Marsupial position on life-history continua and the potential contribution of life-history traits to population growth

Previous studies have suggested that mammal life history varies along the fast-slow continuum and that, in eutherians, this continuum is linked to variation in the potential contribution of survival and reproduction to population growth rate (λ). Fast eutherians mature early, have large litters and short lifespans, and exhibit high potential contribution of age at first reproduction and fertility to λ, while slow eutherians show high potential contribution of survival to λ. However, marsupials have typically been overlooked in comparative tests of mammalian life-history evolution. Here, we tested whether the eutherian life-history pattern extends to marsupials, and show that marsupial life-history trade-offs are organized along two major axes: (i) the reproductive output and dispersion axis, and (ii) the fast-slow continuum, with an additional association between adult survival and body mass. Life-history traits that potentially drive changes in λ are similar in eutherians and marsupials with slow life histories, but differ in fast marsupials; age at first reproduction is the most important trait contributing to λ and fertility contributes little. Marsupials have slower life histories than eutherians, and differences between these clades may derive from their contrasting reproductive modes; marsupials have slower development, growth and metabolism than eutherians of equivalent size.

What are the main drivers of survival and recruitment in tropical forest marsupials? A 16-year study

Tropical forest marsupials exhibit large interannual variation in population sizes, with direct negative density dependence capturing the essential features of their dynamics. However, the demographic mechanisms underlying population growth rate and driving both survival and reproduction are still unclear. We used a 16-year capture-mark-recapture data set for five tropical forest marsupials to test for seasonal and interannual density dependence in survival and recruitment. Hypotheses regarding the effects of exogenous (rainfall and minimum temperature) factors on survival, recruitment, and reproductive parameters (fecundity, litter size, and proportion of reproductive females) were also tested. Population size negatively affected survival in three of five species. High population sizes in a given year reduced survival rates in the following year, with strong detrimental effects on males. Recruitment and proportion of reproductive females were highly dependent on weather variables, and were not affected by previous population sizes (except for Metachirus nudicaudatus). Fecundity (number of female offspring/female) was related negatively to population size only in the black-eared opossum (Didelphis aurita), while litter size was a relatively conservative parameter, largely independent of external conditions. Our analyses indicate that density-dependent survival is the mechanism that regulates population size of tropical forest marsupials, either through a reduction in survival or an increase in emigration rates. This general regulatory mechanism may be common to other marsupials in the Atlantic Forest and other tropical forests.

Marsupiais de florestas tropicais exibem grande variação interanual nos tamanhos populacionais, com dependência negativa e direta da densidade capturando a essência de sua dinâmica populacional. No entanto, os mecanismos demográficos subjacentes à taxa de crescimento populacional e determinantes da sobrevivência e reprodução ainda são incertos. Nós usamos 16 anos de dados de captura-marcação-recaptura de cinco espécies de marsupiais de florestas tropicais para avaliar a dependência de densidade sazonal e interanual na sobrevivência e recrutamento. Hipóteses sobre efeitos de fatores exógenos (pluviosidade e temperatura mínima) na sobrevivência, recrutamento e parâmetros reprodutivos (fecundidade, tamanho da ninhada e proporção de fêmeas reprodutivas) também foram testadas. O tamanho da população afetou negativamente a sobrevivência em três das cinco espécies. O tamanho populacional elevado em um ano reduziu as taxas de sobrevivência no ano seguinte, com efeitos mais negativos nos machos. Recrutamento e proporção de fêmeas reprodutivas foram dependentes das variáveis climáticas e não foram afetados pelos tamanhos populacionais anteriores (com exceção de Metachirus nudicaudatus). A fecundidade (número de filhotes fêmea/fêmea) foi relacionada negativamente ao tamanho da população do gambá-de-orelha-preta (Didelphis aurita), enquanto o tamanho da ninhada foi um parâmetro relativamente conservador e independente das condições externas. Nossas análises indicam que a sobrevivência dependente da densidade regula as populações de marsupiais em florestas tropicais, seja através da redução na sobrevivência ou no aumento da emigração. Esse mecanismo regulatório pode ser comum a outros marsupiais na Mata Atlântica e outras florestas tropicais.

The impact of mating systems and dispersal on fine-scale genetic structure at maternally, paternally and biparentally inherited markers

For decades, studies have focused on how dispersal and mating systems influence genetic structure across populations or social groups. However, we still lack a thorough understanding of how these processes and their interaction shape spatial genetic patterns over a finer scale (tens-hundreds of metres). Using uniparentally inherited markers may help answer these questions, yet their potential has not been fully explored. Here, we use individual-level simulations to investigate the effects of dispersal and mating system on fine-scale genetic structure at autosomal, mitochondrial and Y chromosome markers. Using genetic spatial autocorrelation analysis, we found that dispersal was the major driver of fine-scale genetic structure across maternally, paternally and biparentally inherited markers. However, when dispersal was restricted (mean distance = 100 m), variation in mating behaviour created strong differences in the comparative level of structure detected at maternally and paternally inherited markers. Promiscuity reduced spatial genetic structure at Y chromosome loci (relative to monogamy), whereas structure increased under polygyny. In contrast, mitochondrial and autosomal markers were robust to differences in the specific mating system, although genetic structure increased across all markers when reproductive success was skewed towards fewer individuals. Comparing males and females at Y chromosome vs. mitochondrial markers, respectively, revealed that some mating systems can generate similar patterns to those expected under sex-biased dispersal. This demonstrates the need for caution when inferring ecological and behavioural processes from genetic results. Comparing patterns between the sexes, across a range of marker types, may help us tease apart the processes shaping fine-scale genetic structure.

Novel Consequences of Bird Pollination for Plant Mating

Pollinator behaviour has profound effects on plant mating. Pollinators are predicted to minimise energetic costs during foraging bouts by moving between nearby flowers. However, a review of plant mating system studies reveals a mismatch between behavioural predictions and pollen-mediated gene dispersal in bird-pollinated plants. Paternal diversity of these plants is twice that of plants pollinated solely by insects. Comparison with the behaviour of other pollinator groups suggests that birds promote pollen dispersal through a combination of high mobility, limited grooming, and intra- and interspecies aggression. Future opportunities to test these predictions include seed paternity assignment following pollinator exclusion experiments, single pollen grain genotyping, new tracking technologies for small pollinators, and motion-triggered cameras and ethological experimentation for quantifying pollinator behaviour.

Inbreeding avoidance, patch isolation and matrix permeability influence dispersal and settlement choices by male agile antechinus in a fragmented landscape

Animal dispersal is highly non-random and has important implications for the dynamics of populations in fragmented habitat. We identified interpatch dispersal events from genetic tagging, parentage analyses and assignment tests and modelled the factors associated with apparent emigration and post-dispersal settlement choices by individual male agile antechinus (Antechinus agilis, a marsupial carnivore of south-east Australian forests). Emigration decisions were best modelled with on data patch isolation and inbreeding risk. The choice of dispersal destination by males was influenced by inbreeding risk, female abundance, patch size, patch quality and matrix permeability (variation in land cover). Males were less likely to settle in patches without highly unrelated females. Our findings highlight the importance of individual-level dispersal data for understanding how multiple processes drive non-randomness in dispersal in modified landscapes. Fragmented landscapes present novel environmental, demographic and genetic contexts in which dispersal decisions are made, so the major factors affecting dispersal decisions in fragmented habitat may differ considerably from unfragmented landscapes. We show that the spatial scale of genetic neighbourhoods can be large in fragmented habitat, such that dispersing males can potentially settle in the presence of genetically similar females after moving considerable distances, thereby necessitating both a choice to emigrate and a choice of where to settle to avoid inbreeding.

Costs of reproduction and terminal investment by females in a semelparous marsupial

Evolutionary explanations for life history diversity are based on the idea of costs of reproduction, particularly on the concept of a trade-off between age-specific reproduction and parental survival, and between expenditure on current and future offspring. Such trade-offs are often difficult to detect in population studies of wild mammals. Terminal investment theory predicts that reproductive effort by older parents should increase, because individual offspring become more valuable to parents as the conflict between current versus potential future offspring declines with age. In order to demonstrate this phenomenon in females, there must be an increase in maternal expenditure on offspring with age, imposing a fitness cost on the mother. Clear evidence of both the expenditure and fitness cost components has rarely been found. In this study, we quantify costs of reproduction throughout the lifespan of female antechinuses. Antechinuses are nocturnal, insectivorous, forest-dwelling small (20–40 g) marsupials, which nest in tree hollows. They have a single synchronized mating season of around three weeks, which occurs on predictable dates each year in a population. Females produce only one litter per year. Unlike almost all other mammals, all males, and in the smaller species, most females are semelparous. We show that increased allocation to current reproduction reduces maternal survival, and that offspring growth and survival in the first breeding season is traded-off with performance of the second litter in iteroparous females. In iteroparous females, increased allocation to second litters is associated with severe weight loss in late lactation and post-lactation death of mothers, but increased offspring growth in late lactation and survival to weaning. These findings are consistent with terminal investment. Iteroparity did not increase lifetime reproductive success, indicating that terminal investment in the first breeding season at the expense of maternal survival (i.e. semelparity) is likely to be advantageous for females.

Mammalian hair as an accumulative bioindicator of metal bioavailability in Australian terrestrial environments

The current study represents the first investigation of the suitability of marsupial and eutherian mammalian hair as indicator tissue for metal exposure and accumulation within contaminated Australian terrestrial ecosystems. A soil metal contamination gradient was established across 22 sites at increasing distances from a decommissioned Lead/Zinc smelter in NSW, Australia. Within each site, soil and small mammal populations were sampled. An Australian native marsupial, the insectivorous Brown Antechinus, Antechinus stuartii: Dasyuridae, and introduced rodents, the omnivorous Brown or Norway Rat, Rattus norvegicus: Muridae and the Black Rat, Rattus rattus: Muridae were assessed for hair concentrations of Cadmium (Cd), Copper (Cu), Lead (Pb) and Zinc (Zn). Metals in soil were most elevated at sites within close proximity to the smelter, with soil metal concentrations decreasing with distance from the smelter. The non-essential metals Pb and Cd were accumulated in hair, both metals exhibiting positive linear relationships with environmental exposure (soil metal concentrations). When the variables of weight and snout-vent length were considered, no further contribution in terms of explaining the variability in hair Cd or Pb was observed for all species examined. The essential metals Cu and Zn were regulated in hair, remaining similar across the metal contamination gradient. A significant negative correlation between snout-vent length and hair Cu concentration was found for the Brown Rat; greater hair Cu concentrations were found in smaller individuals of this species. Accumulation of Pb to hair was similar among species while concentrations of Cd in Brown Rat hair were higher than both Black Rat and Brown Antechinus hair. As each of the three aforementioned species exhibit similar bioaccumulation relationships for Pb, we suggest that sampling hair from introduced rodents (pest species) may provide a suitable proxy for the assessment of Pb bioavailability for a range of small mammals within Australian urban remnants.

Natal dispersal and social organization of the swamp antechinus (Antechinus minimus) in a high-density island population

Radiotelemetry, mark–recapture trapping, and microsatellite analysis of genetic variation among three subpopulations were used to investigate the natal dispersal patterns, genetic structure, and social organization within a high-density island population of an insectivorous marsupial, the swamp antechinus ( Antechinus minimus (É. Geoffroy Saint-Hilaire, 1803)). Both demographic and genetic data indicated a high degree of philopatry for both sexes. Associated with high philopatry is low gene flow, often leading to high population genetic structuring. However, there was only weak evidence to support this; allele frequencies and genotype composition of one population tended to be different from the other two. Interestingly, timing of breeding of this subpopulation was delayed compared with the other two subpopulations. The philopatry of both sexes and the apparent lack of kin avoidance behaviour could lead to inbreeding. However, no apparent inbreeding effects were observed and offspring survival was high compared with mainland populations. The fact that male home-range size increased during the breeding season, overlapping with several females, and single females were found nesting with different males at the time of mating (and vice versa) indicates a promiscuous mating system. Potentially, this may reduce inbreeding to some extent. Alternatively, inbreeding may not be purposefully avoided, potentially leading to purging of detrimental alleles, thereby reducing their damaging effects on inbreeding.

Dispersal and the influence of rainfall on a population of the carnivorous marsupial swamp antechinus (Antechinus minimus maritimus)

Although there is evidence that recent climatic changes have had significant impacts on a wide range of species in the Northern Hemisphere, the influence of climate change, particularly drought, on Australian native small mammal species is poorly understood. In this study we investigated dispersal patterns and the influence of rainfall on the swamp antechinus (Antechinus minimus maritimus). Peak abundance occurred after the highest total annual rainfall for two decades, in 2001. A year later the population had declined to 10% of the peak. Birth dates appeared to occur three weeks earlier following a year of high rainfall. The dispersal of nine litters of pouch young (n = 62) was assessed following two breeding seasons. Young males remained on the natal site until December–January and dispersed before the breeding season. New males entered the population between January and June. More than 50% of females were residents and remained on the site to breed; the remaining females were trapped only once. After the male die-off the movements of pregnant females increased and they appeared to expand their home ranges. A. minimus exhibits philopatry of females and dispersal of males, as observed in other Antechinus species but dispersal occurs 2–3 months after weaning. This contrasts with juveniles of other Antechinus species that disperse abruptly after weaning. This study provides evidence that precipitation does have a major effect on the abundance of dasyurid species, making them vulnerable to drought and local or regional extinctions, particularly in areas of fragmented habitat and drying climates.

Sex, season and melatonin administration affects daily activity rhythms in a marsupial, the brown antechinus, Antechinus stuartii

The carnivorous marsupial Antechinus stuartii relies on photoperiodic changes to time reproductive activities, including behaviour, in spring. Similar to other mammals, the administration of the hormone melatonin is known to affect the synchronisation of reproduction in A. stuartii. The present study sought to explore the alterations in locomotor activity from the winter solstice in both males (body mass 35 g) and females (body mass 20 g) as a result of the influences of the changes in the natural photocycle and also of melatonin administration while under the natural photocycle. The total daily activity was found to differ between sexes, with males more active than females, irrespective of melatonin or control treatments. Daily activity patterns were significantly different between male groups but not female treatment groups. Activity patterns were also found to differ between males and females. The significance of these differences is discussed with relation to the profound physiological differences between the sexes, in this mammal where an irreversible stress response is part of the complete post-mating mortality of all males, but not females.

Genetic reconstruction of the population dynamics of a carnivorous marsupial (Antechinus flavipes) in response to floods

Human activities such as regulating river flows, logging and removing fallen timber adversely affect floodplain ecosystems around the world. Studies of the dynamics of floodplain-dwelling populations will help to understand the effects of altered flood regimes and to manage and restore floodplains. The yellow-footed antechinus (Antechinus flavipes) is the only small, native, carnivorous mammal (Marsupialia) on many degraded floodplains in south-eastern Australia, where its abundance appears to increase with proximity to floods, which is partly due to enhanced survival (as inferred from increased abundance of second-year females). We analysed population genetic patterns and maternity among samples collected following the period of postnatal dispersal, in the years preceding and following planned floods, at different distances from flood locations along the Murray River. Our genic and genotypic analyses of mitochondrial DNA (mtDNA) control region haplotypes and 11 microsatellite loci demonstrated high immigration rates into sites in close proximity to floods. All sampled males emigrated from their natal sites to points of capture, while some females were philopatric. There were high rates of dispersal of males among all sites within a partially flooded forest, while females dispersed more to locations closest to inundations rather than to distant places. These results suggest that environmental flows are beneficial to antechinus both by enhancing adult survival and promoting dispersal of females.

Variation in ectoparasite infestation on the brown antechinus, Antechinus stuartii, with regard to host, habitat and environmental parameters

We studied ectoparasite infestation patterns in a wild population of brown antechinuses, Antechinus stuartii (Dasyuridae) at three sites in south-eastern New South Wales, over a period of seven months. Ectoparasite diversity on brown antechinuses was particularly high for a small mammal: 18 species of ectoparasites were recorded, including one undescribed species of mite. Ectoparasite infestation varied between study sites and changed seasonally. Variation between sites was related to habitat and environmental parameters, and the highest ectoparasite infestations were encountered in the coolest habitat with greatest depth of leaf litter. Host population densities varied only slightly between sites, and host density was not considered to be a primary factor influencing ectoparasite infestation. This is the first study to provide a host record of ectoparasites of Antechinus stuartii sensu stricto.

Post-mating sexual selection increases lifetime fitness of polyandrous females in the wild

Females often mate with several males before producing offspring. Field studies of vertebrates suggest, and laboratory experiments on invertebrates confirm, that even when males provide no material benefits, polyandry can enhance offspring survival. This enhancement is widely attributed to genetic benefits that arise whenever paternity is biased towards males that sire more viable offspring. Field studies suggest that post-mating sexual selection biases fertilization towards genetically more compatible males and one controlled experiment has shown that, when females mate with close kin, polyandry reduces the relative number of inbred offspring. Another potential genetic benefit of polyandry is that it increases offspring survival because males with more competitive ejaculates sire more viable offspring. Surprisingly, however, there is no unequivocal evidence for this process. Here, by experimentally assigning mates to females, we show that polyandry greatly increases offspring survival in the Australian marsupial Antechinus stuartii. DNA profiling shows that males that gain high paternity under sperm competition sire offspring that are more viable. This beneficial effect occurs in both the laboratory and the wild. Crucially, there are no confounding non-genetic maternal effects that could arise if polyandry increases female investment in a particular reproductive event because A. stuartii is effectively semelparous. Our results therefore show that polyandry improves female lifetime fitness in nature. The threefold increase in offspring survival is not negated by a decline in maternal lifespan and is too large to be offset by an equivalent decline in the reproductive performance of surviving offspring.