Enforced monoandry over generations induces a reduction of female investment into reproduction in a promiscuous bird.

While uncovering the costs and benefits of polyandry has attracted considerable attention, assessing the net effect of sexual selection on population fitness requires the experimental manipulation of female mating over generations, which is usually only achievable in laboratory populations of arthropods. However, knowing if sexual selection improves or impairs the expression of life-history traits is key for the management of captive populations of endangered species, which are mostly long-lived birds and mammals. It might therefore be questionable to extrapolate the results gathered on laboratory populations of insects to infer the net effect of sexual selection on populations of endangered species. Here, we used a longitudinal dataset that has been collected on a long-lived bird, the houbara bustard, kept in a conservation breeding program, to investigate the effect of enforced monoandry on female investment into reproduction. In captivity, female houbara bustards are artificially inseminated with sperm collected from a single male (enforced monoandry), or sequentially inseminated with semen of different males (polyandry), allowing postcopulatory sexual selection to operate. We identified female lines that were produced either by monoandrous or polyandrous inseminations over three generations, and we compared reproductive investment of females from the two mating system groups. We found that females in the polyandrous lines had higher investment into reproduction as they laid more eggs per season and produced heavier hatchlings. Higher reproductive investment into reproduction in the polyandrous lines did not result from inherited differences from females initially included in the two mating system groups. These results show that removal of sexual selection can alter reproductive investment after only few generations, potentially hindering population fitness and the success of conservation breeding programs.

Paternal age negatively affects sperm production of the progeny.

Parental age has profound consequences for offspring's phenotype. However, whether patrilineal age affects offspring sperm production remains unknown, despite the importance of sperm production for male reproductive success in species facing post-copulatory sexual selection. Using a longitudinal dataset on ejaculate attributes of the houbara bustard, we showed that offspring sired by old fathers had different age-dependent trajectories of sperm production compared to offspring sired by young fathers. Specifically, they produced less sperm (-48%) in their first year of life, and 14% less during their lifetime. Paternal age had the strongest effect, with weak evidence for grandpaternal or great grandpaternal age effects. These results show that paternal age can affect offspring reproductive success by reducing sperm production, establishing an intergenerational link between ageing and sexual selection.

Post-copulatory sexual selection allows females to alleviate the fitness costs incurred when mating with senescing males.

Male senescence has detrimental effects on reproductive success and offspring fitness. When females mate with multiple males during the same reproductive bout, post-copulatory sexual selection that operates either through sperm competition or cryptic female choice might allow females to skew fertilization success towards young males and as such limit the fitness costs incurred when eggs are fertilized by senescing males. Here, we experimentally tested this hypothesis. We artificially inseminated female North African houbara bustards with sperm from dyads of males of different (young and old) or similar ages (either young or old). Then, we assessed whether siring success was biased towards young males and we measured several life-history traits of the progeny to evaluate the fitness costs due to advanced paternal age. In agreement with the prediction, we found that siring success was biased towards young males, and offspring sired by old males had impaired hatching success, growth and post-release survival (in females). Overall, our results support the hypothesis that post-copulatory sexual selection might represent an effective mechanism allowing females to avoid the fitness costs of fertilization by senescing partners.

Elevated basal corticosterone levels increase disappearance risk of light but not heavy individuals in a long-term monitored rodent population.

According to the cort-fitness hypothesis, glucocorticoid levels correlate negatively with fitness. However, field studies found mixed support for this hypothesis, potentially because the association between glucocorticoids and fitness might depend on prevailing environmental conditions. Based on the long-term monitoring of a natural rodent population, we tested whether individuals with elevated corticosterone levels were more likely to disappear, accounting for individual condition and among-year variation in food availability, population density and predation pressure. We used basal corticosterone levels measured at the onset of the pre-breeding season in 331 African striped mice from six generations. While basal corticosterone levels were highly repeatable within individuals, between-individual variation was large. Survival analysis revealed that disappearance risk over the pre-breeding season increased with elevated basal corticosterone levels for light but not for heavy individuals. High levels of corticosterone may be more deleterious to smaller individuals (i.e. through allostatic overload), eventually increasing their mortality risk, and disappearance would represent actual death. An alternative non-exclusive explanation could be that high levels of corticosterone selectively trigger dispersal in light individuals, and disappearance would rather reflect their departure from the population. Although environmental conditions varied considerably among generations, none of the interactions between corticosterone and environmental variables were significant. Disappearance probability was positively correlated with both predation pressure and with food availability, a factor favoring dispersal. In sum, elevated basal corticosterone levels increased disappearance in light striped mice, either directly via reduced survival prospects and/or indirectly via dispersal.

Sperm competition accentuates selection on ejaculate attributes.

Ejaculate attributes are important factors driving the probability of fertilizing eggs. When females mate with several males, competition between sperm to fertilize eggs should accentuate selection on ejaculate attributes. We tested this hypothesis in the North African houbara bustard ( Chlamydotis undulata undulata) by comparing the strength of selection acting on two ejaculate attributes when sperm from single males or sperm from different males were used for insemination. In agreement with the prediction, we found that selection on ejaculate attributes was stronger when sperm of different males competed for egg fertilization. These findings provide the first direct comparison of the strength of selection acting on ejaculate attributes under competitive and non-competitive fertilizations, confirming that sperm competition is a major selective force driving the evolution of ejaculate characteristics.

No evidence for prezygotic postcopulatory avoidance of kin despite high inbreeding depression.

Offspring resulting from mating among close relatives can suffer from impaired fitness through the expression of recessive alleles with deleterious effects. Postcopulatory sperm selection (a prezygotic mechanism of cryptic female choice) has been suggested to be an effective way to avoid inbreeding. To investigate whether postcopulatory female choice allows the avoidance of fertilization by close kin, we performed artificial inseminations in a promiscuous bird, the houbara bustard (Chlamydotis undulata undulata). Females were inseminated with a mix of sperm from triads of males, each constituted of a male genetically unrelated to the female, a first cousin and a half-sibling. When counting the number of eggs sired by unrelated males, cousins or half-siblings, we found a significant deviation from the expected random distribution, with more eggs sired by unrelated males. However, offspring sired by cousins, and especially by half-siblings suffered from high prehatching mortality, suggesting that the observed paternity skew towards unrelated males might reflect differential offspring mortality rather than sperm selection. In agreement with this hypothesis, within-triad siring success was similar for the three parental relatedness categories, but the relationship between siring and hatching success differed across categories. In clutches with high hatching success, unrelated males had the highest success while in clutches with high failure rate, half-siblings had the highest success. Offspring sired by half-siblings also suffered from reduced growth rate during the first three months and higher posthatching mortality. Hence, despite substantial fitness costs associated with fertilization by close relatives, females do not seem to select sperm of unrelated males.

The neoteny-helper hypothesis: When to expect and when not to expect endocrine mechanisms to regulate allo-parental care?

Family groups with helpers occur in several species of fish, birds and mammals. In such cooperatively breeding species all group members help with raising the offspring, i.e. parents and offspring from previous litters. While the ecological reasons and ultimate consequences of allo-parental care have been studied in detail, we know little about its physiological regulation. We propose three alternative hypotheses for the endocrine regulation of allo-parental care. 1. The neoteny-helper hypothesis predicts that helpers that did not undergo adolescence yet show helping behavior without any endocrine mechanisms activating it, as helping is the default response towards infant stimuli. The endocrine changes during adolescence would then deactivate helping behavior. 2. The parent-helper hypothesis predicts that helpers undergo the same endocrine changes as parents (increased prolactin and corticosterone levels; decreased testosterone in males but increased estrogen in females). We predict that this hypothesis is especially important in post-adolescent helpers. 3. The helper-specific hypothesis predicts that there are specific endocrine mechanisms that only exist in helpers but not in breeders. We review evidence for these three hypotheses in 23 species of fish, birds, and mammals. We found no evidence for the helper-specific hypothesis but for both other hypotheses. As predicted, this depended on whether helpers were pre- or post-adolescent, but information on whether or not helpers underwent adolescence was often missing. Thus, future studies should investigate whether or not helpers have reached sexual maturity, differentiate between pre- and post-adolescent helpers, and study behavioral changes in helping behavior during adolescence. We conclude that the neurobiological circuits in the brain necessary for allo-parental care might often be the default stage in helpers from cooperative breeding species, which might be deactivated by specific endocrine mechanisms during adolescence, and then would need reactivation for allo-parental and parental care.

Dietary Supplementation with n-3 Polyunsaturated Fatty Acids Reduces Torpor Use in a Tropical Daily Heterotherm.

Polyunsaturated fatty acids (PUFAs) are involved in a variety of physiological mechanisms, including heterothermy preparation and expression. However, the effects of the two major classes of PUFAs, n-6 and n-3, can differ substantially. While n-6 PUFAs enhance torpor expression, n-3 PUFAs reduce the ability to decrease body temperature. This negative impact of n-3 PUFAs has been revealed in temperate hibernators only. Yet because tropical heterotherms generally experience higher ambient temperature and exhibit higher minimum body temperature during heterothermy, they may not be affected as much by PUFAs as their temperate counterparts. We tested whether n-3 PUFAs constrain torpor use in a tropical daily heterotherm (Microcebus murinus). We expected dietary n-3 PUFA supplementation to induce a reduction in torpor use and for this effect to appear rapidly given the time required for dietary fatty acids to be assimilated into phospholipids. n-3 PUFA supplementation reduced torpor use, and its effect appeared in the first days of the experiment. Within 2 wk, control animals progressively deepened their torpor bouts, whereas supplemented ones never entered torpor but rather expressed only constant, shallow reductions in body temperature. For the rest of the experiment, the effect of n-3 PUFA supplementation on torpor use remained constant through time. Even though supplemented animals also started to express torpor, they exhibited higher minimum body temperature by 2°-3°C and spent two fewer hours in a torpid state per day than control individuals, on average. Our study supports the view that a higher dietary content in n-3 PUFAs negatively affects torpor use in general, not only in cold-acclimated hibernators.

When to initiate torpor use? Food availability times the transition to winter phenotype in a tropical heterotherm.

Timing of winter phenotype expression determines individual chances of survival until the next reproductive season. Environmental cues triggering this seasonal phenotypic transition have rarely been investigated, although they play a central role in the compensation of climatic fluctuations via plastic phenotypic adjustments. Initiation of winter daily torpor use-a widespread energy-saving phenotype-could be primarily timed according to anticipatory seasonal cues (anticipatory cues hypothesis), or flexibly fine-tuned according to actual energy availability (food shortage hypothesis). We conducted a food supplementation experiment on wild heterothermic primates (grey mouse lemurs, Microcebus murinus) at the transition to the food-limited dry season, i.e. the austral winter. As expected under the food shortage hypothesis, food-supplemented individuals postponed the seasonal transition to normal torpor use by 1-2 month(s), spent four times less torpid, and exhibited minimal skin temperature 6 °C higher than control animals. This study provides the first in situ experimental evidence that food availability, rather than abiotic cues, times the launching of torpor use. Fine-tuning of the timing of seasonal phenotypic transitions according to actual food shortage should provide heterotherms with a flexible adaptive mechanism to survive unexpected environmental fluctuations.

Food shortage can drive body temperature regulation in wild heterothermic vertebrates.

Food availability is expected to trigger hibernation and torpor (ie heterothermy) use. Yet, laboratory experiments under controlled conditions dominate, and this hypothesis remains largely untested under natural conditions. Further experimental manipulations of food availability must therefore be conducted in the wild, accounting for other covarying environmental stressors.

Shallow hypothermia depends on the level of fatty acid unsaturation in adipose and liver tissues in a tropical heterothermic primate.

Optimal levels of unsaturated fatty acids have positive impacts on the use of prolonged bouts of hypothermia in mammalian hibernators, which generally have to face low winter ambient temperatures. Unsaturated fatty acids can maintain the fluidity of fat and membrane phospholipids at low body temperatures. However, less attention has been paid to their role in the regulation of shallow hypothermia, and in tropical species, which may be challenged more by seasonal energetic and/or water shortages than by low temperatures. The present study assessed the relationship between the fatty acids content of white adipose and liver tissues and the expression of shallow hypothermia in a tropical heterothermic primate, the gray mouse lemur (Microcebus murinus). The adipose tissue is the main tissue for fat storage and the liver is involved in lipid metabolism, so both tissues were expected to influence hypothermia dependence on fatty acids. As mouse lemurs largely avoid deep hypothermia (i.e. torpor) use under standard captive conditions, the expression of hypothermia was triggered by food-restricting experimental animals. Hypothermia depth increased with time, with a stronger increase for individuals that exhibited higher contents of unsaturated fatty acids suggesting that they were more flexible in their use of hypothermia. However these same animals delayed the use of long hypothermia bouts relative to individuals with a higher level of saturated fatty acids. This study evidences for the first time that body fatty acids unsaturation levels influence the regulation of body temperature not only in cold-exposed hibernators but also in tropical, facultative heterotherms.

Field evidence for a proximate role of food shortage in the regulation of hibernation and daily torpor: a review.

Hibernation and daily torpor (heterothermy) have long been assumed to be adaptive responses to seasonal energy shortage. Laboratory studies have demonstrated that food shortage alone can trigger the use of heterothermy. However, their potential to predict heterothermic responses in the wild is limited, and few field studies demonstrate the dependence of heterothermy on food availability under natural conditions. Thus, the view of heterothermy as an energy saving strategy to compensate for food shortage largely remains an untested hypothesis. In this paper, we review published evidence on the proximate role of food availability in heterothermy regulation by endotherms, and emphasize alternative hypotheses that remain to be tested. Most studies have relied on correlative evidence. Manipulations of food availability, that demonstrate the proximate role of food availability, have been conducted in only five free-ranging heterotherms. Several other metabolic constraints covary with food availability and can confound its effect. Shortage in water availability, the nutritional composition of food, or subsequent conversion of food in fat storage all could be actual proximate drivers of heterothermy regulation, rather than food shortage. Social interactions, competition for food and predation also likely modulate the relative strength of food shortage between individuals. The ecological relevance of the dependence of heterothermy on food availability remains to be assessed in field experiments that account for the confounding effects of covarying environmental and internal factors.

Geographic variations of life history traits and potential trade-offs in different populations of the parasitoid Leptopilina heterotoma.

Energy allocation is determined by resource availability and trade-offs among traits, and so organisms have to give some traits priority over others to maximize their fitness according to their environment. In this study, we investigated the geographic variations in life history traits and potential trade-offs in populations of the parasitoid Leptopilina heterotoma (Hymenoptera: Figitidae) originating from the north and the south of the Rhône-Saône valley (over a gradient of 300 km, South-East France). We measured a set of traits related to reproduction, maintenance, and mobility using several estimators of each of these main functions determined at different times. We did not find any clear differences between populations from contrasting areas, whereas the southern populations, which were all assumed to be exposed to similar environmental conditions, displayed contrasting patterns of energy allocation. Thus, the most likely explanation seems to be that the evolution of the life history of L. heterotoma is probably shaped by local selective pressures, such as microclimate, microhabitats, or intensity of competition, rather than by regional ecological conditions. Using our study as an example, we discuss the interest of considering several traits and using different ways of measuring them, concluding that multiple measurements should be performed in future studies to ensure the robustness of the results.