and per capita reproductive success of willow ptarmigan over 5–15 years in 42 survey areas, nested within 5 spatially separated populations in south-central Norway. Our aims were to (1) examine spatial and temporal patterns of variation in densities of adult birds (i.e., the breeding densities) and reproductive success (juveniles/pair) measured in autumn and (2) evaluate which habitat distribution model best described the distribution of willow ptarmigan across heterogeneous mountain landscapes. Variation in density of adult birds was primarily attributable to variation between survey areas which could arise from spatial heterogeneity in adult survival or as a consequence of spacing behavior of juveniles during the settlement stage. In contrast, reproductive success was more variable between years and did not vary consistently between survey areas once year effects were accounted for. The lack of any relationship between the density of adult birds and reproductive success supported the predictions of an ideal free distribution (IFD), implying that within years, the mean reproductive success was approximately equal across survey areas. However, analysis based on Taylor's power law (i.e., the relationship between logarithms of spatial variance and mean density of adult birds) suggested that aggregation was stronger than expected under IFD. This implies that the relative change in density of adult birds was larger in areas with high mean densities than in areas with low densities. The exact mechanisms causing this statistical pattern are unclear, but based on the breeding biology of willow ptarmigan we suggest that yearlings are attracted to areas of high densities during the settlement period in spring. Our study was conducted during a period of low overall density and we suggest that this pattern might be particular to such situations. This implies that the presence of conspecifics might represent a cue signaling high adult survival and thus high habitat quality.

Risk‐sensitive reproductive allocation: fitness consequences of body mass losses in two contrasting environments

For long-lived organisms, the fitness value of survival is greater than that of current reproduction. Asymmetric fitness rewards suggest that organisms inhabiting unpredictable environments should adopt a risk-sensitive life history, predicting that it is adaptive to allocate resources to increase their own body reserves at the expense of reproduction. We tested this using data from reindeer populations inhabiting contrasting environments and using winter body mass development as a proxy for the combined effect of winter severity and density dependence. Individuals in good and harsh environments responded similarly: Females who lost large amounts of winter body mass gained more body mass the coming summer compared with females losing less mass during winter. Additionally, females experienced a cost of reproduction: On average, barren females gained more body mass than lactating females. Winter body mass development positively affected both the females' reproductive success and offspring body mass. Finally, we discuss the relevance of our findings with respect to scenarios for future climate change.

On the evolutionary stability of partial migration

The evolution of partial migration in birds is typically assumed to be the result of an optimization process. The fitness rewards for individuals choosing to migrate are balanced against the rewards of remaining in the breeding area all year around. This balancing is often thought to result in an evolutionarily stable strategy (ESS) such that an optimal fraction of the population becomes migratory through adaptive evolution. Here I show that this solution can indeed be reached through adaptive evolution, but that the equilibrium is a neutral or “weak” ESS. The equilibrium fraction of migrants is more reminiscent of the Fisherian sex ratio. I also show that this individual-based evolutionary solution may deviate significantly from the optimal solution for the population (maximum population size), quite in line with previous findings. Finally, I show that partial migration is very unlikely without density- or frequency-dependent selection.

The Effect of Participation in a Fitness Rewards Program on Medical Care Expenditures in an Employee Population

In 2008, the University of Minnesota began offering the Fitness Rewards Program (FRP) that provides a monthly credit toward fitness center dues for individuals who exercise at a participating facility at least 8 times per month. This study evaluates whether participation in the FRP is associated with a decrease in medical spending. Using a difference-in-differences approach, we estimate multivariate regression models of average monthly medical expenditures for 2006 through 2008. We find positive benefits from reduced medical spending among highly persistent FRP exercisers. However, the results are sensitive to the inclusion or exclusion of individuals who had very high medical expenditures in any year. Offering exercise-focused wellness initiatives may have economic benefits for employers, but further research is needed to understand the sensitivity of results to the inclusion of outliers.

The evolution of social discounting in hierarchically clustered populations

The expression of a social behaviour may affect the fitness of actors and recipients living in the present and in the future of the population. When there is a risk that a future reward will not be experienced in such a context, the value of that reward should be discounted; but by how much? Here, we evaluate social discount rates for delayed fitness rewards to group of recipients living at different positions in both space and time than the actor in a hierarchically clustered population. This is a population where individuals are grouped into families, families into villages, villages into clans, and so on, possibly ad infinitum. The group-wide fitness effects are assumed to either increase or decrease the fecundity or the survival of recipients and can be arbitrarily extended in space and time. We find that actions changing the survival of individuals living in the future are generally more strongly discounted than fecundity-changing actions for all future times and that the value of future rewards increases as individuals live longer. We also find that delayed fitness effects may not only be discounted by a constant factor per unit delay (exponential discounting), but that, as soon as there is localized dispersal in a population, discounting per unit delay is likely to fall rapidly for small delays and then slowly for longer delays (hyperbolic discounting). As dispersal tends to be localized in natural populations, our results suggest that evolution is likely to favour individuals that express present-biased behaviours and that may be time-inconsistent with respect to their group-wide effects.

Small but smart: the interaction between environmental cues and internal state modulates host-patch exploitation in a parasitic wasp

The reproductive success of insect parasitoids depends on two activities, searching for hosts to obtain immediate fitness rewards (offspring) and searching for food to extend life span and enhance future reproductive opportunities. Models suggest that to deal with this trade-off and to cope with the variability of the resources they depend on, parasitoids should simultaneously integrate information originating from three distinct sources: host and food availabilities in the environment (environmental information) and the metabolic reserves of the parasitoid (internal information). We tested whether, in the parasitic wasp Venturia canescens, these three types of information are perceived simultaneously and interact during host-patch exploitation. Experiments were performed with strains originating from two different geographical locations and with individuals of the two reproductive modes, arrhenotoky and thelytoky. We hypothesised that there would be differences between modes of reproduction as they thrive in different ecological conditions. Our results show that metabolic state, food availability and the number of ovipositions performed simultaneously modulate patch residence time in V. canescens of both populations and reproductive modes. Unfed wasps left their host patch earlier than fed ones. The tendency to leave the patch increased with an increasing probability of finding food. Our study confirms that globally, each oviposition increases the patch-leaving tendency (decremental mechanism). This effect was modulated by both the metabolic reserves and food availability, and the relationship depended on the geographic origin of populations. Individuals of one of the populations switched from a decremental to an incremental patch-leaving mechanism depending on the presence or absence of food in the vicinity. Differences between reproductive modes in the responses to environmental cues can be explained by the different ecological conditions they live in.

Physical Fitness Testing and Rewards

Physical Fitness Testing and Rewards

Tests of Density‐Dependent Habitat Selection in a Patchy Environment

A simple regression analysis can be used to assess the response of animal density to differences in habitat quality. The same test can evaluate general predictions of habitat selection theory as well as search for differences in the shapes of habitat suitability density functions, something previous tests have been unable to do. Combined with demographic or other estimates of fitness, regression tests can provide new insights into the evolution of habitat selection. Regression and fitness tests were used to explore the pattern of density—dependent habitat use in two temperate—zone rodents. The intensity of population regulation appeared to be inversely related to a habitat's carrying capacity. Variation in density—dependent habitat choice suggests new and unexpected dispersal strategies that vary with habitat heterogeneity. The predictions of the theory are complicated when habitat quality varies independently of population density. Sweepstake fitness rewards may be reaped by animals that would previously have been assumed to have made a suboptimal habitat choice. At the level of microhabitat, habitat selection models appeared to be much less capable of predicting variation in population density. Such a relation could be due to complex and inverse relationships between microhabitat quality and carrying capacity, or it could simply reflect scaling patterns in habitat selection.