Primate longevity: Its place in the mammalian scheme

Data on captive longevity in 587 mammalian species were analyzed in order to evaluate primate longevity in the context of general mammalian life history patterns. Contrary to some recurrent claims in the literature, we found that 1) primates are not the longest-lived mammalian order, either by absolute longevity, longevity corrected for body size, or metabolic expenditure per lifetime; 2) although relative brain size is highly correlated with longevity in primates, this is an aberrant trend for mammals in general, and other body organs account for an even greater amount of variation in longevity; and 3) there has been no progressive evolution of increased longevity among the primate superfamilies. The exceptional magnitude of primate longevity may, in keeping with evolutionary senescence theory, be due to an evolutionary history of low vulnerability to environmentally imposed death due to their body size, arboreal habit, and propensity to live in social groups. © 1992 Wiley-Liss, Inc.     

Effects of body size on biochemical characteristics of trabecular cardiac muscle and plasma of rainbow trout (Oncorhynchus mykiss)

The objective of this study was to characterize biochemical indices of oxidative metabolism in trabecular cardiac muscle of female rainbow trout over a 200-fold size range. We also examined scaling effects on plasma concentrations of protein, albumin, and non-esterified fatty acids (NEFA). Animals were sampled from four size categories (<20 g, 100–200 g, 300–400 g and >2 kg). Ventricle mass relative to body mass was size-dependent, with the smallest trout having smaller hearts. Total protein in cardiac tissue was 20% higher in animals weighing over 300 g compared to fish less than 200 g. Plasma albumin and protein were increased 50–70% in trout over 100 g compared to fish smaller than 20 g. NEFA in plasma were similar for all animals. Activities of carnitine palmitoyltransferase and cytochrome oxidase were elevated 53% and 38%, respectively, in trabecular cardiac muscle of the largest trout compared to the smallest animals. Citrate synthase activity was independent of heart size, suggesting that the increase in oxidative capacity was not due to an increase in mitochondrial density. The increased capacity to bind and transport fatty acids in blood and the higher oxidative capacity of cardiac muscle may reflect a metabolic adaptation for increased oxidation of fatty acids and ventricular performance in larger female trout. These findings are not consistent with the scaling paradigm of oxidative metabolism and may reflect changes in ventricular architecture with ontogeny.     

Evolving ecological networks and the emergence of biodiversity patterns across temperature gradients

In ectothermic organisms, it is hypothesized that metabolic rates mediate influences of temperature on the ecological and evolutionary processes governing biodiversity. However, it is unclear how and to what extent the influence of temperature on metabolism scales up to shape large-scale diversity patterns. In order to clarify the roles of temperature and metabolism, new theory is needed. Here, we establish such theory and model eco-evolutionary dynamics of trophic networks along a broad temperature gradient. In the model temperature can influence, via metabolism, resource supply, consumers' vital rates and mutation rate. Mutation causes heritable variation in consumer body size, which diversifies and governs consumer function in the ecological network. The model predicts diversity to increase with temperature if resource supply is temperature-dependent, whereas temperature-dependent consumer vital rates cause diversity to decrease with increasing temperature. When combining both thermal dependencies, a unimodal temperature-diversity pattern evolves, which is reinforced by temperature-dependent mutation rate. Studying coexistence criteria for two consumers showed that these outcomes are owing to temperature effects on mutual invasibility and facilitation. Our theory shows how and why metabolism can influence diversity, generates predictions useful for understanding biodiversity gradients and represents an extendable framework that could include factors such as colonization history and niche conservatism.     

Atmospheric oxygen level and the evolution of insect body size

Insects are small relative to vertebrates, possibly owing to limitations or costs associated with their blind-ended tracheal respiratory system. The giant insects of the late Palaeozoic occurred when atmospheric PO₂ (aPO₂) was hyperoxic, supporting a role for oxygen in the evolution of insect body size. The paucity of the insect fossil record and the complex interactions between atmospheric oxygen level, organisms and their communities makes it impossible to definitively accept or reject the historical oxygen-size link, and multiple alternative hypotheses exist. However, a variety of recent empirical findings support a link between oxygen and insect size, including: (i) most insects develop smaller body sizes in hypoxia, and some develop and evolve larger sizes in hyperoxia; (ii) insects developmentally and evolutionarily reduce their proportional investment in the tracheal system when living in higher aPO₂, suggesting that there are significant costs associated with tracheal system structure and function; and (iii) larger insects invest more of their body in the tracheal system, potentially leading to greater effects of aPO₂ on larger insects. Together, these provide a wealth of plausible mechanisms by which tracheal oxygen delivery may be centrally involved in setting the relatively small size of insects and for hyperoxia-enabled Palaeozoic gigantism.     

Miniaturization in plethodontid salamanders (Gaudata: Plethodontidae) and its consequences for the brain and visual system

In seven species of plethodontid salamanders (Desmognathus ochrophaeus, Eurycea bislineata, Plethodon cinereus, Batrachoseps attenuatus, Hydromantes italicus, Thorius narisovalis and Bolitoglossa subpalmata), absolute and relative volumes of the eye, the brain, major regions of the brain, and regions containing the major visual and visuomotor centres (i.e. thalamus, praetectum, tectum and tegmentum mesencephali), and the density and number of neurons in these regions were determined. The seven species range from moderately large to extremely small in body size and from the smallest to the largest genome sizes found in terrestrial salamanders. The following processes were observed in miniaturized salamanders with intermediate to large genome and cell sizes (Batrachoseps, Thorius) as compared to small and medium-sized salamanders with small genome and cell sizes: (1) increase in the relative size of the brain, from 3.9 to 12.4% of head volume; (2) reduction in relative size of the ventricles from 10.9 to 5.8% of brain volume; (3) increase in relative volume of those brain regions containing the major visual and visuomotor centres from 29.2 to 37% of brain volume; (4) increase in volume of grey matter relative to white matter, from 33.2 to 44.4% of midbrain volume; (5) increase in volume of tectal relative to tegmental grey matter, from 54.8 to 76.8% of total midbrain volume; (6) increase in neuron packing density in the regions containing the visual centres, from 16 to 31.5%. Because of these compensatory processes, Thorius, the smallest species with a head 1/27 and a brain 1/9 the size of that of the largest one, Hydromantes, has 1/3 as many central visual neurons (58 000 vs. 187 000). Some of these processes found in miniaturized salamanders, such as increase in tectal cell density, also occur in large salamanders with very large genome and cell sizes, viz. in Bolitoglossa (25%) and Hydromantes (29%). Thus, increase in genome size and cell size seem to pose functional problems similar to miniaturization; both cases involve an increase in cell size relative to overall organismal structure.     

The relationship between genome size, development rate, and body size in copepods

Freshwater cyclopoid copepods exhibit at least a fivefold range in somatic genome size and a mechanism, chromatin diminution, which could account for much of this interspecific variation. These attributes suggest that copepods are well suited to studies of genome size evolution. We tested the nucleotypic hypothesis of genome size evolution, which poses that variation in genome size is adaptive due to the bulk effects of both coding and noncoding DNA on cell size and division rates, and their correlates. We found a significant inverse correlation between genome size and developmental (growth) rate in five freshwater cyclopoid species at three temperatures. That is, species with smaller genomes developed faster. Species with smaller genomes had significantly smaller bodies at 22 °C, but not at cooler and warmer temperatures. Species with smaller genomes developed faster at all three temperatures, but had smaller bodies only at 22 °C. We propose a model of life history evolution that adds genome size and cell cycle dynamics to the suite of characters on which selection may act to mold life histories and to influence the distribution of traits among different habitats.     

Are rates of species diversification correlated with rates of morphological evolution?

Some major evolutionary theories predict a relationship between rates of proliferation of new species (species diversification) and rates of morphological divergence between them. However, this relationship has not been rigorously tested using phylogeny-based approaches. Here, we test this relationship with morphological and phylogenetic data from 190 species of plethodontid salamanders. Surprisingly, we find that rates of species diversification and morphological evolution are not significantly correlated, such that rapid diversification can occur with little morphological change, and vice versa. We also find that most clades have undergone remarkably similar patterns of morphological evolution (despite extensive sympatry) and that those relatively novel phenotypes are not associated with rapid diversification. Finally, we find a strong relationship between rates of size and shape evolution, which has not been previously tested.