A two-resource model of terminal investment

Experimental evolution for improved postinfection survival selects for increased disease resistance in Drosophila melanogaster

Disease resistance (defined as the host capacity to limit systemic infection intensity) and disease tolerance (defined as the host capacity to limit infection-induced damage) are 2 complementary defense strategies that help the hosts maximize their survival and fitness when infected with pathogens and parasites. In addition to the underlying physiological mechanisms, the existing theory postulates that these 2 strategies differ in terms of the conditions under which each strategy evolves in the host populations, their evolutionary dynamics, and the ecological and epidemiological consequences of their evolution. Here, we explored if one or both of these strategies evolve when host populations are subjected to selection for increased postinfection survival. We experimentally evolved Drosophila melanogaster populations, selecting for the flies that survived an infection with the entomopathogen Enterococcus faecalis. We found that the host populations evolved increased disease resistance in response to selection for increased survival. This was despite the physiological costs associated with increased resistance, the expression of which varied with the phase of infection. We did not find evidence of any change in disease tolerance in the evolved host populations.

A dynamic threshold model for terminal investment

Although reproductive strategies can be influenced by a variety of intrinsic and extrinsic factors, life history theory provides a rigorous framework for explaining variation in reproductive effort. The terminal investment hypothesis proposes that a decreased expectation of future reproduction (as might arise from a mortality threat) should precipitate increased investment in current reproduction. Terminal investment has been widely studied, and a variety of intrinsic and extrinsic cues that elicit such a response have been identified across an array of taxa. Although terminal investment is often treated as a static strategy, the level at which a cue of decreased future reproduction is sufficient to trigger increased current reproductive effort (i.e., the terminal investment threshold) may depend on context, including the internal state of the organism or its current external environment, independent of the cue that triggers a shift in reproductive investment. Here, we review empirical studies that address the terminal investment hypothesis, exploring both the intrinsic and extrinsic factors that mediate its expression. Based on these studies, we propose a novel framework within which to view the strategy of terminal investment, incorporating factors that influence an individual's residual reproductive value beyond a terminal investment trigger - the dynamic terminal investment threshold.

Between semelparity and iteroparity: Empirical evidence for a continuum of modes of parity

The number of times an organism reproduces (i.e., its mode of parity) is a fundamental life-history character, and evolutionary and ecological models that compare the relative fitnesses of different modes of parity are common in life-history theory and theoretical biology. Despite the success of mathematical models designed to compare intrinsic rates of increase (i.e., density-independent growth rates) between annual-semelparous and perennial-iteroparous reproductive schedules, there is widespread evidence that variation in reproductive allocation among semelparous and iteroparous organisms alike is continuous. This study reviews the ecological and molecular evidence for the continuity and plasticity of modes of parity-that is, the idea that annual-semelparous and perennial-iteroparous life histories are better understood as endpoints along a continuum of possible strategies. I conclude that parity should be understood as a continuum of different modes of parity, which differ by the degree to which they disperse or concentrate reproductive effort in time. I further argue that there are three main implications of this conclusion: (1) that seasonality should not be conflated with parity; (2) that mathematical models purporting to explain the general evolution of semelparous life histories from iteroparous ones (or vice versa) should not assume that organisms can only display either an annual-semelparous life history or a perennial-iteroparous one; and (3) that evolutionary ecologists should base explanations of how different life-history strategies evolve on the physiological or molecular basis of traits underlying different modes of parity.

An ontogenetic perspective on individual differences

Phenotypic differences among individuals can arise during any stage of life. Although several distinct processes underlying individual differences have been defined and studied (e.g. parental effects, senescence), we lack an explicit, unified perspective for understanding how these processes contribute separately and synergistically to observed variation in functional traits. We propose a conceptual framework based on a developmental view of life-history variation, linking each ontogenetic stage with the types of individual differences originating during that period. In our view, the salient differences among these types are encapsulated by three key criteria: timing of onset, when fitness consequences are realized, and potential for reversibility. To fill a critical gap in this framework, we formulate a new term to refer to individual differences generated during adulthood-reversible state effects. We define these as 'reversible changes in a functional trait resulting from life-history trade-offs during adulthood that affect fitness', highlighting how the adult phenotype can be repeatedly altered in response to environmental variation. Defining individual differences in terms of trade-offs allows explicit predictions regarding when and where fitness consequences should be expected. Moreover, viewing individual differences in a developmental context highlights how different processes can work in concert to shape phenotype and fitness, and lays a foundation for research linking individual differences to ecological and evolutionary theory.

When is it socially acceptable to feel sick?

Disease is a ubiquitous and powerful evolutionary force. Hosts have evolved behavioural and physiological responses to disease that are associated with increased survival. Behavioural modifications, known as 'sickness behaviours', frequently involve symptoms such as lethargy, somnolence and anorexia. Current research has demonstrated that the social environment is a potent modulator of these behaviours: when conflicting social opportunities arise, animals can decrease or entirely forgo experiencing sickness symptoms. Here, I review how different social contexts, such as the presence of mates, caring for offspring, competing for territories or maintaining social status, affect the expression of sickness behaviours. Exploiting the circumstances that promote this behavioural plasticity will provide new insights into the evolutionary ecology of social behaviours. A deeper understanding of when and how this modulation takes place may lead to better tools to treat symptoms of infection and be relevant for the development of more efficient disease control programmes.

Resource availability as a proxy for terminal investment in a beetle

Terminal investment hypothesis is a longstanding theoretical idea that organisms should increase their reproductive effort as their prospects for survival and reproduction decline. However, numerous attempts to test the terminal investment in reproduction have yielded contradictory results. This study reports an experimental confirmation of the terminal investment hypothesis. It was predicted that immune-challenged yellow mealworm beetles (Tenebrio molitor) are more likely to follow terminal investment strategy when their food resources are limited. Our results suggest the key role of food resources while making decisions to follow a terminal investment strategy. We found that male individuals invested in their sexual attractiveness at the expense of immune response and survival when food was not available. In contrast, the beetles did not decrease their lifespan and did not invest in the attractiveness of their sex odours under conditions of food ad libitum. Our results show the importance of food availability and quality in understanding the evolution of reproductive strategies.