Universal scaling of production rates across mammalian lineages

Quantifying maternal investment in mammals using allometry

Maternal investment influences the survival and reproduction of both mothers and their progeny and plays a crucial role in understanding individuals' life-history and population ecology. To reveal the complex mechanisms associated with reproduction and investment, it is necessary to examine variations in maternal investment across species. Comparisons across species call for a standardised method to quantify maternal investment, which remained to be developed. This paper addresses this limitation by introducing the maternal investment metric - MI - for mammalian species, established through the allometric scaling of the litter mass at weaning age by the adult mass and investment duration (i.e. gestation + lactation duration) of a species. Using a database encompassing hundreds of mammalian species, we show that the metric is not highly sensitive to the regression method used to fit the allometric relationship or to the proxy used for adult body mass. The comparison of the maternal investment metric between mammalian subclasses and orders reveals strong differences across taxa. For example, our metric confirms that Eutheria have a higher maternal investment than Metatheria. We discuss how further research could use the maternal investment metric as a valuable tool to understand variation in reproductive strategies.

Non-uniform temporal scaling of developmental processes in the mammalian cortex

The time that it takes the brain to develop is highly variable across animals. Although staging systems equate major developmental milestones between mammalian species, it remains unclear how distinct processes of cortical development scale within these timeframes. Here, we compare the timing of cortical development in two mammals of similar size but different developmental pace: eutherian mice and marsupial fat-tailed dunnarts. Our results reveal that the temporal relationship between cell birth and laminar specification aligns to equivalent stages between these species, but that migration and axon extension do not scale uniformly according to the developmental stages, and are relatively more advanced in dunnarts. We identify a lack of basal intermediate progenitor cells in dunnarts that likely contributes in part to this timing difference. These findings demonstrate temporal limitations and differential plasticity of cortical developmental processes between similarly sized Therians and provide insight into subtle temporal changes that may have contributed to the early diversification of the mammalian brain.

The Relevance of Time in Biological Scaling

Various phenotypic traits relate to the size of a living system in regular but often disproportionate (allometric) ways. These "biological scaling" relationships have been studied by biologists for over a century, but their causes remain hotly debated. Here, I focus on the patterns and possible causes of the body-mass scaling of the rates/durations of various biological processes and life-history events, i.e., the "pace of life". Many biologists have regarded the rate of metabolism or energy use as the master driver of the "pace of life" and its scaling with body size. Although this "energy perspective" has provided valuable insight, here I argue that a "time perspective" may be equally or even more important. I evaluate various major ways that time may be relevant in biological scaling, including as (1) an independent "fourth dimension" in biological dimensional analyses, (2) a universal "biological clock" that synchronizes various biological rates/durations, (3) a scaling method that uses various biological time periods (allochrony) as scaling metrics, rather than various measures of physical size (allometry), as traditionally performed, (4) an ultimate body-size-related constraint on the rates/timing of biological processes/events that is set by the inevitability of death, and (5) a geological "deep time" approach for viewing the evolution of biological scaling patterns. Although previously proposed universal four-dimensional space-time and "biological clock" views of biological scaling are problematic, novel approaches using allochronic analyses and time perspectives based on size-related rates of individual mortality and species origination/extinction may provide new valuable insights.

Reproductive dispersion and damping time scale with life-history speed

Iteroparous species may reproduce at many different ages, resulting in a reproductive dispersion that affects the damping of population perturbations, and varies among life histories. Since generation time ( T c $$ {T}_c $$ ) is known to capture aspects of life-history variation, such as life-history speed, does T c $$ {T}_c $$ also determine reproductive dispersion ( S $$ S $$ ) or damping time ( τ $$ \tau $$ )? Using phylogenetically corrected analyses on 633 species of animals and plants, we find, firstly, that reproductive dispersion S $$ S $$ scales isometrically with T c $$ {T}_c $$ . Secondly, and unexpectedly, we find that the damping time ( τ $$ \tau $$ ) does not scale isometrically with generation time, but instead changes only as T c b $$ {T}_c^b $$ with b < 1 $$ b<1 $$ (also, there is a similar scaling with S $$ S $$ ). This non-isometric scaling implies a novel demographic contrast: increasing generation times correspond to a proportional increase in reproductive dispersion, but only to a slower increase in the damping time. Thus, damping times are partly decoupled from the slow-fast continuum, and are determined by factors other than allometric constraints.

The Pace of Life: Metabolic Energy, Biological Time, and Life History

New biophysical theory and electronic databases raise the prospect of deriving fundamental rules of life, a conceptual framework for how the structures and functions of molecules, cells and individual organisms give rise to emergent patterns and processes of ecology, evolution and biodiversity. This framework is very general, applying across taxa of animals from 10-10 g protists to 108 g whales, and across environments from deserts and abyssal depths to rain forests and coral reefs. It has several hallmarks: 1) Energy is the ultimate limiting resource for organisms and the currency of biological fitness. 2) Most organisms are nearly equally fit, because in each generation at steady state they transfer an equal quantity of energy (22.4 kJ/g) and biomass (1 g/g) to surviving offspring. This is the equal fitness paradigm (EFP) of Brown et al. (2018). 3) The enormous diversity of life histories is due largely to variation in metabolic rates (e.g., energy uptake and expenditure via assimilation, respiration and production) and biological times (e.g., generation time). As in standard allometric and metabolic theory, most physiological and life history traits scale approximately as quarter-power functions of body mass, m (rates as ∼m-1/4 and times as ∼m1/4), and as exponential functions of temperature. 4) Time is the fourth dimension of life. Generation time is the pace of life. 5) There is, however, considerable variation not accounted for by the above scalings and existing theories. Much of this "unexplained" variation is due to natural selection on life history traits to adapt the biological times of generations to the clock times of geochronological environmental cycles. 7) Most work on biological scaling and metabolic ecology has focused on respiration rate. The emerging synthesis applies conceptual foundations of energetics and the EFP to shift the focus to production rate and generation time.

The ecology and evolution of the monito del monte, a relict species from the southern South America temperate forests

The arboreal marsupial monito del monte (genus Dromiciops, with two recognized species) is a paradigmatic mammal. It is the sole living representative of the order Microbiotheria, the ancestor lineage of Australian marsupials. Also, this marsupial is the unique frugivorous mammal in the temperate rainforest, being the main seed disperser of several endemic plants of this ecosystem, thus acting as keystone species. Dromiciops is also one of the few hibernating mammals in South America, spending half of the year in a physiological dormancy where metabolism is reduced to 10% of normal levels. This capacity to reduce energy expenditure in winter contrasts with the enormous energy turnover rate they experience in spring and summer. The unique life history strategies of this living Microbiotheria, characterized by an alternation of life in the slow and fast lanes, putatively represent ancestral traits that permitted these cold‐adapted mammals to survive in this environment. Here, we describe the ecological role of this emblematic marsupial, summarizing the ecophysiology of hibernation and sociality, updated phylogeographic relationships, reproductive cycle, trophic relationships, mutualisms, conservation, and threats. This marsupial shows high densities, despite presenting slow reproductive rates, a paradox explained by the unique characteristics of its three‐dimensional habitat. We finally suggest immediate actions to protect these species that may be threatened in the near future due to habitat destruction and climate change.

Landscape rules predict optimal superhighways for the first peopling of Sahul

Archaeological data and demographic modelling suggest that the peopling of Sahul required substantial populations, occurred rapidly within a few thousand years and encompassed environments ranging from hyper-arid deserts to temperate uplands and tropical rainforests. How this migration occurred and how humans responded to the physical environments they encountered have, however, remained largely speculative. By constructing a high-resolution digital elevation model for Sahul and coupling it with fine-scale viewshed analysis of landscape prominence, least-cost pedestrian travel modelling and high-performance computing, we create over 125 billion potential migratory pathways, whereby the most parsimonious routes traversed emerge. Our analysis revealed several major pathways-superhighways-transecting the continent, that we evaluated using archaeological data. These results suggest that the earliest Australian ancestors adopted a set of fundamental rules shaped by physiological capacity, attraction to visually prominent landscape features and freshwater distribution to maximize survival, even without previous experience of the landscapes they encountered.

Interspecies chimeric conditions affect the developmental rate of human pluripotent stem cells

Human pluripotent stem cells hold significant promise for regenerative medicine. However, long differentiation protocols and immature characteristics of stem cell-derived cell types remain challenges to the development of many therapeutic applications. In contrast to the slow differentiation of human stem cells in vitro that mirrors a nine-month gestation period, mouse stem cells develop according to a much faster three-week gestation timeline. Here, we tested if co-differentiation with mouse pluripotent stem cells could accelerate the differentiation speed of human embryonic stem cells. Following a six-week RNA-sequencing time course of neural differentiation, we identified 929 human genes that were upregulated earlier and 535 genes that exhibited earlier peaked expression profiles in chimeric cell cultures than in human cell cultures alone. Genes with accelerated upregulation were significantly enriched in Gene Ontology terms associated with neurogenesis, neuron differentiation and maturation, and synapse signaling. Moreover, chimeric mixed samples correlated with in utero human embryonic samples earlier than human cells alone, and acceleration was dose-dependent on human-mouse co-culture ratios. The altered gene expression patterns and developmental rates described in this report have implications for accelerating human stem cell differentiation and the use of interspecies chimeric embryos in developing human organs for transplantation.

Different solutions lead to similar life history traits across the great divides of the amniote tree of life

Amniote vertebrates share a suite of extra-embryonic membranes that distinguish them from anamniotes. Other than that, however, their reproductive characteristics could not be more different. They differ in basic ectothermic vs endothermic physiology, in that two clades evolved powered flight, and one clade evolved a protective shell. In terms of reproductive strategies, some produce eggs and others give birth to live young, at various degrees of development. Crucially, endotherms provide lengthy parental care, including thermal and food provisioning—whereas ectotherms seldom do. These differences could be expected to manifest themselves in major differences between clades in quantitative reproductive traits. We review the reproductive characteristics, and the distributions of brood sizes, breeding frequencies, offspring sizes and their derivatives (yearly fecundity and biomass production rates) of the four major amniote clades (mammals, birds, turtles and squamates), and several major subclades (birds: Palaeognathae, Galloanserae, Neoaves; mammals: Metatheria and Eutheria). While there are differences between these clades in some of these traits, they generally show similar ranges, distribution shapes and central tendencies across birds, placental mammals and squamates. Marsupials and turtles, however, differ in having smaller offspring, a strategy which subsequently influences other traits.

Basic considerations on seasonal breeding in mammals including their testing by comparing natural habitats and zoos

Seasonal reproduction is common in mammals. Whereas specific conditions triggering a seasonal response can only be identified in controlled experiments, large-scale comparisons of reproduction in natural habitats and zoos can advance knowledge for taxa unavailable for experimentation. We outline how such a comparison can identify species whose seasonal physiology is linked to photoperiodic triggers, and those whose perceived seasonality in the wild is the consequence of fluctuating resources without a photoperiodic trigger. This concept groups species into those that do not change their aseasonal pattern between natural habitats and zoos because they are not constrained by resources in the wild, those that do not change a seasonal pattern between natural habitats and zoos because they are triggered by photoperiod irrespective of resources, and those that change from a more seasonal pattern in the natural habitat to an aseasonal pattern in zoos because the zoo environment alleviates resource limitations experienced in the wild. We explain how detailed comparisons of mating season timing in both environments can provide clues whether a specific daylength or a specific number of days after an equinox or solstice is the likely phototrigger for a taxon. We outline relationships between life history strategies and seasonality, with special focus on relative shortening of gestation periods in more seasonal mammals. Irrespective of whether such shortening results from the adaptive value of fitting a reproductive cycle within one seasonal cycle (minimizing ‘lost opportunity’), or from benefits deriving from separating birth and mating (to optimize resource use, or to reduce infanticide), reproductive seasonality may emerge as a relevant driver of life history acceleration. Comparisons of data from natural habitats and zoos will facilitate testing some of the resulting hypotheses.

Monotocy and the evolution of plural breeding in mammals

In many mammals, breeding females are intolerant of each other and seldom associate closely but, in some, they aggregate in groups that vary in size, stability, and kinship structure. Aggregation frequently increases competition for food, and interspecific differences in female sociality among mammals are commonly attributed to contrasts in ecological parameters, including variation in activity timing, the distribution of resources, as well as the risk of predation. However, there is increasing indication that differences in female sociality are also associated with phylogenetic relationships and with contrasts in life-history parameters. We show here that evolutionary transitions from systems where breeding females usually occupy separate ranges ("singular breeding") to systems where breeding females usually aggregate ("plural breeding") have occurred more frequently in monotocous lineages where females produce single young than in polytocous ones where they produce litters. A likely explanation of this association is that competition between breeding females for resources is reduced where they produce single young and is more intense where they produce litters. Our findings reinforce evidence that variation in life-history parameters plays an important role in shaping the evolution of social behavior.

Quantitative Analysis of the Timing of Development of the Cerebellum and Precerebellar Nuclei in Monotremes, Metatherians, Rodents, and Humans

We have used a quantitative statistical approach to compare the pace of development in the cerebellum and precerebellar systems relative to body size in monotremes and metatherians with that in eutherians (rodents and humans). Embryos, fetuses, and early postnatal mammals were scored on whether key structural events had been reached in the development of the cerebellum itself (CC-corpus cerebelli; 10 milestones), or the pontine and inferior olivary precerebellar nuclear groups (PC; 4 milestones). We found that many early cerebellar and precerebellar milestones (e.g., formation of Purkinje cell layer and deep cerebellar nuclei) were reached at a smaller absolute body length in both metatherians and eutherians together, compared to monotremes. Some later milestones (e.g., formation of the external granular layer and primary fissuration) were reached at a smaller body length in metatherians than eutherians. When the analysis was performed with proportional body length expressed as a natural log-transformed ratio of length at birth, milestones were reached at a much smaller proportional body length in rodents and humans than in the metatherians or monotremes. The findings are consistent with the slower pace of metabolic activity and embryonic development in monotremes. They also indicate slightly advanced maturation of some early features of the cerebellum in some metatherians (i.e., early cerebellar development in dasyurids relative to body size), but do not support the notion of an accelerated development of the cerebellum to cope with the demands of early birth. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1998-2013, 2020. © 2019 American Association for Anatomy.

Prioritizing surveillance of Nipah virus in India

The 2018 outbreak of Nipah virus in Kerala, India, highlights the need for global surveillance of henipaviruses in bats, which are the reservoir hosts for this and other viruses. Nipah virus, an emerging paramyxovirus in the genus Henipavirus, causes severe disease and stuttering chains of transmission in humans and is considered a potential pandemic threat. In May 2018, an outbreak of Nipah virus began in Kerala, > 1800 km from the sites of previous outbreaks in eastern India in 2001 and 2007. Twenty-three people were infected and 21 people died (16 deaths and 18 cases were laboratory confirmed). Initial surveillance focused on insectivorous bats (Megaderma spasma), whereas follow-up surveys within Kerala found evidence of Nipah virus in fruit bats (Pteropus medius). P. medius is the confirmed host in Bangladesh and is now a confirmed host in India. However, other bat species may also serve as reservoir hosts of henipaviruses. To inform surveillance of Nipah virus in bats, we reviewed and analyzed the published records of Nipah virus surveillance globally. We applied a trait-based machine learning approach to a subset of species that occur in Asia, Australia, and Oceana. In addition to seven species in Kerala that were previously identified as Nipah virus seropositive, we identified at least four bat species that, on the basis of trait similarity with known Nipah virus-seropositive species, have a relatively high likelihood of exposure to Nipah or Nipah-like viruses in India. These machine-learning approaches provide the first step in the sequence of studies required to assess the risk of Nipah virus spillover in India. Nipah virus surveillance not only within Kerala but also elsewhere in India would benefit from a research pipeline that included surveys of known and predicted reservoirs for serological evidence of past infection with Nipah virus (or cross reacting henipaviruses). Serosurveys should then be followed by longitudinal spatial and temporal studies to detect shedding and isolate virus from species with evidence of infection. Ecological studies will then be required to understand the dynamics governing prevalence and shedding in bats and the contacts that could pose a risk to public health.

Critical factors for the recovery of marine mammals

{en} Over the past decades, much research has focused on understanding the critical factors for marine extinctions with the aim of preventing further species losses in the oceans. Although conservation and management strategies are enabling several species and populations to recover, others remain at low abundance levels or continue to decline. To understand these discrepancies, we used a published database on abundance trends of 137 populations of marine mammals worldwide and compiled data on 28 potentially critical factors for recovery. We then applied random forests and additive mixed models to determine which intrinsic and extrinsic factors are critical for the recovery of marine mammals. A mix of life-history characteristics, ecological traits, phylogenetic relatedness, population size, geographic range, human impacts, and management efforts explained why populations recovered or not. Consistently, species with lower age at maturity and intermediate habitat area were more likely to recover, which is consistent with life-history and ecological theory. Body size, trophic level, social interactions, dominant habitat, ocean basin, and habitat disturbance also explained some differences in recovery patterns. Overall, a variety of intrinsic and extrinsic factors were important for species' recovery, pointing to cumulative effects. Our results provide insight for improving conservation and management strategies to enhance recoveries in the future.

PHYSIOLOGY AND ENDOCRINOLOGY SYMPOSIUM: Postnatal reproductive development and the lactocrine hypothesis

Maternal effects on development can program cell fate and dictate offspring phenotype. Such effects do not end at birth, but extend into postnatal life through signals communicated from mother to offspring in first milk (colostrum). Transmission of bioactive factors from mother to offspring as a specific consequence of nursing defines a lactocrine mechanism. The female reproductive tract is not fully formed at birth (postnatal day = PND 0). Data for ungulates and mice indicate that disruption of development during neonatal life can have lasting effects on the form and function of uterine tissues. Uterine growth and histogenesis proceed in an ovary-independent manner shortly after birth, suggesting that extra-ovarian inputs are important in this process. Data for the pig indicate that lactocrine signals communicated within 12 to 48 h from birth constitute one source of such uterotrophic support. Disruption of lactocrine signaling, either naturally, by limited colostrum consumption, or experimentally, by milk replacer feeding, alters neonatal porcine uterine development and can have negative consequences for reproductive performance in adults. Substantial differences in endometrial and uterine gene expression between colostrum- and replacer-fed gilts were evident by PND 2, when RNA sequencing revealed over 800 differentially expressed, lactocrine-sensitive genes. Lactocrine-sensitive biological processes identified through transcriptomic studies and integrated microRNA-mRNA pathway analyses included those associated with both cell-cell and ESR1 signaling, and tissue development. Evidence for the pig indicates that colostrum consumption and lactocrine signaling are required to establish a normal uterine developmental program and optimal uterine developmental trajectory.

Geography of current and future global mammal extinction risk

Identifying which species are at greatest risk, what makes them vulnerable, and where they are distributed are central goals for conservation science. While knowledge of which factors influence extinction risk is increasingly available for some taxonomic groups, a deeper understanding of extinction correlates and the geography of risk remains lacking. Here, we develop a predictive random forest model using both geospatial and mammalian species' trait data to uncover the statistical and geographic distributions of extinction correlates. We also explore how this geography of risk may change under a rapidly warming climate. We found distinctive macroecological relationships between species-level risk and extinction correlates, including the intrinsic biological traits of geographic range size, body size and taxonomy, and extrinsic geographic settings such as seasonality, habitat type, land use and human population density. Each extinction correlate exhibited ranges of values that were especially associated with risk, and the importance of different risk factors was not geographically uniform across the globe. We also found that about 10% of mammals not currently recognized as at-risk have biological traits and occur in environments that predispose them towards extinction. Southeast Asia had the most actually and potentially threatened species, underscoring the urgent need for conservation in this region. Additionally, nearly 40% of currently threatened species were predicted to experience rapid climate change at 0.5 km/year or more. Biological and environmental correlates of mammalian extinction risk exhibit distinct statistical and geographic distributions. These results provide insight into species-level patterns and processes underlying geographic variation in extinction risk. They also offer guidance for future conservation research focused on specific geographic regions, or evaluating the degree to which species-level patterns mirror spatial variation in the pressures faced by populations within the ranges of individual species. The added impacts from climate change may increase the susceptibility of at-risk species to extinction and expand the regions where mammals are most vulnerable globally.

Birth mass is the key to understanding the negative correlation between lifespan and body size in dogs

Larger dog breeds live shorter than the smaller ones, opposite of the mass-lifespan relationship observed across mammalian species. Here we use data from 90 dog breeds and a theoretical model based on the first principles of energy conservation and life history tradeoffs to explain the negative correlation between longevity and body size in dogs. We found that the birth/adult mass ratio of dogs scales negatively with adult size, which is different than the weak interspecific scaling in mammals. Using the model, we show that this ratio, as an index of energy required for growth, is the key to understanding why the lifespan of dogs scales negatively with body size. The model also predicts that the difference in mass-specific lifetime metabolic energy usage between dog breeds is proportional to the difference in birth/adult mass ratio. Empirical data on lifespan, body mass, and metabolic scaling law of dogs strongly supports this prediction.

Generation time and fitness tradeoffs during the evolution of multicellularity

The evolution of multicellular organisms from their unicellular ancestors is an example of an evolutionary transition in individuality (ETI), i.e. a change in the units of selection and adaptation. The theory of ETIs poses particular challenges because, by definition, key theoretical constructs such as fitness are shifting during an ETI. Past work emphasized the importance of life history tradeoffs during ETIs in which lower level units form groups and become individuals at a higher level. In particular, it has been hypothesized that the convexity of the lower-level tradeoff between viability and fecundity changes with group size and determines the optimality of lower-level specialization in the fitness components of the group. This is important because lower-level specialization is a key indicator of higher-level individuality. Here we show that increasing generation time can increase the convexity of the lower-level viability-fecundity tradeoff. This effect is a novel hypothesis for the positive association between cell-group size and cellular specialization in a major model system for ETIs, the volvocine algae. The pattern in this clade is thought to be an example of a more general size-complexity rule. Our hypothesis is that larger groups have longer generation times and longer generation times lead to more convex lower-level viability-fecundity tradeoffs, which could account for specialization being optimal only in larger cell groups (colonies). We discuss the robustness of this effect to various changes in the assumptions of our model. Our work is important for the study of ETIs in general because viability and fecundity are fundamental components of fitness in all systems and because generation time is expected to be changing during many ETIs.

Undiscovered Bat Hosts of Filoviruses

Ebola and other filoviruses pose significant public health and conservation threats by causing high mortality in primates, including humans. Preventing future outbreaks of ebolavirus depends on identifying wildlife reservoirs, but extraordinarily high biodiversity of potential hosts in temporally dynamic environments of equatorial Africa contributes to sporadic, unpredictable outbreaks that have hampered efforts to identify wild reservoirs for nearly 40 years. Using a machine learning algorithm, generalized boosted regression, we characterize potential filovirus-positive bat species with estimated 87% accuracy. Our model produces two specific outputs with immediate utility for guiding filovirus surveillance in the wild. First, we report a profile of intrinsic traits that discriminates hosts from non-hosts, providing a biological caricature of a filovirus-positive bat species. This profile emphasizes traits describing adult and neonate body sizes and rates of reproductive fitness, as well as species’ geographic range overlap with regions of high mammalian diversity. Second, we identify several bat species ranked most likely to be filovirus-positive on the basis of intrinsic trait similarity with known filovirus-positive bats. New bat species predicted to be positive for filoviruses are widely distributed outside of equatorial Africa, with a majority of species overlapping in Southeast Asia. Taken together, these results spotlight several potential host species and geographical regions as high-probability targets for future filovirus surveillance.

Preventing future outbreaks of ebolaviruses in humans and other vulnerable animal populations will require identifying the natural reservoirs of filoviruses. Accumulating indirect evidence points to certain bat species as prime suspects. To guide the search for natural filovirus reservoirs, we mined intrinsic biological data on the world’s bat species to determine what features best predict filovirus hosts compared to bats at large. We report a suite of traits that distinguishes seropositive bat species from all others with an estimated 87% accuracy. We also identify several bat species not currently known to be filovirus hosts whose trait profiles indicate should be surveillance targets. Geographic regions where numerous potential filovirus hosts co-occur (potential filovirus hotspots) suggest that filovirus distribution and diversity may be greater than previously thought.

Allometric scaling of the elevation of maternal energy intake during lactation

Background

In most mammals, lactating mothers dramatically increase their food intake after parturition and reach a peak intake rate after a certain time while their offspring continue to grow. A common view, perpetuated by the metabolic theory of ecology, is that the allometric scaling of maternal metabolic rate with body mass limits the changes in energy intake and expenditure. Therefore these potential effects of metabolic scaling should be reflected in the elevation of maternal energy intake during lactation. To test this hypothesis, we collected published data on 24 species (13 domesticated) and established scaling relationships for several characteristics of the patterns of energy intake elevation (amplitude of the elevation, time to peak, and cumulative elevation to peak).

Results

A curvilinear allometric scaling relationship with maternal body mass (in double-logarithmic space) was found for the amplitude of maternal energy intake elevation, similarly to what has been observed for scaling relationships of basal metabolic rate in non-breeding mammals. This result indirectly supports the metabolic theory of ecology. However, this curvilinear allometric scaling does not seem to drive the scaling relationships found for the other characteristics of maternal energy intake. Both the duration and shape of the energy intake patterns showed substantial variation independently of species’ body mass.

Conclusions

Data available for a few mammals, mostly domesticated, provides little evidence for the hypothesis that a single law of metabolic scaling governs the elevation of maternal energy intake after parturition. Obtaining further food intake data in wild species will be crucial to unravel the general mechanisms underlying variation in this unique adaptation of mammalian females.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-016-0164-y) contains supplementary material, which is available to authorized users.

Allometric and temporal scaling of movement characteristics in Galapagos tortoises

Understanding how individual movement scales with body size is of fundamental importance in predicting ecological relationships for diverse species. One-dimensional movement metrics scale consistently with body size yet vary over different temporal scales. Knowing how temporal scale influences the relationship between animal body size and movement would better inform hypotheses about the efficiency of foraging behaviour, the ontogeny of energy budgets, and numerous life-history trade-offs. We investigated how the temporal scaling of allometric patterns in movement varies over the course of a year, specifically during periods of motivated (directional and fast movement) and unmotivated (stationary and tortuous movement) behaviour. We focused on a recently diverged group of species that displays wide variation in movement behaviour - giant Galapagos tortoises (Chelonoidis spp.) - to test how movement metrics estimated on a monthly basis scaled with body size. We used state-space modelling to estimate seven different movement metrics of Galapagos tortoises. We used log-log regression of the power law to evaluate allometric scaling for these movement metrics and contrasted relationships by species and sex. Allometric scaling of movement was more apparent during motivated periods of movement. During this period, allometry was revealed at multiple temporal intervals (hourly, daily and monthly), with values observed at daily and monthly intervals corresponding most closely to the expected one-fourth scaling coefficient, albeit with wide credible intervals. We further detected differences in the magnitude of scaling among taxa uncoupled from observed differences in the temporal structuring of their movement rates. Our results indicate that the definition of temporal scales is fundamental to the detection of allometry of movement and should be given more attention in movement studies. Our approach not only provides new conceptual insights into temporal attributes in one-dimensional scaling of movement, but also generates valuable insights into the movement ecology of iconic yet poorly understood Galapagos giant tortoises.

Rodent reservoirs of future zoonotic diseases

The increasing frequency of zoonotic disease events underscores a need to develop forecasting tools toward a more preemptive approach to outbreak investigation. We apply machine learning to data describing the traits and zoonotic pathogen diversity of the most speciose group of mammals, the rodents, which also comprise a disproportionate number of zoonotic disease reservoirs. Our models predict reservoir status in this group with over 90% accuracy, identifying species with high probabilities of harboring undiscovered zoonotic pathogens based on trait profiles that may serve as rules of thumb to distinguish reservoirs from nonreservoir species. Key predictors of zoonotic reservoirs include biogeographical properties, such as range size, as well as intrinsic host traits associated with lifetime reproductive output. Predicted hotspots of novel rodent reservoir diversity occur in the Middle East and Central Asia and the Midwestern United States.

Bioactive Functions of Milk Proteins: a Comparative Genomics Approach

The composition of milk includes factors required to provide appropriate nutrition for the growth of the neonate. However, it is now clear that milk has many functions and comprises bioactive molecules that play a central role in regulating developmental processes in the young while providing a protective function for both the suckled young and the mammary gland during the lactation cycle. Identifying these bioactives and their physiological function in eutherians can be difficult and requires extensive screening of milk components that may function to improve well-being and options for prevention and treatment of disease. New animal models with unique reproductive strategies are now becoming increasingly relevant to search for these factors.

Monotreme lactation protein is highly expressed in monotreme milk and provides antimicrobial protection

Monotremes (platypus and echidna) are the descendants of the oldest ancestor of all extant mammals distinguished from other mammals by mode of reproduction. Monotremes lay eggs following a short gestation period and after an even briefer incubation period, altricial hatchlings are nourished over a long lactation period with milk secreted by nipple-less mammary patches located on the female’s abdomen. Milk is the sole source of nutrition and immune protection for the developing young until weaning. Using transcriptome and mass spectrometry analysis of milk cells and milk proteins, respectively, a novel Monotreme Lactation Protein (MLP) was identified as a major secreted protein in milk. We show that platypus and short-beaked echidna MLP genes show significant homology and are unique to monotremes. The MLP transcript was shown to be expressed in a variety of tissues; however, highest expression was observed in milk cells and was expressed constitutively from early to late lactation. Analysis of recombinant MLP showed that it is an N-linked glycosylated protein and biophysical studies predicted that MLP is an amphipathic, α-helical protein, a typical feature of antimicrobial proteins. Functional analysis revealed MLP antibacterial activity against both opportunistic pathogenic Staphylococcus aureus and commensal Enterococcus faecalis bacteria but showed no effect on Escherichia coli, Pseudomonas aeruginosa, Staphylococcus epidermidis, and Salmonella enterica. Our data suggest that MLP is an evolutionarily ancient component of milk-mediated innate immunity absent in other mammals. We propose that MLP evolved specifically in the monotreme lineage supporting the evolution of lactation in these species to provide bacterial protection, at a time when mammals lacked nipples.

Basal metabolic rate and maternal energetic investment durations in mammals

Background

The Metabolic Theory of Ecology (MTE) predicts that gestation duration, lactation duration, and their sum, total development time, are constrained by mass-specific basal metabolic rate such that they should scale with body mass with an exponent of 0.25. However, tests of the MTE’s predictions have yielded mixed results. In an effort to resolve this uncertainty, we used phylogenetically-controlled regression to investigate the allometries of gestation duration, lactation duration, and total development time in four well-studied mammalian orders, Artiodactyla, Carnivora, Primates, and Rodentia.

Results

The results we obtained are not consistent with the predictions of the MTE. Gestation duration scaling exponents are below 0.25 in all four orders. The scaling exponent for lactation duration is below 0.25 in Carnivora and Rodentia, indistinguishable from 0.25 in Artiodactyls, and steeper than 0.25 in Primates. Total development time scales with body mass as predicted by the MTE in Primates, but not in artiodactyls, carnivores, and rodents. In the latter three orders, the exponent is 0.15.

Conclusions

Together, these results indicate that the influence of basal metabolic rate on mammalian maternal investment durations must be more complicated than the MTE envisages, and that other factors must play an important role. Future research needs to allow for the possibility that different factors drive gestation duration and lactation duration, and that the drivers of the two durations may differ among orders.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-014-0194-z) contains supplementary material, which is available to authorized users.

Dinosaur physiology. Evidence for mesothermy in dinosaurs

Were dinosaurs ectotherms or fast-metabolizing endotherms whose activities were unconstrained by temperature? To date, some of the strongest evidence for endothermy comes from the rapid growth rates derived from the analysis of fossil bones. However, these studies are constrained by a lack of comparative data and an appropriate energetic framework. Here we compile data on ontogenetic growth for extant and fossil vertebrates, including all major dinosaur clades. Using a metabolic scaling approach, we find that growth and metabolic rates follow theoretical predictions across clades, although some groups deviate. Moreover, when the effects of size and temperature are considered, dinosaur metabolic rates were intermediate to those of endotherms and ectotherms and closest to those of extant mesotherms. Our results suggest that the modern dichotomy of endothermic versus ectothermic is overly simplistic.

Revisiting non-offspring nursing: allonursing evolves when the costs are low

Allonursing, the nursing of another female's offspring, is commonly assumed to have evolved through the benefits of kin selection or reciprocity. The evolution of allonursing may also be influenced by variation in the possible costs to allonurses. The relative influence of costs and benefits on the incidence of allonursing in mammals remains unexplored. We show, using comparative analyses, that where females group with kin, the presence or the absence of allonursing is not associated with further variation in relatedness. Allonursing is most common where females produce litters; here the relative investment per offspring is low, and the costs of nursing additional young are likely to be reduced. Our results suggest that variation in the potential benefits is not associated with the distribution of allonursing, but that allonursing can quickly evolve when the costs to allonurses of nursing additional offspring are low.

How body mass and lifestyle affect juvenile biomass production in placental mammals

In mammals, the mass-specific rate of biomass production during gestation and lactation, here called maternal productivity, has been shown to vary with body size and lifestyle. Metabolic theory predicts that post-weaning growth of offspring, here termed juvenile productivity, should be higher than maternal productivity, and juveniles of smaller species should be more productive than those of larger species. Furthermore because juveniles generally have similar lifestyles to their mothers, across species juvenile and maternal productivities should be correlated. We evaluated these predictions with data from 270 species of placental mammals in 14 taxonomic/lifestyle groups. All three predictions were supported. Lagomorphs, perissodactyls and artiodactyls were very productive both as juveniles and as mothers as expected from the abundance and reliability of their foods. Primates and bats were unproductive as juveniles and as mothers, as expected as an indirect consequence of their low predation risk and consequent low mortality. Our results point the way to a mechanistic explanation for the suite of correlated life-history traits that has been called the slow-fast continuum.

Reproduction in the marsupial dibbler, Parantechinus apicalis; differences between island and mainland populations

Details of the reproductive endocrinology of the dibbler, Parantechinus apicalis, an endangered member of the Family Dasyuridae, are presented from two geographically-separated populations, living either on the mainland or on islands in Jurien Bay, Western Australia. Plasma free cortisol in males measured in the island population during 1998/9 did not differ between the breeding and non-breeding season, but during the March rut in 2000, when males died after breeding, free cortisol levels were significantly raised. Post-mating mortality in dibbler males is facultative, rather than obligatory and the cortisol data implicate the same physiological sequelae described in other dasyurids. In females, a single annual oestrus was recorded during late summer to autumn in both populations with an onset earlier by 12 days in the mainland animals. Faecal steroids excreted as progesterone metabolites (PM) and oestradiol-17β were measured during the annual oestrous period and showed significantly higher PM concentrations in island animals. Oestradiol, although raised, was not different between the two populations. A profile of PM levels throughout gestation revealed a small peak at the time of ovulation, followed by slowly rising levels to peak 8 days before birth, indicating slow development of the corpora lutea. Using collective data, the presumptive day of ovulation could be identified, allowing the calculation of a presumptive gestation length of 45days in dibblers from mainland populations. This gestation length compares with that of a related species, Pseudantechinus macdonnellensis, reported at 45-55 days. A surprising finding is the significantly shorter gestation period of approximately 38 days in island animals compared with those from the mainland. This and other differences between reproductive parameters of island and mainland populations are discussed in the context of the 'island syndrome'.

Evidence that gestation duration and lactation duration are coupled traits in primates

Gestation duration and lactation duration are usually treated as independently evolving traits in primates, but the metabolic theory of ecology (MTE) suggests both durations should be determined by metabolic rate. We used phylogenetic generalized least-squares linear regression to test these different perspectives. We found that the allometries of the durations are divergent from each other and different from the scaling exponent predicted by the MTE (0.25). Gestation duration increases much more slowly (0.06 < m < 0.12), and lactation duration much more quickly (0.36 < m < 0.52) with body mass than the MTE predicts. By contrast, we found that the combined duration of gestation and lactation is consistent with the MTE's predictions (0.22 < m < 0.35). These results suggest that gestation duration and lactation duration might best be viewed as distinct but coupled adaptations. When transferring energy to their offspring, primate mothers must meet metabolically dictated physiological requirements while optimizing the timing of the switch from gestation to lactation in relation to some as-yet-unidentified body-size-related factor.

The maximum rate of mammal evolution

How fast can a mammal evolve from the size of a mouse to the size of an elephant? Achieving such a large transformation calls for major biological reorganization. Thus, the speed at which this occurs has important implications for extensive faunal changes, including adaptive radiations and recovery from mass extinctions. To quantify the pace of large-scale evolution we developed a metric, clade maximum rate, which represents the maximum evolutionary rate of a trait within a clade. We applied this metric to body mass evolution in mammals over the last 70 million years, during which multiple large evolutionary transitions occurred in oceans and on continents and islands. Our computations suggest that it took a minimum of 1.6, 5.1, and 10 million generations for terrestrial mammal mass to increase 100-, and 1,000-, and 5,000-fold, respectively. Values for whales were down to half the length (i.e., 1.1, 3, and 5 million generations), perhaps due to the reduced mechanical constraints of living in an aquatic environment. When differences in generation time are considered, we find an exponential increase in maximum mammal body mass during the 35 million years following the Cretaceous-Paleogene (K-Pg) extinction event. Our results also indicate a basic asymmetry in macroevolution: very large decreases (such as extreme insular dwarfism) can happen at more than 10 times the rate of increases. Our findings allow more rigorous comparisons of microevolutionary and macroevolutionary patterns and processes.

Drivers and hotspots of extinction risk in marine mammals

The world's oceans are undergoing profound changes as a result of human activities. However, the consequences of escalating human impacts on marine mammal biodiversity remain poorly understood. The International Union for the Conservation of Nature (IUCN) identifies 25% of marine mammals as at risk of extinction, but the conservation status of nearly 40% of marine mammals remains unknown due to insufficient data. Predictive models of extinction risk are crucial to informing present and future conservation needs, yet such models have not been developed for marine mammals. In this paper, we: (i) used powerful machine-learning and spatial-modeling approaches to understand the intrinsic and extrinsic drivers of marine mammal extinction risk; (ii) used this information to predict risk across all marine mammals, including IUCN "Data Deficient" species; and (iii) conducted a spatially explicit assessment of these results to understand how risk is distributed across the world's oceans. Rate of offspring production was the most important predictor of risk. Additional predictors included taxonomic group, small geographic range area, and small social group size. Although the interaction of both intrinsic and extrinsic variables was important in predicting risk, overall, intrinsic traits were more important than extrinsic variables. In addition to the 32 species already on the IUCN Red List, our model identified 15 more species, suggesting that 37% of all marine mammals are at risk of extinction. Most at-risk species occur in coastal areas and in productive regions of the high seas. We identify 13 global hotspots of risk and show how they overlap with human impacts and Marine Protected Areas.

The cost of sex: quantifying energetic investment in gamete production by males and females

The relative energetic investment in reproduction between the sexes forms the basis of sexual selection and life history theories in evolutionary biology. It is often assumed that males invest considerably less in gametes than females, but quantifying the energetic cost of gamete production in both sexes has remained a difficult challenge. For a broad diversity of species (invertebrates, reptiles, amphibians, fishes, birds, and mammals), we compared the cost of gamete production between the sexes in terms of the investment in gonad tissue and the rate of gamete biomass production. Investment in gonad biomass was nearly proportional to body mass in both sexes, but gamete biomass production rate was approximately two to four orders of magnitude higher in females. In both males and females, gamete biomass production rate increased with organism mass as a power law, much like individual metabolic rate. This suggests that whole-organism energetics may act as a primary constraint on gamete production among species. Residual variation in sperm production rate was positively correlated with relative testes size. Together, these results suggest that understanding the heterogeneity in rates of gamete production among species requires joint consideration of the effects of gonad mass and metabolism.