Biotic resistance to invasion is ubiquitous across ecosystems of the United States

The biotic resistance hypothesis predicts that diverse native communities are more resistant to invasion. However, past studies vary in their support for this hypothesis due to an apparent contradiction between experimental studies, which support biotic resistance, and observational studies, which find that native and non‐native species richness are positively related at broad scales (small‐scale studies are more variable). Here, we present a novel analysis of the biotic resistance hypothesis using 24 456 observations of plant richness spanning four community types and seven ecoregions of the United States. Non‐native plant occurrence was negatively related to native plant richness across all community types and ecoregions, although the strength of biotic resistance varied across different ecological, anthropogenic and climatic contexts. Our results strongly support the biotic resistance hypothesis, thus reconciling differences between experimental and observational studies and providing evidence for the shared benefits between invasive species management and native biodiversity conservation.     

Two hypotheses on the causes of male homosexuality and paedophilia

This note considers two hypotheses on the causes of homosexuality and paedophilia in men, viz. the hypotheses of maternal immunity and of postnatal learning. According to the maternal immune hypothesis, there is progressive immunization of some mothers to male-specific antigens by each succeeding male fetus, and there are concomitantly increasing effects of anti-male antibodies on the sexual differentiation of the brain in each succeeding male fetus. An attempt is made to assess the status of this hypothesis within immunology. Knowledge of the properties of anti-male antibodies is meagre and there has been little direct experimentation on them, let alone on their effects on the developing male fetal brain. Moreover until the relevant antigens are identified, it will not be possible to test mothers of male homosexuals or paedophiles for the presence of such antibodies. Yet until this experimentation has been done, it would seem premature to regard the hypothesis as more than a very provisional explanatory tool. The evidence in relation to the postnatal learning hypothesis is quite different. There is an abundance of data suggesting that male homosexuals and paedophiles report having experienced more sexual abuse (however defined) in childhood (CSA) than do heterosexual controls. The question revolves round the interpretation of these data. Many (though not all) of these studies are correlational and thus subject to the usual qualifications concerning such data. However, there are grounds for supposing that some of the reports are veridical, and there is support from a longitudinal study reporting a small but significant increase in paedophilia in adulthood following CSA. To summarize: most boys who experience CSA do not later develop into homosexuals or paedophiles. However, the available evidence suggests that a few do so as a result of the abuse.     

The cause(s) of the fraternal birth order effect in male homosexuality

It has been established that the probability that a man is homosexual is positively related to his number of older brothers, but not older sisters when the brothers are accounted for. This is known as the 'fraternal birth order' effect. In the past, efforts have been made to explain this phenomenon in terms of several alternative biological hypotheses and a psychosocial hypothesis. This note examines how well these hypotheses accommodate the fraternal birth order effect. It is concluded that: (1) the evidence for the hypothesis of maternal immunoreactivity to the male fetus is weak; (2) the evidence for the intrauterine hormone exposure hypothesis is also weak; (3) the evidence for the hypothesis of postnatal learning is stronger. Lastly, there seem likely to be causes common to male homosexuality and paedophilia. They may include sexual (or quasi-sexual) experience in childhood or adolescence.     

Vitamin E and heart disease: basic science to clinical intervention trials

A review is presented of studies on the effects of vitamin E on heart disease, studies encompassing basic science, animal studies, epidemiological and observational studies, and four intervention trials. The in vitro, cellular, and animal studies, which are impressive both in quantity and quality, leave no doubt that vitamin E, the most important fat-soluble antioxidant, protects animals against a variety of types of oxidative stress. The hypothesis that links vitamin E to the prevention of cardiovascular disease (CVD) postulates that the oxidation of unsaturated lipids in the low-density lipoprotein (LDL) particle initiates a complex sequence of events that leads to the development of atherosclerotic plaque. This hypothesis is supported by numerous studies in vitro, in animals, and in humans. There is some evidence that the ex vivo oxidizability of a subject's LDL is predictive of future heart events. This background in basic science and observational studies, coupled with the safety of vitamin E, led to the initiation of clinical intervention trials. The three trials that have been reported in detail are, on balance, supportive of the proposal that supplemental vitamin E can reduce the risk for heart disease, and the fourth trial, which has just been reported, showed small, but not statistically significant, benefits. Subgroup analyses of cohorts from the older three trials, as well as evidence from smaller trials, indicate that vitamin E provides protection against a number of medical conditions, including some that are indicative of atherosclerosis (such as intermittent claudication). Vitamin E supplementation also produces an improvement in the immune system and protection against diseases other than cardiovascular disease (such as prostate cancer). Vitamin E at the supplemental levels being used in the current trials, 100 to 800 IU/d, is safe, and there is little likelihood that increased risk will be found for those taking supplements. About one half of American cardiologists take supplemental vitamin E, about the same number as take aspirin. In fact, one study suggests that aspirin plus vitamin E is more effective than aspirin alone. There are a substantial number of trials involving vitamin E that are in progress. However, it is possible, or even likely, that each condition for which vitamin E provides benefit will have a unique dose-effect curve. Furthermore, different antioxidants appear to act synergistically, so supplementation with vitamin E might be more effective if combined with other micronutrients. It will be extremely difficult to do trials that adequately probe the dose-effect curve for vitamin E for each condition that it might affect, or to do studies of all the possible combinations of other micronutrients that might act with vitamin E to improve its effectiveness. Therefore, the scientific community must recognize that there never will be a time when the science is "complete." At some point, the weight of the scientific evidence must be judged adequate; although some may regard it as early to that judgement now, clearly we are very close. In view of the very low risk of reasonable supplementation with vitamin E, and the difficulty in obtaining more than about 30 IU/day from a balanced diet, some supplementation appears prudent now.     

The enemy release hypothesis as a hierarchy of hypotheses

In ecology, a hypothesis is usually not discarded if a few studies reject it, as long as there are other studies supporting it. How to assess the usefulness of ecological hypotheses is therefore not straightforward. Using the enemy release hypothesis as an example, we show how creating a hierarchy of hypotheses (HoH) can help reviewing and evaluating evidence for and against an ecological hypothesis. In a HoH, a broad, overarching hypothesis branches into more specific and better testable sub‐hypotheses. The enemy release hypothesis is a major hypothesis in invasion ecology and posits that the absence of enemies in the exotic range of an alien species is a cause of its invasion success. Based on a systematic review of empirical tests of this hypothesis, we divided it into sub‐hypotheses, differentiating among 1) indicators for enemy release, 2) types of comparisons, and 3) types of enemies. We identified 176 empirical tests and weighted each test according to the number of alien species studied and the research method (experimental vs observational, field vs enclosure vs laboratory). For the broadly formulated enemy release hypothesis, we found nearly as much supporting (36%) as questioning evidence (43%). At the sub‐hypotheses level, however, we found that some sub‐hypotheses are strongly supported by empirical evidence, whereas others receive hardly any support. These differences are further emphasized for some types of habitat and focal taxonomic groups. Our findings suggest that several specific formulations (i.e. sub‐hypotheses) of the broad enemy release hypothesis are useful, whereas other formulations should be viewed more critically. In general, the approach outlined here can help evaluate major ecological hypotheses and their specific sub‐hypotheses. Our study also highlights the need for a scientific debate on how much supporting evidence is sufficient to consider an ecological hypothesis to be useful.     

Correlational selection does not explain the evolution of a behavioural syndrome

Correlated suites of behaviours, or behavioural syndromes, appear to be widespread, and yet few studies have explored how they arise and are maintained. One possibility holds that correlational selection can generate and maintain behavioural syndrome if certain behavioural combinations enjoy greater fitness than other combinations. Here we test this correlational selection hypothesis by comparing behavioural syndrome structure with a multivariate fitness surface based on reproductive success of male water striders. We measured the structure of a behavioural syndrome including dispersal ability, exploration behaviour, latency to remount and sex recognition sensitivity in males. We then measured the relationship between these behaviours and mating success in a range of sex ratio environments. Despite the presence of some significant correlational selection, behavioural syndrome structure was not associated with correlational selection on behaviours. Although we cannot conclusively reject the correlational selection hypothesis, our evidence suggests that correlational selection and resulting linkage disequilibrium might not be responsible for maintaining the strong correlations between behaviours. Instead, we suggest alternative ways in which this behavioural syndrome may have arisen and outline the need for physiological and quantitative genetic tests of these suggestions.     

Personality and basal metabolic rate in a wild bird population

Personality and metabolic rate are predicted to show covariance on methodological and functional grounds, but empirical studies at the individual level are rare, especially in natural populations. Here we assess the relationship between exploration behaviour, an important axis of personality, and basal metabolic rate (BMR) for 680 free‐living great tits Parus major, studied over three years. We find that exploration behaviour is weakly negatively related to BMR among female, but not male, birds. Moreover, we find exploration behaviour to be independent of methodological aspects of BMR measurements (e.g. activity levels, time to acclimatize) which have been suggested to be indicative of personality‐related activity or stress levels during measurement. This suggests that the weak link between exploration behaviour and BMR found here is functional rather than methodological. We therefore test the hypothesis that selection favours covariance between exploration behaviour and metabolic rate, but find no evidence for correlational survival or fecundity selection. Our data therefore provide at best only very weak evidence for a functional link between personality and metabolic rate, and we suggest that studies of personality and metabolic strategies, or personality and daily energy expenditure, are required to further resolve the link between personality and metabolic rate.     

The Comparative Method in Cross-Cultural and Cross-Species Research

Comparative methodology is controversial in biology and the related field of research on behavioral and psychological traits across human cultures. We critically examine this controversy. We argue that the widely held opinion of non-independence among historically-related cultures and species errs by not recognizing and incorporating into research the two distinct and complementary categories of causation that account for an extant trait—the phylogenetic origin of the trait (cultural or otherwise) and the maintenance of the trait after its phylogenetic origin. Phylogenetic correction for non-independence is required in the comparative study of a trait’s phylogenetic origin, but is irrelevant for the comparative study of the causation of a trait’s maintenance after its phylogenetic origin. Across cultures, even closely related cultures, causes that are specific to each culture act to maintain cultural similarities, which makes such similarities independent. Among species, even closely related species, similarities are maintained by lineage-specific, and hence independent, evolutionary causal processes. Comparative methodology has been criticized as being inferior to other types of scientific methodology (e.g., experimentation), because it is based on correlational data. This criticism is erroneous because, all scientific findings are correlational, and in hypothesis testing, the ability of an empirical finding to address causation depends solely upon the control of confounding variables, not the type of scientific method itself.     

The role of parasites in the dynamics of a reindeer population

Even though theoretical models show that parasites may regulate host population densities, few empirical studies have given support to this hypothesis. We present experimental and observational evidence for a host-parasite interaction where the parasite has sufficient impact on host population dynamics for regulation to occur. During a six year study of the Svalbard reindeer and its parasitic gastrointestinal nematode Ostertagia gruehneri we found that anthelminthic treatment in April-May increased the probability of a reindeer having a calf in the next year, compared with untreated controls. However, treatment did not influence the over-winter survival of the reindeer. The annual variation in the degree to which parasites depressed fecundity was positively related to the abundance of O. gruehneri infection the previous October, which in turn was related to host density two years earlier. In addition to the treatment effect, there was a strong negative effect of winter precipitation on the probability of female reindeer having a calf. A simple matrix model was parameterized using estimates from our experimental and observational data. This model shows that the parasite-mediated effect on fecundity was sufficient to regulate reindeer densities around observed host densities.     

Telomere attrition due to infection

Background

Telomeres–the terminal caps of chromosomes–become shorter as individuals age, and there is much interest in determining what causes telomere attrition since this process may play a role in biological aging. The leading hypothesis is that telomere attrition is due to inflammation, exposure to infectious agents, and other types of oxidative stress, which damage telomeres and impair their repair mechanisms. Several lines of evidence support this hypothesis, including observational findings that people exposed to infectious diseases have shorter telomeres. Experimental tests are still needed, however, to distinguish whether infectious diseases actually cause telomere attrition or whether telomere attrition increases susceptibility to infection. Experiments are also needed to determine whether telomere erosion reduces longevity.

Methodology/Principal Findings

We experimentally tested whether repeated exposure to an infectious agent, Salmonella enterica, causes telomere attrition in wild-derived house mice (Mus musculus musculus). We repeatedly infected mice with a genetically diverse cocktail of five different S. enterica strains over seven months, and compared changes in telomere length with sham-infected sibling controls. We measured changes in telomere length of white blood cells (WBC) after five infections using a real-time PCR method. Our results show that repeated Salmonella infections cause telomere attrition in WBCs, and particularly for males, which appeared less disease resistant than females. Interestingly, we also found that individuals having long WBC telomeres at early age were relatively disease resistant during later life. Finally, we found evidence that more rapid telomere attrition increases mortality risk, although this trend was not significant.

Conclusions/Significance

Our results indicate that infectious diseases can cause telomere attrition, and support the idea that telomere length could provide a molecular biomarker for assessing exposure and ability to cope with infectious diseases.     

Phi Index: A New Metric to Test the Flush Early and Avoid the Rush Hypothesis

Optimal escape theory states that animals should counterbalance the costs and benefits of flight when escaping from a potential predator. However, in apparent contradiction with this well-established optimality model, birds and mammals generally initiate escape soon after beginning to monitor an approaching threat, a phenomena codified as the “Flush Early and Avoid the Rush” (FEAR) hypothesis. Typically, the FEAR hypothesis is tested using correlational statistics and is supported when there is a strong relationship between the distance at which an individual first responds behaviorally to an approaching predator (alert distance, AD), and its flight initiation distance (the distance at which it flees the approaching predator, FID). However, such correlational statistics are both inadequate to analyze relationships constrained by an envelope (such as that in the AD-FID relationship) and are sensitive to outliers with high leverage, which can lead one to erroneous conclusions. To overcome these statistical concerns we develop the phi index (Φ), a distribution-free metric to evaluate the goodness of fit of a 1∶1 relationship in a constraint envelope (the prediction of the FEAR hypothesis). Using both simulation and empirical data, we conclude that Φ is superior to traditional correlational analyses because it explicitly tests the FEAR prediction, is robust to outliers, and it controls for the disproportionate influence of observations from large predictor values (caused by the constrained envelope in AD-FID relationship). Importantly, by analyzing the empirical data we corroborate the strong effect that alertness has on flight as stated by the FEAR hypothesis.     

Selection of trait combinations through bee and fly visitation to flowers of Polemonium foliosissimum

Pollinators are known to exert natural selection on floral traits, but the extent to which combinations of floral traits are subject to correlational selection (nonadditive effects of two traits on fitness) is not well understood. Over two years, we used phenotypic manipulations of plant traits to test for effects of flower colour, flower shape and their interaction on rates of pollinator visitation to Polemonium foliosissimum. We also tested for correlational selection based on weighting visitation by the amount of conspecific pollen delivered per visit by each category of insect visitor. Although bumblebees were the presumed pollinators, solitary bees and flies contributed substantially (42%) to pollination. In manipulations of one trait at a time, insects visited flowers presenting the natural colour and shape over flowers manipulated to present artificial mutants with either paler colour or a more open or more tubular flower. When both colour and shape were manipulated in combination, selection on both traits arose, with bumblebees responding mainly to colour and flies responding mainly to shape. Despite selection on both floral traits, in a year with many bumblebees, we saw no evidence for correlational selection of these traits. In a year when flies predominated, fly visitation showed a pattern of correlational selection, but not favouring the natural phenotype, and correlational selection was still not detected for expected pollen receipt. These results show that flower colour and shape are subject to pollinator‐mediated selection and that correlational selection can be generated based on pollinator visitation alone, but provide no evidence for correlational selection specifically for the current phenotype.     

Leisure activities, cognition and dementia

Accumulated evidence shows that leisure activities have a positive impact on cognitive function and dementia. This review aimed to systematically summarize the current evidence on this topic taking into account the limitations of the studies and biological plausibility for the underlying mechanisms linking cognition, dementia and leisure activities, with special attention on mental, physical and social activities. We included only longitudinal studies, with a follow-up time of at least 2 years, published in English from 1991 to March 2011 on leisure activities and cognition (n = 29) or dementia (n = 23) and provided some evidence from intervention studies on the topic. A protective effect of mental activity on cognitive function has been consistently reported in both observational and interventional studies. The association of mental activity with the risk of dementia was robust in observational studies but inconsistent in clinical trials. The protective effect of physical activity on the risk of cognitive decline and dementia has been reported in most observational studies, but has been less evident in interventional studies. Current evidence concerning the beneficial effect of other types of leisure activities on the risk of dementia is still limited and results are inconsistent. For future studies it is imperative that the assessment of leisure activities is standardized, for example, the frequency, intensity, duration and the type of activity; and also that the cognitive test batteries and the definition of cognitive decline are harmonized/standardized. Further, well designed studies with long follow-up times are necessary. This article is part of a Special Issue entitled: Imaging Brain Aging and Neurodegenerative disease.     

Meta-analyses of adverse effects data derived from randomised controlled trials as compared to observational studies: methodological overview

Background

There is considerable debate as to the relative merits of using randomised controlled trial (RCT) data as opposed to observational data in systematic reviews of adverse effects. This meta-analysis of meta-analyses aimed to assess the level of agreement or disagreement in the estimates of harm derived from meta-analysis of RCTs as compared to meta-analysis of observational studies.

Methods and Findings

Searches were carried out in ten databases in addition to reference checking, contacting experts, citation searches, and hand-searching key journals, conference proceedings, and Web sites. Studies were included where a pooled relative measure of an adverse effect (odds ratio or risk ratio) from RCTs could be directly compared, using the ratio of odds ratios, with the pooled estimate for the same adverse effect arising from observational studies. Nineteen studies, yielding 58 meta-analyses, were identified for inclusion. The pooled ratio of odds ratios of RCTs compared to observational studies was estimated to be 1.03 (95% confidence interval 0.93–1.15). There was less discrepancy with larger studies. The symmetric funnel plot suggests that there is no consistent difference between risk estimates from meta-analysis of RCT data and those from meta-analysis of observational studies. In almost all instances, the estimates of harm from meta-analyses of the different study designs had 95% confidence intervals that overlapped (54/58, 93%). In terms of statistical significance, in nearly two-thirds (37/58, 64%), the results agreed (both studies showing a significant increase or significant decrease or both showing no significant difference). In only one meta-analysis about one adverse effect was there opposing statistical significance.

Conclusions

Empirical evidence from this overview indicates that there is no difference on average in the risk estimate of adverse effects of an intervention derived from meta-analyses of RCTs and meta-analyses of observational studies. This suggests that systematic reviews of adverse effects should not be restricted to specific study types. Please see later in the article for the Editors'' Summary     

Combining limnological and palaeolimnological data to disentangle the effects of nutrient pollution and climate change on lake ecosystems: problems and potential

1. As long‐term observational lake records continue to lengthen, the historical overlap with lake sediment records grows, providing increasing opportunities for placing the contemporary ecological status of lakes in a temporal perspective. 2. Comparisons between long‐term data sets and sediment records, however, require lake sediments to be accurately dated and for sediment accumulation rates to be sufficiently rapid to allow precise matching with observational data. 3. The critical role of the sediment record in this context is its value in tracking the changing impact of human activity on a lake from a pre‐disturbance reference through to the present day. 4. Here, we use data from a range of lakes across Europe presented as case studies in this Special Section. The seven sites considered all possess both long‐term observational records and high‐quality sediment records. Our objective is to assess whether recent climate change is having an impact on their trophic status and in particular whether that impact can be disentangled from the changes associated with nutrient pollution. 5. The palaeo‐data show clear evidence for the beginning of nutrient pollution varying from the mid‐nineteenth century at Loch Leven to the early and middle twentieth century at other sites. The monitoring data show different degrees of recovery when judged against the palaeo‐reference. 6. The reason for limited recovery is attributed to continuing high nutrient concentrations related to an increase in diffuse nutrient loading or to internal P recycling, but there is some evidence that climate change may be playing a role in offsetting recovery at some sites. If this is the case, then lake ecosystems suffering from eutrophication may not necessarily return to their pre‐eutrophication reference status despite the measures that have been taken to reduce external nutrient loading. 7. The extent to which future warming might further limit such recovery can be evaluated only by continued monitoring combined with the use of palaeo‐records that set the pre‐eutrophication reference.     

Canine size and shape in male anthropoid primates

While studies of canine dimorphism in primates are common, only a few have examined canine tooth size independently within each sex. Recently, Greenfield and Washburn (Am. J. Phys. Anthropol. 84:17–34, 1991) proposed that there are two types of male canines which reflect different allometric scaling patterns of canine crown height against canine occlusal dimensions. They also suggest that proportional canine shape, measured as canine crown height (or projection) relative to the occlusal dimensions, provides an estimate of the canine's function as a weapon, though they provide no test of this hypothesis. This analysis critically examines the claim that there are two types of male canines among anthropoids. It then tests the hypothesis that relative male canine size (measured against body weight) and proportional canine shape are related to estimates of intermale competition, diet, and substrate (used as a surrogate measure of predation pressure). While there is strong taxonomic variation in canine size and shape among male anthropoids, no evidence is found for two discrete canine types. Rather, within families and subfamilies, canine dimensions scale isometrically against body weight and against each other, with a continuum of canine shapes among different taxa. While variation in male canine size is associated with intermale competition and substrate, even when taxonomic variation is controlled, variation in proportional canine shape is not. Neither canine size nor shape are generally associated with variation in diet. © 1993 Wiley-Liss, Inc.     

Geographic variation in calcium and clutch size

For over a half century numerous hypotheses have surfaced aimed at explaining a key life history trait, the evolution of clutch size in birds. A principal goal has been to explain why clutch size generally increases with latitude both within species and among closely related species. Most hypotheses have stressed food limitation, predation, or seasonality. I present a novel hypothesis to explain geographic variation: a limitation of calcium resulting from broad scale variation in this element. Because the storage capacity of avian medullary bone is limited or nonexistent, during egg formation, female birds must intake supplemental calcium. Yet calcium and other exchangeable bases are much rarer in tropical soils. I briefly review the abundant experimental and observational evidence supporting how calcium limitation affects clutch size and other life history traits, and I present a series of predictions (and apparent support for them), stemming from the calcium variation and limitation hypothesis. The balance of evidence suggests that variability in availability of environmental calcium plays a proximate and ultimate role in the evolution of clutch size. Although this hypothesis is not mutually exclusive with others, it highlights another factor that needs to be considered in studies of the geographic variation in clutch size.     

A test of the hypothesis that correlational selection generates genetic correlations

Theory predicts that correlational selection on two traits will cause the major axis of the bivariate G matrix to orient itself in the same direction as the correlational selection gradient. Two testable predictions follow from this: for a given pair of traits, (1) the sign of correlational selection gradient should be the same as that of the genetic correlation, and (2) the correlational selection gradient should be positively correlated with the value of the genetic correlation. We test this hypothesis with a meta-analysis utilizing empirical estimates of correlational selection gradients and measures of the correlation between the two focal traits. Our results are consistent with both predictions and hence support the underlying hypothesis that correlational selection generates a genetic correlation between the two traits and hence orients the bivariate G matrix.     

Cloning of hexokinase isoenzyme PI from Saccharomyces cerevisiae: PI transformants confirm the unique role of hexokinase isoenzyme PII for glucose repression in yeasts

Summary Hexokinase isoenzyme PI was cloned using a gene pool obtained from a yeast strain having only one functional hexokinase, isoenzyme PI. The gene was characterized using 20 restriction enzymes and located within a region of 2.0 kbp. The PI plasmid strongly hybridized with the PII plasmids isolated previously (Fröhlich et al. 1984). Hence there was a close relationship between the two genes, one of which must have been derived from the other by gene duplication. In conrrast, glucose repression was restored only in hexokinase PII transformants; PI transformants remained non-repressible. This observation provided additional evidence for the hypothesis of Entian (1980) that only hexokinase PII is necessary for glucose repression. Furthermore, glucose phosphorylating activity in PI transformants exceeded that of wild-type cells, giving clear evidence that the phosphorylating capacity is not important for glucose repression.     

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.