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1. Individual life histories: neither slow nor fast, just diverse

   https://doi.org/10.1098/rspb.2023.0511  

   Van de Walle, Joanie; Fay, Rémi; Gaillard, Jean-Michel; Pelletier, Fanie; Hamel, Sandra; Gamelon, Marlène; Barbraud, Christophe; Blanchet, F. Guillaume; Blumstein, Daniel T.; Charmantier, Anne; et al (July 2023, Proceedings of the Royal Society B: Biological Sciences)

   The slow–fast continuum is a commonly used framework to describe variation in life-history strategies across species. Individual life histories have also been assumed to follow a similar pattern, especially in the pace-of-life syndrome literature. However, whether a slow–fast continuum commonly explains life-history variation among individuals within a population remains unclear. Here, we formally tested for the presence of a slow–fast continuum of life histories both within populations and across species using detailed long-term individual-based demographic data for 17 bird and mammal species with markedly different life histories. We estimated adult lifespan, age at first reproduction, annual breeding frequency, and annual fecundity, and identified the main axes of life-history variation using principal component analyses. Across species, we retrieved the slow–fast continuum as the main axis of life-history variation. However, within populations, the patterns of individual life-history variation did not align with a slow–fast continuum in any species. Thus, a continuum ranking individuals from slow to fast living is unlikely to shape individual differences in life histories within populations. Rather, individual life-history variation is likely idiosyncratic across species, potentially because of processes such as stochasticity, density dependence, and individual differences in resource acquisition that affect species differently and generate non-generalizable patterns across species. 

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2. Environmental, evolutionary, and ecological drivers of slow growth in deep-sea demersal teleosts

   https://doi.org/10.3354/meps13591  

   Black, JA; Neuheimer, AB; Horn, PL; Tracey, DM; Drazen, JC (January 2021, Marine Ecology Progress Series)

   null (Ed.)

   The deep sea (>500 m ocean depth) is the largest global habitat, characterized by cool temperatures, low ambient light, and food-poor conditions relative to shallower waters. Deep-sea teleosts generally grow more slowly than those inhabiting shallow water. However, this is a generalization, and even amongst deep-sea teleosts, there is a broad continuum of growth rates. The importance of potential drivers of growth rate variability amongst deep-sea species, such as temperature, food availability, oxygen concentration, metabolic rate, and phylogeny, have yet to be fully evaluated. We present a meta-analysis in which age and size data were collected for 53 species of teleosts whose collective depth ranges span from surface waters to 4000 m. We calculated growth metrics using both calendar and thermal age, and compared them with environmental, ecological, and phylogenetic variables. Temperature alone explained up to 30% of variation in the von Bertalanffy growth coefficient ( K , yr -1 ), and 21% of the variation in the average annual increase in mass (AIM, %), a metric of growth prior to maturity. After correcting for temperature effects, depth was still a significant driver of growth, explaining up to 20 and 10% of the remaining variation in K and AIM, respectively. Oxygen concentration also explained ~11% of remaining variation in AIM following temperature correction. Relatively minor amounts of variation may be explained by food availability, phylogeny, and the locomotory mode of the teleosts. We also found strong correlation between growth and metabolic rate, which may be an underlying driver also related to temperature, depth, and other factors, or the 2 parameters may simply covary as a result of being linked by evolutionary pressures. Evaluating the influence of ecological and/or environmental drivers of growth is a vital step in understanding both the evolution of life history parameters across the depth continuum as well as their implications for species’ resilience to increasing anthropogenic stressors. 

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3. Oxygen supply capacity in animals evolves to meet maximum demand at the current oxygen partial pressure regardless of size or temperature

   https://doi.org/10.1101/701417  

   Seibel, B. (January 2020, Journal of experimental biology)

   null (Ed.)

   The capacity to extract oxygen from the environment and transport it to respiring tissues in support of metabolic demand reportedly has implications for species’ thermal tolerance, body size, diversity and biogeography. Here, we derived a quantifiable linkage between maximum and basal metabolic rate and their oxygen, temperature and size dependencies. We show that, regardless of size or temperature, the physiological capacity for oxygen supply precisely matches the maximum evolved demand at the highest persistently available oxygen pressure and this is the critical PO2 for the maximum metabolic rate, Pcrit-max. For most terrestrial and shallow-living marine species, Pcrit-max is the current atmospheric pressure, 21 kPa. Any reduction in oxygen partial pressure from current values will result in a calculable decrement in maximum metabolic performance. However, oxygen supply capacity has evolved to match demand across temperatures and body sizes and so does not constrain thermal tolerance or cause the well-known reduction in mass-specific metabolic rate with increasing body mass. The critical oxygen pressure for resting metabolic rate, typically viewed as an indicator of hypoxia tolerance, is, instead, simply a rate-specific reflection of the oxygen supply capacity. A compensatory reduction in maintenance metabolic costs in warm-adapted species constrains factorial aerobic scope and the critical PO2 to a similar range, between ∼2 and 6, across each species’ natural temperature range. The simple new relationship described here redefines many important physiological concepts and alters their ecological interpretation. 

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4. Longevity, Not Stream Flow, Explains Variation in Freshwater Mussel Growth Rates Across Four Rivers

   https://doi.org/10.1111/fwb.14373  

   Brunetz, Ian_A; Lopez, Jonathan_W; Hopper, Garrett_W; González, Irene_Sánchez; Atkinson, Carla_L (December 2024, Freshwater Biology)

   ABSTRACT Freshwater mussels (Bivalvia: Unionida) are among the most imperilled freshwater taxa. Yet, there is a lack of basic life history information for mussels, including data on their growth and longevity. These data help inform conservation efforts, as they can indicate whether species or populations may be vulnerable to decline and inform which species may be best adapted to certain habitats. We aimed to quantify growth and longevity in five mussel species from four river systems in the southeastern United States and test whether growth was related to stream flow. We also interpreted our findings in the context of life history theory.To model mussel growth and longevity, we cut radial thick sections from the shells of mussels and used high‐resolution photography to image the shells. We identified annual growth rings (annuli) and used von Bertalanffy growth models to estimate growth rate (K) and maximum age (A_max) across 13 mussel populations. We then used biochronological methods to remove age‐related variation in annual growth in each shell. We tested whether annual growth was correlated with stream flow using discharge‐based statistics.We found substantial variation inKandA_maxamong species and among populations of the same species.Kwas negatively related toA_max. We did not find consistent correlations between annual growth and stream flow.Our estimates ofKandA_maxalign with previous studies on closely related species and populations. They also match the eco‐evolutionary prediction that growth rate and longevity are negatively related. Life history theory predicts that short‐lived species with higher growth rates should be better adapted to environments with cyclical disturbance regimes, whereas longer‐lived species with low growth rates should be better adapted to stable environments. The lack of correlation between annual growth and stream flow suggests that mussel growth may be limited by other factors in our study system.While some species seem to have relatively narrow ranges for growth and longevity, other species show wide variation among populations. This highlights the need for species‐ and population‐specific conservation efforts. Fundamental life history information can be integrated with other species traits to predict how freshwater taxa may respond to ecological threats. 

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5. Slower environmental cycles maintain greater life‐history variation within populations

   https://doi.org/10.1111/ele.13867  

   Park, John S.; Wootton, J. Timothy; Gaillard, ed., Jean‐Michel (September 2021, Ecology Letters)

   Abstract Populations in nature are comprised of individual life histories, whose variation underpins ecological and evolutionary processes. Yet the forces of environmental selection that shape intrapopulation life‐history variation are still not well‐understood, and efforts have largely focused on random (stochastic) fluctuations of the environment. However, a ubiquitous mode of environmental fluctuation in nature is cyclical, whose periodicities can change independently of stochasticity. Here, we test theoretically based hypotheses for whether shortened (‘Fast’) or lengthened (‘Slow’) environmental cycles should generate higher intrapopulation variation of life history phenotypes. We show, through a combination of agent‐based modelling and a multi‐generational laboratory selection experiment using the tidepool copepodTigriopus californicus, that slower environmental cycles maintain higher levels of intrapopulation variation. Surprisingly, the effect of environmental periodicity on variation was much stronger than that of stochasticity. Thus, our results show that periodicity is an important facet of fluctuating environments for life‐history variation. 

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