Jia Zheng

Species with diverse parental care systems exhibit flexibility in response to environmental changes. Such changes can influence parental care decisions by altering the trade‐offs between current and future reproduction. While previous studies have revealed correlations between specific social and ecological factors and parental care systems, the joint effects of multiple factors could be complex and may result in different outcomes within and across populations and between sexes. In this study, we observed parental care systems and their seasonal variations across three geographic populations in a polygamous species, the Chinese penduline tits ( Remiz consobrinus ), by monitoring breeding events over 8 years. In this species, biparental care, female‐only care, male‐only care and biparental desertion coexist in one population. To investigate whether and how sex‐specific parental care patterns correlate with local conditions, we compared breeding phenology, season length, mating opportunity and reproductive success and their associations with the frequency of parental care patterns observed within and across the populations. We found striking population differences in the parental care system: biparental care predominated in one population, whereas female‐only care was more common in the other two. These divergent parental care systems were shaped by sex‐specific responses to the interacting local conditions. In all three populations, females synchronised egg‐laying and offered males a short but highly successful re‐mating window. This resulted in rapidly decreased male mating opportunities due to male collective clutch desertion. Males were more likely to care when re‐mating opportunities were low. In contrast, females consistently provided care, regardless of variations in female mating opportunities. Additionally, males deserted clutches more in populations where desertion resulted in no fitness cost to the current broods. We found that desertion costs on brood fitness matter more than achieving a high reproductive reward in driving uniparental care, contrary to the theory that uniparental care only occurs when single parents can raise young efficiently. Our study demonstrates that parental care system variance across populations could result from the combined influences of social and ecological conditions, and their interactions. We highlight the consistency of sex‐specific responses to local conditions in parental care.

Biodiversity loss driven by climate change and human activities poses a critical global challenge. Population restoration and reintroduction programs are essential for mitigating this threat to endangered species, yet their outcomes often remain unpredictable due to poorly understood success factors, such as the inevitable inbreeding during bottleneck events. The conservation programme of the crested ibis (Nipponia nippon) marks a successful example where the population rose from just seven individuals to over 9000 in the past four decades. By developing an individual-based model that simulates the restoration process and incorporates species-specific demography and inbreeding data, we conclude that this successful restoration is largely deterministic, as our results closely mirror empirical recovery time and population-level inbreeding coefficients. To establish general guidelines for reintroduction programs, we compare how inbreeding depression influences the recovery success of two reintroduction strategies involving small founder populations. Our simulations reveal that the ‘firework’ approach (one-source translocations) outperforms the ‘sequential’ (serial translocations) approach in restoration effectiveness. Furthermore, by expanding analyses over a broad demographic space, we demonstrate that the net effect of inbreeding varies with species-specific demography and highlight the importance of considering their interaction when interpreting conservation outcomes and designing future reintroduction programs.

Parental cooperation is not self-evident, as conflicts often arise over individual contributions. Evolutionary game theory suggests this conflict may be resolved through negotiation, where parents adjust their care level based on their partner’s contribution. However, mathematical negotiation models typically predict low parental cooperation. As these models are not dynamically explicit and mostly neglect stochasticity, we employ individual-based simulations to investigate how parental negotiation strategies evolve and shape care patterns. Our results differ markedly from earlier analytical predictions. Parental negotiation strategies readily evolve, resulting in four alternative care patterns: uniparental care, sex-biased care and two types of egalitarian biparental care. Effective cooperation evolves regularly but, contrary to common expectations, always relies on a Tit-for-Tat strategy rather than parental compensation. Our study underscores that diverse cooperative patterns in animals can emerge from sex-specific negotiation strategies, even in the absence of initial sex roles and environmental variation.

Parental cooperation is not self-evident, as conflicts often arise over individual contributions. Evolutionary game theory suggests this conflict may be resolved through negotiation, where parents adjust their care level based on their partner’s contribution. However, mathematical negotiation models typically predict low parental cooperation. As these models are not dynamically explicit and mostly neglect stochasticity, we employ individual-based simulations to investigate how parental negotiation strategies evolve and shape care patterns. Our results differ markedly from earlier analytical predictions. Parental negotiation strategies readily evolve, producing four alternative care patterns: uniparental care, sex-biased care and two types of egalitarian biparental care, that occasionally emerge as transitional stages over evolutionary time. Effective cooperation evolved regularly but, contrary to common expectations, always relied on a Tit-for-Tat strategy rather than parental compensation. Our study underscores that diverse cooperative patterns in animals can emerge from sex-specific negotiation strategies, even in the absence of initial sex roles and environmental variation.

Biodiversity loss driven by climate change and human activities poses a critical global challenge. Population restoration and reintroduction programs are essential for mitigating this threat, yet their outcomes are often unpredictable due to poorly understood success factors. The conservation program of the crested ibis ( Nipponia nippon ) marks a successful example where the population rose from seven survivors to over 9,000 in the past four decades. To learn whether this successful restoration was due to chance or largely repeatable, we developed an individual-based model that simulates the restoration process by incorporating life-history parameters from empirical data. Our simulation results closely mirror empirical findings, including the time taken to reach the current population size and population-level inbreeding coefficients. We further analyzed the model to compare the effectiveness of two reintroduction strategies and analyzed how inbreeding depression interacts with demography to influence the chance of recovery from bottlenecks. The reintroduction simulations reveal that the ‘firework’ approach (one-source translocations) outperforms the ‘stepping-stone’ (serial translocations) approach in restoration effectiveness. Our simulations over broad demographic parameters demonstrate that the net effect of inbreeding varies with species-specific demography, and highlight the importance of considering this interaction when interpreting conservation outcomes and designing future reintroduction programs.

The evolution of parental cooperation is challenging to explain because caring for offspring is costly, and parents face an evolutionary conflict of interests over how much care each should contribute. Evolutionary game theory suggests that this conflict may be resolved through parental negotiation, where the parents make their care level dependent on the care provided by their partner. However, mathematical negotiation models typically predict a low level of parental cooperation. As these models are not dynamically explicit and tend to neglect stochasticity, we here investigate the evolution of parental negotiation strategies by means of individual-based simulations. Our results differ markedly from earlier analytical predictions. Parental negotiation strategies readily evolve, but replicate simulations result in four alternative care patterns: uniparental care, sex-biased care and egalitarian biparental care with constant or oscillatory care levels. Hence, pronounced sex differences in parental care can evolve even in the absence of sexual selection and uncertainty of paternity. Effective parental cooperation is the most frequent outcome, but in contrast to the earlier models, it is not based on parental compensation but on a tit-for-tat negotiation strategy. Actually, compensation destabilises cooperation and leads to less effective outcomes where one parental sex exploits the other one.

Parents confront multiple aspects of offspring demands and need to coordinate different parental care tasks. Biparental care is considered to evolve under circumstances where one parent is not competent for all tasks and cannot efficiently raise offspring. However, this hypothesis is difficult to test, as uniparental and biparental care rarely coexist. Chinese penduline tits (Remiz consobrinus) provide such a system where both parental care types occur. Here, we experimentally investigated whether parents in biparental nests are less capable of caring than parents in uniparental nests. We monitored parenting efforts at (1) naturally uniparental and biparental nests and (2) biparental nests before and during the temporary removal of a parent. Given the relatively small sample sizes, we have employed various statistical analyses confirming the robustness of our results. We found that total feeding frequency and brooding duration were similar for natural uniparental and biparental nests. Feeding frequency, but not brooding duration, contributed significantly to nestling mass. In line with this, a temporary parental removal revealed that the remaining parents at biparental nests fully compensated for the partner's feeding absence but not for brooding duration. This reflects that the manipulated parents are confronted with a trade‐off between feeding and brooding and were selected to invest in the more influential one. However, such a trade‐off may not occur in parents of natural uniparental care nests. The different capabilities of a parent independently coordinating feeding and brooding tasks suggest that parents from biparental and uniparental nests were exposed to different resource conditions, thereby foraging efficiency may differ between care types.

Parents confront multiple aspects of offspring demands and need to coordinate different parental care tasks. Biparental care is considered to evolve under circumstances where one parent is not competent for all tasks and cannot efficiently raise offspring. However, this hypothesis is difficult to test, as uniparental and biparental care rarely coexist. Chinese penduline tits (Remiz consobrinus) provide such a system where both parental care types occur. Here, we experimentally investigated whether parents in biparental nests is less capable of caring than parents in uniparental nests. We monitored parenting efforts at (1) naturally uniparental and biparental nests and (2) biparental nests before and during the temporary removal of a parent. We found that total feeding frequency and brooding duration were similar for natural uniparental and biparental nests. Feeding frequency, but not brooding duration, contributed significantly to nestling mass. In line with this, parental removal revealed that the remaining parents at biparental nests fully compensated for the partner’s feeding absence but not for brooding duration. This reflects that the manipulated parents are confronted with a trade-off between feeding and brooding and were selected to invest in the more influential one. However, such a trade-off was not observed in parents of natural uniparental care nests. The different capabilities of a parent independently coordinating feeding and brooding tasks indicated that parents from biparental and uniparental nests experienced different environmental conditions.

1. Parental care patterns differ enormously among and even within species. This is exemplified by Chinese penduline tits Remiz consobrinus, where biparental care, female only care, male only care and biparental desertion all occur in the same population; moreover, the distribution of these care patterns differs systematically between populations. The eco-evolutionary determinants of this diversity are largely unknown.2. We developed an individual based model that allows us to investigate the effects of season length and offspring needs (expressed by the efficacy with which a clutch can be raised by a single parent) on the evolution of parental care patterns. The model is largely conceptual, aiming at general conclusions. However, to keep the model realistic, its set up and the choice of parameters are motivated by field studies on Chinese penduline tits. Exploring a wide range of parameters, we investigate how parental care patterns are affected by season length and offspring needs and whether and under what conditions diverse parental care patterns can stably coexist.3. We report five main findings. First, under a broad range of conditions, different care patterns (e.g. male care and biparental care) coexist at equilibrium. Second, for the same parameters, alternative evolutionary equilibria are possible; this can explain differences in care patterns across populations. Third, rapid evolutionary transitions can occur between alternative equilibria; this can explain the often reported evolutionary lability of parental care patterns. Fourth, season length has a strong but nonmonotonic effect on the evolved care patterns. Fifth, when uniparental care efficacy is low, biparental care tends to evolve; however, in many scenarios uniparental care is still common at equilibrium. In addition, our study sheds new light on Trivers' hypothesis that the sex with the highest prezygotic investment is predestined to invest more postzygotically as well.4. Our study highlights that diversity in parental care can readily evolve and it shows that even in the absence of environmental change parental care patterns can be evolutionary labile. In the presence of directional environmental change, systematic shifts in care patterns are to be expected.