Adaptive sex allocation in relation to life-history in the common brushtail possum, Trichosurus vulpecula

Richardson's ground squirrel litter size-sex ratio trade-off reveals conditional adaptive sex allocation

Trivers and Willard proposed that female mammals should adjust their investment in male versus female offspring relative to their ability to produce high-quality offspring. We tested whether litter size-sex ratio trade-offs predicted by Adaptive Sex Allocation (ASA) theory occur among Richardson's ground squirrel (Urocitellus richardsonii) dams over 10 distinct breeding years in a population where individuals experienced variability in food availability and habitat disruption. Litters of primiparous dams became increasingly female-biased with increasing litter size, but that trend waned among second litters born to dams, and reversed among third litters, with larger litters becoming more male-biased, suggesting that ASA is a product of interacting selection pressures. Trade-offs were not associated with habitat disruption, the availability of supplementary food, or dam age. An association between habitat disruption and male-biased sex ratios, the prevalence of litter size-sex ratio trade-offs and placental scar counts exceeding the number of juveniles at weaning in our population, but not in a geographically distinct population of conspecifics exposed to different environmental conditions reveal that the expression of ASA varies among populations and among years within populations, illustrating the conditional nature of ASA.

Different maternal investment strategies for male and female calves in a polygynous mammal

Maternal effects occur when the phenotype of the mother influences that of the young to the detriment of her survival, growth or fitness. The investment of the mother can be affected by maternal body condition and/or experience. Trivers-Willard Hypothesis (TWH) and Local Resource Competition Hypothesis (LRCH) are the main hypotheses used to explain bias in birth sex-ratios in mammals, as well as for sex-biased maternal investment. Both hypotheses suggest that a different amount of investment must be expected according to the sex of the young. However, recent studies suggest that these differences are not in quantity but in the strategies: mechanisms and objectives may differ for each sex. We studied how maternal characteristics (age, body mass, body condition, and dominance status) influence relevant aspects of the birth and early growth of the calf (birth date, birth body mass, body mass at weaning, and body condition at weaning) separately for each sex; and how that investment is mediated by milk production and composition (lactose, fat, and protein). One hundred eighty-eight newborns from 75 captive red deer hinds aged from 2 to 19 years were analyzed. The main differential investment observed was related to birth date: when producing a female, hinds give birth earlier in the season only if they have a good body condition; however, when gestating a male it is the older hinds those which deliver earlier. Subsequently, milk production and composition are correlated with birth body mass in female calves, but to weaning body mass in males. Thus, only hind body mass affects the weaning body mass of female calves, compared with age and hind body mass in males. These results suggest that while TWH fits the maternal investment strategy found for male calves, it is LRCH which correlates with the maternal investment patterns observed for females.

Kin and birth order effects on male child mortality: three East Asian populations, 1716-1945

Human child survival depends on adult investment, typically from parents. However, in spite of recent research advances on kin influence and birth order effects on human infant and child mortality, studies that directly examine the interaction of kin context and birth order on sibling differences in child mortality are still rare. Our study supplements this literature with new findings from large-scale individual-level panel data for three East Asian historical populations from northeast China (1789-1909), northeast Japan (1716-1870), and north Taiwan (1906-1945), where preference for sons and first-borns is common. We examine and compare male child mortality risks by presence/absence of co-resident parents, grandparents, and other kin, as well as their interaction effects with birth order. We apply discrete-time event-history analysis on over 172,000 observations of 69,125 boys aged 1-9 years old. We find that in all three populations, while the presence of parents is important for child survival, it is more beneficial to first/early-borns than to later-borns. Effects of other co-resident kin are however null or inconsistent between populations. Our findings underscore the importance of birth order in understanding how differential parental investment may produce child survival differentials between siblings.

Sex allocation and secondary sex ratio in Cuban boa (Chilabothrus angulifer): mother's body size affects the ratio between sons and daughters

Secondary sex ratios of animals with genetically determined sex may considerably deviate from equality. These deviations may be attributed to several proximate and ultimate factors. Sex ratio theory explains some of them as strategic decisions of mothers improving their fitness by selective investment in sons or daughters, e.g. local resource competition hypothesis (LRC) suggests that philopatric females tend to produce litters with male-biased sex ratios to avoid future competition with their daughters. Until now, only little attention has been paid to examine predictions of sex ratio theory in snakes possessing genetic sex determination and exhibiting large variance in allocation of maternal investment. Cuban boa is an endemic viviparous snake producing large-bodied newborns (∼200 g). Extremely high maternal investment in each offspring increases importance of sex allocation. In a captive colony, we collected breeding records of 42 mothers, 62 litters and 306 newborns and examined secondary sex ratios (SR) and sexual size dimorphism (SSD) of newborns. None of the examined morphometric traits of neonates appeared sexually dimorphic. The sex ratio was slightly male biased (174 males versus 132 females) and litter sex ratio significantly decreased with female snout-vent length. We interpret this relationship as an additional support for LRC as competition between mothers and daughters increases with similarity of body sizes between competing snakes.

Costs of Rearing the Wrong Sex: Cross-Fostering to Manipulate Offspring Sex in Tammar Wallabies

Sex allocation theory assumes that offspring sex (son vs. daughter) has consequences for maternal fitness. The most compelling experiment to test this theory would involve manipulating offspring sex and measuring the fitness consequences of having the "wrong" sex. Unfortunately, the logistical challenges of such an experiment limit its application. In tammar wallabies (Macropus eugenii), previous evidence suggests that mothers in good body condition are more likely to produce sons compared to mothers in poor condition, in support of the Trivers-Willard Hypothesis (TW) of condition-dependent sex allocation. More recently, we have found in our population of tammar wallabies that females with seemingly poor access to resources (based on condition loss over the dry summer) are more likely to produce sons, consistent with predictions from the Local Resource Competition (LRC) hypothesis, which proposes that production of sons or daughters is driven by the level of potential competition between mothers and philopatric daughters. We conducted a cross-fostering experiment in free-ranging tammar wallabies to disassociate the effects of rearing and birthing offspring of each sex. This allowed us to test the prediction of the LRC hypothesis that rearing daughters reduces the future direct fitness of mothers post-weaning and the prediction of the TW hypothesis that rearing sons requires more energy during lactation. Overall, we found limited costs to the mother of rearing the "wrong" sex, with switching of offspring sex only reducing the likelihood of a mother having a pouch young the following year. Thus, we found some support for both hypotheses in that rearing an unexpected son or an unexpected daughter both lead to reduced future maternal fitness. The study suggests that there may be context-specific costs associated with rearing the "wrong" sex.

Litter sex ratios in Richardson's ground squirrels: long-term data support random sex allocation and homeostasis

When costs of producing male versus female offspring differ, parents may vary allocation of resources between sons and daughters. We tested leading sex-allocation theories using an information-theoretic approach and Bayesian hierarchical models to analyse litter sex ratios (proportion males) at weaning for 1,049 litters over 24 years from a population of Richardson's ground squirrels (Urocitellus richardsonii), a polygynandrous, annually reproducing mammal in which litter size averages from six to seven offspring and sons are significantly heavier than daughters at birth and weaning. The model representing random Mendelian sex-chromosome assortment fit the data best; a homeostatic model received similar support but other models performed poorly. Embryo resorption was rare, and 5 years of litter data in a second population revealed no differences in litter size or litter sex ratio between birth and weaning, suggesting that litter size and sex ratio are determined in early pregnancy. Sex ratio did not vary with litter size at weaning in any of 29 years, and the observed distribution of sex ratios did not differ significantly from the binomial distribution for any litter size. For 1,580 weaned litters in the two populations, average sex ratio deviated from parity in only 3 of 29 years. Heavier females made a greater reproductive investment than lighter females, weaning larger and heavier litters composed of smaller sons and daughters, but litter sex ratio was positively related to maternal mass in only 2 of 29 years. Such occasional significant patterns emphasize the importance of multi-season studies in distinguishing infrequent events from normal patterns.

Brushtail possums (Trichosurus vulpecula) in metropolitan Sydney: population biology and response to Suprelorin contraceptive implants

The cohabitation of the common brushtail possum (Trichosurus vulpecula) with people in urban areas often causes conflict. Basic biological parameters are needed to evaluate potential new management options such as fertility control. This study investigated the biology of an urban brushtail possum population and the effects of Suprelorin contraceptive implants on individual females within that population. Trapping success remained constant over time, with 2.8 ± 0.2 individuals trapped per residential property. Recapture rates for males declined rapidly over time, with only 30% of males recaptured on the same property 12 months after initial capture (n = 30) and no males recaptured after 18 months (compared with female recapture rates of 58% after 18 months, n = 33). These data, combined with the preponderance of males observed within the lower age classes, suggests that male possum turnover is high within urban areas. This may be partially compensated for by the male-bias (1.8 : 1) observed in pouch young. Breeding was seasonal with the main peak of births in autumn, and a secondary smaller peak in spring. Suprelorin contraceptive treatment effectively inhibited reproduction in adult females for a minimum duration of 519 ± 7 (n = 5) and ≥700 ± 20 (n = 5) days after administration of one or two 4.7-mg implants, respectively, with no negative side-effects obvious. The concurrent collection of data on contraceptive efficacy and population-specific life-history parameters provides a unique opportunity to highlight the importance of understanding local population dynamics when evaluating the likely efficacy and implementation of fertility control programs to manage problem wildlife.

Reproductive ecology of wild tammar wallabies in natural and developed habitats on Garden Island, Western Australia

Reproduction may be influenced by major environmental changes experienced by an entire population as well as variation within a population in maternal resource availability or quality. We examined relationships between body condition and reproductive traits in two wild populations of the seasonally breeding tammar wallaby (Macropus eugenii) on Garden Island, Western Australia, that differed in access to supplemental food resources. Body condition changed predictably over the year, with females losing condition over the dry summer and gaining condition over the winters, when lactation occurs and most annual precipitation falls. Body condition influenced reproduction, with females of greater body condition more likely to have a pouch young, and to have a larger pouch young, early in the reproductive season. This intrapopulation pattern was opposite to that seen across populations – females in a native bushland were in poorer condition yet were more likely to have pouch young, and had larger pouch young, in March than did females living in a water-supplemented habitat on a naval base. Body condition did not influence the probability of weaning a pouch young in a year, nor did reproductive success influence changes in body condition across seasons. Instead, annual variation in precipitation had a dramatic effect on population weaning success in the native bushland, but not on the naval base. In summary, individual variation in body condition as well as large climatic variation influenced the reproduction of tammar wallabies on Garden Island, but the nature of the effect depended on the different habitats experienced by the two study populations.

Sex-biased parental investment is correlated with mate ornamentation in eastern bluebirds

Males typically have greater variance in reproductive success than females, so mothers should benefit by producing sons under favorable conditions. Being paired with a better-than-average mate is one such favorable circumstance. High-quality fathers can improve conditions for their offspring by providing good genes, good resources, or both, so females paired to such males should invest preferentially in sons. Ornamentation has been linked to male quality in many birds and, in support of differential allocation theory, females of several avian species invest more in entire broods when paired to attractive mates. Additionally, the females of some bird species apparently manipulate the primary sex-ratio of their broods in relation to the attractiveness of their mates. However, empirical support for a link between mate ornamentation and preferential feeding of sons (another form of biased investment) is lacking. We tested for correlations between sex-biased parental investment and mate plumage colour in the eastern bluebird (Sialia sialis), a species in which juveniles have sexually dichromatic UV-blue plumage. We found that the proportion of maternal feeding attempts to fledgling sons (versus fledgling daughters) was positively correlated with structurally coloured plumage ornamentation of fathers. Additionally, paternal feeding attempts to sons were correlated with plumage ornamentation of mothers and increased in fathers exhibiting breast plumage characteristics typical of older males. These results provide further support for the idea that parental strategies are influenced by mate attractiveness and provide the first evidence that mate ornamentation can influence parental behavior even after offspring have left the nest.

The role of host and environmental factors in the epidemiology of rumpwear in brushtail possums

Rumpwear is a common, but poorly defined and understood disease of brushtail possums (Trichosurus spp.) in Australia and New Zealand, resulting in varying degrees of fur loss. Populations of Trichosurus cunninghami (mountain brushtail possum) and T. vulpecula (common brushtail possum) were studied over two years at Boho South (Victoria), to describe the disease, investigate its prevalence and explore its epidemiology. The main type of lesion observed was coat damage in the lumbo-sacral region, and increased severity of rumpwear was related to decreased coat length and increased hair breakage. Significant skin lesions were rare and hair breakage patterns suggested mechanical damage. Rumpwear was very prevalent in both T. cunninghami and T. vulpecula, but very severe rumpwear was rare. There was no true seasonal pattern to the prevalence of rumpwear in T. cunninghami and sex and habitat did not affect prevalence or severity. Prevalence and severity of rumpwear did, however, increase with age. There were no significant relationships between the number of previous captures or body condition index with rumpwear.

Maternal effects in fish populations

Recently, the importance of the female to population dynamics-especially her non-genetic contribution to offspring fitness or maternal effect-has received much attention in studies of a diverse collection of animal and plant taxa. Of particular interest to fisheries scientists and managers is the role of the demographic structure of the adult component of fish populations in the formation of future year classes. Traditionally, fisheries managers tended to assess whole populations without regard to variation between the individuals within the population. In doing so, they overlooked the variation in spawning production between individual females as a source of variation to recruitment magnitude and fluctuation. Indeed, intensive and/or selective harvesting of larger and older females, those that may produce more-and higher quality-offspring, has been implicated in the collapse of a number of important fish stocks. In a fisheries resource management context, whether capture fisheries or aquaculture, female demographics and inter-female differences warrant serious consideration in developing harvesting and breeding strategies, and in understanding general population dynamics. Here I review the range of female traits and environmental conditions females encounter which may influence the number or quality of their offspring via a maternal effect.

Terminal reproductive effort in a marsupial

Life-history theory predicts that as organisms approach the end of their life, they should increase their reproductive effort (RE). However, studies on mammals often find that measures of RE do not vary with maternal age. This might be because offspring have some control over energy transfer which may constrain adaptive variation in RE by mothers, particularly in eutherian mammals where placental function is primarily controlled by offspring. However, in marsupials, energy transfer is primarily by lactation and under maternal control, leaving marsupial mothers free to vary RE. Here, we provide the first analysis, to our knowledge, of age-specific RE in a marsupial, the common brushtail possum. RE, measured as the proportion of maternal mass lost during lactation, was strongly correlated with offspring mass as a yearling. Older females had higher RE, gave birth earlier in the season and were more likely to produce two offspring in a year. Females with high RE in one year were lighter at the beginning of the next breeding season. These results provide the clearest support yet for terminal RE in a mammal.

Maternal condition and facultative sex ratios in populations with overlapping generations

Facultative investment in offspring sex is related to maternal condition in many organisms. In mammals, empirical support for condition-dependent sex allocation is equivocal, and there is some doubt as to theoretical expectations. Much theory has been developed to make predictions for condition-dependent sex ratios in populations with discrete generations. However, the extension of these predictions to populations with overlapping generations (OLGs; e.g., mammals) has been limited, leaving doubt as to the specific prediction for maternal-condition-dependent sex ratios in mammals. We develop a population genetics model that incorporates maternal effects on multiple offspring fitness components in a population with OLGs. Using a rare-gene and evolutionarily stable strategy approach, we demonstrate that sex ratio predictions of this model are identical to those for equivalent discrete generations models. We show that the predicted sex ratios depend on the sex-specific ratio of R(o) (offspring lifetime fitness) for offspring of good and poor mothers. This offspring lifetime fitness rule indicates that empirical research on conditional sex ratios should consider all three components of offspring R(o) (juvenile survival, adult life span, and fertility).

Sexual dimorphism in a marsupial: seasonal and lifetime differences in sex-specific mass

Whereas sexual dimorphism in body mass is common in many vertebrates, the developmental mechanisms leading to diverging growth patterns between the sexes are poorly understood in mammals. In the present study, I investigate seasonal and lifetime differences in sex-specific mass in a marsupial, the common brushtail possum (Trichosurus vulpecula). From the age of 2 years, males were heavier than females, but significant dimorphism was evident only at the age of 4 years. Annual mass gain was greater in males than in females from 2 to 4 years of age. The body mass of both sexes fluctuated throughout the year, although patterns differed: females lost mass during lactation in winter, whereas males lost mass during the breeding season in later summer and autumn. The mass of a juvenile predicted its mass as an adult for males, but not females. Male-biased sexual dimorphism in mass of possums appears to arise through a combination of a greater rate and duration of mass gain in young males, and the diversion of somatic reserves away from growth and into reproduction in females.

Life history and demographics of an island possum

Knowledge of life history and demography is essential for effective management of target species. Here, I describe life history and demographic traits of the common brushtail possum (Trichosurus vulpecula) on Magnetic Island, north Queensland. Data were collected during monthly live-trapping sessions over three years (2001–2004). Density was ~100% higher than reported in most Australian populations, at 5 possums per hectare; losses from the population were more than compensated for by births and immigration. Age-specific survival differed between the sexes: male survival declined earlier and faster than female survival. Males were significantly heavier than females as adults and sexual dimorphism appears to arise through a faster growth rate in males following weaning. Age at maturity in females varied from 1 to 3 years; 61% of females produced their first young at age 2. Males became sexually mature at age 3. Breeding was seasonal, with >50% births occurring in April–May; a smaller birth peak in spring was due to some females producing a second young after they had successfully weaned a first. The Magnetic Island possum population appears to be more similar to New Zealand populations, in terms of their life history and demographics, than to other previously studied mainland Australian populations.