Sperm competition and brain size evolution in mammals

The ‘expensive tissue hypothesis’ predicts a size trade‐off between the brain and other energetically costly organs. A specific version of this hypothesis, the ‘expensive sexual tissue hypothesis’, argues that selection for larger testes under sperm competition constrains brain size evolution. We show here that there is no general evolutionary trade‐off between brain and testis mass in mammals. The predicted negative relationship between these traits is not found for rodents, ungulates, primates, carnivores, or across combined mammalian orders, and neither does total brain mass vary according to the level of sperm competition as determined by mating system classifications. Although we are able to confirm previous reports of a negative relationship between brain and testis mass in echolocating bats, our results suggest that mating system may be a better predictor of brain size in this group. We conclude that the expensive sexual tissue hypothesis accounts for little or none of the variance in brain size in mammals, and suggest that a broader framework is required to understand the costs of brain size evolution and how these are met.     

Encephalization and the evolution of longevity in mammals

Allometric principles account for most of the observed variation in maximum life span among mammals. When body-size effects are controlled for, most of the residual variance in mammalian life span can be explained by variations in brain size, metabolic rate and body temperature. It is shown that species with large brains for a given body size and metabolic rate, such as anthropoid primates, also have long maximum life spans. Conversely, mammals with relatively high metabolic rates and low levels of encephalization, as in most insectivores and rodents, tend to have short life spans. The hypothesis is put forward that encephalization and metabolic rate, which may govern other life history traits, such as growth and reproduction, are the primary determinants directing the evolution of mammalian longevity.     

The digestive tract and life history of small mammals

The type of food, differentiation of the large intestine and stomach, and methane production, as well as life history data, are considered in Insectivora, Rodentia and Lagomorpha. When food containing plant cell wall material is eaten, there is either a differentiation of the stomach or the large intestine. In animals with low body mass and little differentiation of the gastrointestinal tract, methane production is low, but structures essential for microbial digestion of plant cell wall material, such as haustration of the colon or formation of a caecum, can be found in many methane‐producers. Animals with a body mass < 500 g and a weaning time < 20 days are non‐producers of methane. Establishment of a balanced microbial population in the gastrointestinal tract requires some time. Many non‐producers of methane wean their young in < 10 days, but many producers need > 50 days for the weaning process. Caviomorpha, Thryonomyidae and Hystricidae seem to have ‘opened the door’ to the use of low quality food by microbial fermentation, but some of them have to ‘pay’ for this extension of the food range by an extended weaning period, which also means an extended dependency on the mother.     

Life histories and the evolution of cooperative breeding in mammals

While the evolution of cooperative breeding systems (where non-breeding helpers participate in rearing young produced by dominant females) has been restricted to lineages with socially monogamous mating systems where coefficients of relatedness between group members are usually high, not all monogamous lineages have produced species with cooperative breeding systems, suggesting that other factors constrain the evolution of cooperative breeding. Previous studies have suggested that life-history parameters, including longevity, may constrain the evolution of cooperative breeding. Here, we show that transitions to cooperative breeding across the mammalian phylogeny have been restricted to lineages where females produce multiple offspring per birth. We find no support for effects of longevity or of other life-history parameters. We suggest that the evolution of cooperative breeding has been restricted to monogamous lineages where helpers have the potential to increase the reproductive output of breeders.     

Postnatal long bone growth in terrestrial placental mammals: Allometry,life history,and organismal traits

The ontogenetic allometry of long bone proportions is poorly understood in Mammalia. It has previously been suggested that during mammalian ontogeny long bone proportions grow more slender (positive allometry; length ∝ circumference^>1.0), although this conclusion was based upon data from a few small‐bodied taxa. It remains unknown how ontogenetic long bone allometry varies across Mammalia in terms of both taxonomy and body size. We collected long bone length and circumference data for ontogenetic samples of 22 species of mammals spanning six major clades and three orders of magnitude in body mass. Using reduced major axis bivariate regressions to compare bone length to circumference, we found that isometry and positive allometry are the most widespread patterns of growth across mammals. Negative allometry (i.e., bones growing more robust during ontogeny) occurs in mammals but is largely restricted to cetartiodactyls. Using regression slope as a proxy for long bone allometry, we compared long bone allometry to life history and organismal traits. Neonatal body mass, adult body mass, and growth rate have a negative relationship with long bone allometry. At an adult mass of roughly 15–20 kg, long bone growth shifts from positive allometry to mainly isometry and negative allometry. There were no significant relationships between ontogenetic long bone allometry and either cursoriality or basal metabolic rate. J. Morphol. © 2012 Wiley Periodicals, Inc.     

Geographic range size, life history and rates of diversification in Australian mammals

Abstract What causes species richness to vary among different groups of organisms? Two hypotheses are that large geographical ranges and fast life history either reduce extinction rates or raise speciation rates, elevating a clade's rate of diversification. Here we present a comparative analysis of these hypotheses using data on the phylogenetic relationships, geographical ranges and life history of the terrestrial mammal fauna of Australia. By comparing species richness patterns to null models, we show that species are distributed nonrandomly among genera. Using sister‐clade comparisons to control for clade age, we then find that faster diversification is significantly associated with larger geographical ranges and larger litters, but there is no evidence for an effect of body size or age at first breeding on diversification rates. We believe the most likely explanation for these patterns is that larger litters and geographical ranges increase diversification rates because they buffer species from extinction. We also discuss the possibility that positive effects of litter size and range size on diversification rates result from elevated speciation rates.     

Predation, group size and mortality in a cooperative mongoose, Suricata suricatta

1. In social mammals where group members cooperate to detect predators and raise young, members of small groups commonly show higher mortality or lower breeding success than members of large ones. It is generally assumed that this is because large group size allows individuals to detect or repel predators more effectively but other benefits of group size may also be involved, including reduced costs of raising young and more effective competition for resources with neighbouring groups.
2. To investigate the extent to which predation rate affects survival, we compared mortality rates in two populations of suricates ( Suricata suricatta ), one living in an area of high predator density (Kalahari Gemsbok Park) and one living in an area of relatively low predator density (neighbouring ranchland). Most aspects of feeding ecology and growth (including time spent feeding, daily weight gain, growth, adult body weight, breeding frequency and neonatal mortality) were similar in the two populations. In contrast, mortality of animals over 3 months old was 1·7 times higher in the Park than on ranchland.
3. Mortality of juveniles between emergence from the natal burrow and 6 months of age was higher in small groups than large ones in the Park but significantly lower in small groups than large ones on ranchland. Adult mortality declined in larger groups in both areas.
4. The tendency for survival to be low in small groups had far-reaching consequences for the risk of group extinction. During a year of low rainfall in the Park, all groups of less than nine animals became extinct and population density declined to around a third of its initial level. We argue that high group extinction rates are to be expected in species where survival declines in small groups and mortality rates are high.     

Correlates of species richness in mammals: body size, life history, and ecology

We present the most extensive examination to date of proposed correlates of species richness. We use rigorous phylogenetic comparative techniques, data for 1,692 mammal species in four clades, and multivariate statistics to test four hypotheses about species richness and compare the evidence for each. Overall, we find strong support for the life-history model of diversification. Species richness is significantly correlated with shorter gestation period in the carnivores and large litter size in marsupials. These traits and short interbirth intervals are also associated with species richness in a pooled analysis of all four clades. Additionally, we find some support for the abundance hypotheses in different clades of mammals: abundance correlates positively with species richness in primates but negatively in microchiropterans. Our analyses provide no evidence that mammalian species richness is associated with body size or degree of sexual dimorphism.     

Variability and size in mammals and birds

Body size, its variability, and their ecological correlates have long been important topics in evolutionary biology. Yet, the question of whether there is a general relationship between size and size-relative variability has not previously been addressed. Through an analysis of body-mass and length measurements from 65 074 individuals from 351 mammalian species, we show that size-relative variability increases significantly with mean species body size. Analysis of mean body mass and standard deviations for 237 species of birds revealed the same pattern. We present three plausible alternatives explanations and eliminate several others. Of these, the hypothesis that the increase in size-relative variability with mean body mass is related to the scaling of body mass components is most strongly supported. In effect, larger mammals and birds are more variable because their body mass is composed to greater relative degree of components with higher intrinsic variability (bone, fat, and muscle). In contrast, smaller mammals and birds have lower body mass variability because they are composed to a greater relative extent of components (viscera and nervous system) in which size variation is more highly constrained by energetic and functional factors.     

Aspects of the life cycle of Schistosoma margrebowiei infection in laboratory mammals

Schistosoma margrebowiei Le Roux, 1933 has been recorded in a number of mammals in Africa. The parasite was maintained in the laboratory using Bulinus natalensis as intermediate host and hamsters, mice and gerbils as definitive hosts.Worm recovery, growth of paired worms, egg output and egg viability were determined and compared in the three laboratory hosts. The results in the three hosts are discussed and the mouse was observed to be suitable for the long term studies on S. margrebowiei in the laboratory. Hamsters and gerbils were observed to be useful for the study of the acute phase of the disease.     

The evolution of body armor in mammals: plantigrade constraints of large body size

Predictions associated with opposing selection generating minimum variance in basal metabolic rate (BMR) in mammals at a constrained body mass (CBM; 358 g) were tested. The CBM is presumed to be associated with energetic constraints linked to predation and variable resources at intermediate sizes on a logarithmic mass scale. Opposing selection is thought to occur in response to energetic constraints associated with predation and unpredictable resources. As body size approaches and exceeds the CBM, mammals face increasing risks of predation and daily energy requirements. Fast running speeds may require high BMRs, but unpredictable and low resources may select for low BMRs, which also reduce foraging time and distances and thus predation risks. If these two selection forces oppose each other persistently, minimum BMR variance may result. However, extreme BMR outliers at and close to the CBM should be indicative of unbalanced selection and predator avoidance alternatives (escapers vs. defenders), and may therefore provide indirect support for opposing selection. It was confirmed that body armor in defenders evolves at and above the CBM, and armored mammals had significantly lower BMRs than their nonarmored counterparts. However, analyses comparing the BMR of escapers--the fastest nonarmored runners (Lagomorpha)--with similar-sized counterparts were inconclusive and were confounded by limb morphology associated with speed optimization. These analyses suggest that the risks and costs of predation and the speed limitations of the plantigrade foot may constrain the evolution of large body sizes in plantigrade mammals.     

Patterns of maximum body size evolution in Cenozoic land mammals: eco-evolutionary processes and abiotic forcing

There is accumulating evidence that macroevolutionary patterns of mammal evolution during the Cenozoic follow similar trajectories on different continents. This would suggest that such patterns are strongly determined by global abiotic factors, such as climate, or by basic eco-evolutionary processes such as filling of niches by specialization. The similarity of pattern would be expected to extend to the history of individual clades. Here, we investigate the temporal distribution of maximum size observed within individual orders globally and on separate continents. While the maximum size of individual orders of large land mammals show differences and comprise several families, the times at which orders reach their maximum size over time show strong congruence, peaking in the Middle Eocene, the Oligocene and the Plio-Pleistocene. The Eocene peak occurs when global temperature and land mammal diversity are high and is best explained as a result of niche expansion rather than abiotic forcing. Since the Eocene, there is a significant correlation between maximum size frequency and global temperature proxy. The Oligocene peak is not statistically significant and may in part be due to sampling issues. The peak in the Plio-Pleistocene occurs when global temperature and land mammal diversity are low, it is statistically the most robust one and it is best explained by global cooling. We conclude that the macroevolutionary patterns observed are a result of the interplay between eco-evolutionary processes and abiotic forcing.     

The evolution of cooperative breeding in birds: kinship,dispersal and life history

The evolution of cooperation among animals has posed a major problem for evolutionary biologists, and despite decades of research into avian cooperative breeding systems, many questions about the evolution of their societies remain unresolved. A review of the kin structure of avian societies shows that a large majority live in kin-based groups. This is consistent with the proposed evolutionary routes to cooperative breeding via delayed dispersal leading to family formation, or limited dispersal leading to kin neighbourhoods. Hypotheses proposed to explain the evolution of cooperative breeding systems have focused on the role of population viscosity, induced by ecological/demographic constraints or benefits of philopatry, in generating this kin structure. However, comparative analyses have failed to generate robust predictions about the nature of those constraints, nor differentiated between the viscosity of social and non-social populations, except at a coarse level. I consider deficiencies in our understanding of how avian dispersal strategies differ between social and non-social species, and suggest that research has focused too narrowly on population viscosity and that a broader perspective that encompasses life history and demographic processes may provide fresh insights into the evolution of avian societies.     

Body size and measurement of species diversity in large grazing mammals

Species are by definition different from each other. This fact favours ranking rather than additive indices. However, ecologists have measured species diversity in terms of species richness, or by combining species richness with the relative abundance of species within an area. Both methods raise problems: species richness treats all species equally, while relative abundance is not a fixed property of species but varies widely temporally and spatially, and requires a massive sampling effort. The functional aspect of species diversity measurement may be strengthened by incorporating differences between species such as body size as a component of diversity. An index of diversity derived from a measure of variation in body size among species is proposed for large grazing mammals. The proposed diversity index related positively to species abundance, indicating that the use of body size as a surrogate for diversity is adequate. Because the proposed index is based on presence or absence data, the expensive and time consuming counting of individuals per species in each sampling unit is not necessary.     

Energy metabolism, brain size and longevity in mammals

The mathematical relations between basal energy metabolism, brain size, and life span in mammals have been investigated. The evolutionary level of brain development, or encephalization (c), is a function both of brain weight (E) and of body weight (P) according to (formula; see text) Brain weight was found to be a linear function of the product of encephalization and basal metabolic rate. The oxygen consumption of the brain (Mbrain) is proportional to both encephalization and body weight according to (formula; see text) The ratio of metabolic rate in the cerebral cortex to that in the brain as a whole depends solely upon the degree of encephalization and is independent of the size of the animal. The maximum potential life span of a mammal was found to be proportional to the product of its degree of encephalization and the reciprocal of its metabolic rate per unit weight. Life span may be regarded as the algebraic sum of two components: (1) a deduced somatic component (Lb) inversely related to the basal metabolic rate per unit weight, and (2) an encephalization component (Le) related directly to the evolutionary increase of relative brain size.     

Selection,constraint, and the evolution of coloration in African starlings

Colorful plumage plays a prominent role in the evolution of birds, influencing communication (sexual/social selection), and crypsis (natural selection). Comparative studies have focused primarily on these selective pressures, but the mechanisms underlying color production can also be important by constraining the color gamut upon which selection acts. Iridescence is particularly interesting to study the interaction between selection and color‐producing mechanisms because a broad range of colors can be produced with a shared template, and innovations to this template further expand this by increasing the parameters interacting to produce colors. We examine the patterns of ornamentation and dichromatism evolution in African starlings, a group remarkably diverse in color production mechanisms, social systems, and ecologies. We find that the presence of iridescence is ancestral to the group, being predominantly lost in females and cooperative breeders, as well as species with less labile templates. Color‐producing mechanisms interact and are the main predictors of plumage ornamentation and elaboration, with little influence of selective pressures in their evolution. Dichromatism, however is influenced by social system and the loss of iridescence. Our results show the importance of considering both selection and constraints, and the different roles that they may have, in the evolution of ornamentation and dimorphism.     

Nutritional constraints on brain evolution: Sodium and nitrogen limit brain size

Nutrition has been hypothesized as an important constraint on brain evolution. However, it is unclear whether the availability of specific nutrients or the difficulty of locating high-quality diets limits brain evolution, especially over long periods of time. We found that dietary nutrient content predicted brain size across 42 species of butterflies. Brain size, relative to body size, was associated with the sodium and nitrogen content of a species’ diet. There was no evidence that host plant apparency (measured by plant height) was related to brain evolution. The timing of diet shifts across species varied from 3.5 to 90 million years ago, but nutritional constraints did not lessen over time as species adapted to a diet. Although nutrition was linked to overall brain volume, there was no evidence that nutrition was related to the relative size of individual brain regions. Laboratory rearing experiments confirmed the underlying assumption of most comparative studies that the majority of interspecific trait variation stems from genetically based differences across species rather than developmental plasticity. This study highlights a novel role of sodium and nitrogen in brain evolution, which is additionally interesting given current anthropogenic change in the availability of these nutrients.