Communal Nesting Research Articles

Biology and ecology of a deep cave nesting spider wasp, Ageniella evansi Townes, (Hymenoptera: Pompilidae), in Arizona

ABSTRACT Cavernicolous populations of the spider wasp Ageniella evansi Townes, 1957 (Hymenoptera: Pompilidae) nest deep within limestone caves at Colossal Cave Mountain Park near Tucson, Arizona, USA. These remarkable populations are the only truly cavernicolous pompilid wasps currently known. As many as 79 females were documented nesting simultaneously within Arkenstone Cave. The facts that the wasps do not nest near cave entrances but only deep within the interior of the caves, are philopatric to each occupied cave and have been present each year for decades confirm their cavernicolous behaviour as subtroglophiles. Studies conducted for over 30 years revealed many insights into the behaviour of these wasps in addition to their nesting deep within limestone caves, including: intraspecific and interspecific competition, plasticity in host selection, precision solar navigation, evidence that some form of communal nesting may be present and behaviour suggestive of rudimentary cognition in the species. This study demonstrates the importance of assessing all animals found in caves as potential ecological elements and not focusing solely on exotic troglobiotic species.

Clutchesof the Cat-eye snakes genus Leptodeiraspp. (SERPENTES: Dipsadidae), with a bibliographic review and new reports for Panama

The reproduction of Neotropical snakes, such as those of the genus Leptodeira, varies widely influenced by climatic factors, with species ranging from strictly seasonal to continuous reproductive patterns. Recent research reveals significant variability in reproductive cycles, determined by local environmental conditions such as temperature and precipitation. Nesting behaviors remain poorly understood, and snakes use diverse environments, from natural shelters to communal nests of other species. Within the family Dipsadidae, species such as L. maculataand L. punctatademonstrate correlations between female body size (SVL) and clutch size, reflecting adaptive strategies to maximize reproductive success in diverse ecological contexts. Field work in Panama provided new insights, documenting egg-laying and gravid individuals of species such as L. rhombiferaand L. ornata, highlighting further research to elucidate the reproductive dynamics and ecological adaptations particular to each. species. Analytical approaches including Pearson and Spearman correlationsandcluster analysis reveal distinct patterns in clutch size relative to female size, underscoring species adaptations and variability within the genus

Communal nesting shapes the sex-dependent glutamatergic response to early life stress in the rat prefrontal cortex.

Early social environment, either positive or negative, shapes the adult brain. Communal nesting (CN), a naturalistic setting in which 2-3 females keep their pups in a single nest sharing care-giving behavior, provides high level of peer interaction for pups. Early social isolation (ESI) from dam and siblings represents, instead, an adverse condition providing no peer interaction. We investigated whether CN (enrichment setting) might influence the response to ESI (impoverishment setting) in terms of social behavior and glutamate system in the medial prefrontal cortex (mPFC) of adult and adolescent male and female rats. Pinning (a rewarding component of social play behavior) was significantly more pronounced in males than in females exposed to the combination of CN and ESI. CN sensitized the glutamate synapse in the mPFC of ESI-exposed male, but not female, rats. Accordingly, we observed (i) a potentiation of the glutamatergic neurotransmission in the mPFC of both adolescent and adult males, as shown by the recruitment of NMDA receptor subunits together with increased expression/activation of PSD95, SynCAM 1, Synapsin I and αCaMKII; (ii) a de-recruiting of NMDA receptors from active synaptic zones of same-age females, together with reduced expression/activation of the above-mentioned proteins, which might reduce the glutamate transmission. Whether similar sex-dependent glutamate homeostasis modulation occurs in other brain areas remains to be elucidated. CN and ESI interact to shape social behavior and mPFC glutamate synapse homeostasis in an age- and sex-dependent fashion, suggesting that early-life social environment may play a crucial role in regulating the risk to develop psychopathology.

Uncovering floral composition of paper wasp nests (Hymenoptera: Vespidae: Polistes) through DNA metabarcoding

As the social organism, Polistes wasps build a communal nest using woody fibers with saliva for sustaining brood and adult population throughout the season. Limited information exists regarding the identification specific plant materials employed in wasp nest building. Thus, we firstly tested if the DNA metabarcoding approach utilizing rbcL and trnL molecular markers could identify the plant species quantitatively and qualitatively inform the mixed-origin woody samples. A threshold of 0.01 proportion of reads was applied for rbcL and trnL molecular markers, while this threshold for median proportion was 0.0025. In assessing taxa richness, the median proportion demonstrated superior performance, exhibiting higher taxa detection power, however, rbcL marker outperformed in quantitative analysis. Subsequently, we applied DNA metabarcoding to identify the plant materials from the nests of two Polistes species, P. mandarinus and P. rothneyi. The results showed that higher preference of Quercus and Robinia as the major nest building materials regardless of the surrounding plant communities, by two wasp species. Material diversity was higher for P. rothneyi than P. mandarinus, which may explain the abundance of this species possibly with heightened adaptive capacities in their nesting behavior. This study demonstrated that DNA metabarcoding could identify the complex nest-building plant materials of paper wasps and provide insights into their ecological interactions in the natural ecosystem.

Subjective experience of the environment determines serotoninergic antidepressant treatment outcome in male mice

BackgroundThe effects of the selective serotonin reuptake inhibitors (SSRIs), the first-line antidepressant treatment, have been proposed to be affected, at least in part, by the living environment. Since the quality of the environment depends not only on its objective features, but also on the subjective experience, we hypothesized that the latter plays a key role in determining SSRI treatment outcome. MethodsWe chronically administered the SSRI fluoxetine to two groups of adult CD-1 male mice that reportedly show distinct subjective experiences of the environment measured as consistent and significantly different responses to the same emotional and social stimuli. These distinct socioemotional profiles were generated by rearing mice either in standard laboratory conditions (SN) or in a communal nest (CN) where three dams breed together their offspring, sharing caregiving behavior. ResultsAt adulthood, CN mice displayed higher levels of agonistic and anxiety-like behaviors than SN mice, indicating that they experience the environment as more socially challenging and potentially dangerous. We then administered fluoxetine, which increased offensive and anxious response in SN, while producing opposite effects in CN mice. BDNF regulation was modified by the treatment accordingly. LimitationsSubjective experience in mice was assessed as behavioral response to the environment. ConclusionsThese results show that the subjective experience of the environment determines fluoxetine outcome. In a translational perspective, our findings suggest considering not only the objective quality, but also the subjective appraisal, of the patient's living environment for developing effective personalized therapeutic approaches in psychiatry.

Outcomes of early social experiences on glucocorticoid and endocannabinoid systems in the prefrontal cortex of male and female adolescent rats.

Social and emotional experiences differently shape individual's neurodevelopment inducing substantial changes in neurobiological substrates and behavior, particularly when they occur early in life. In this scenario, the present study was aimed at (i) investigating the impact of early social environments on emotional reactivity of adolescent male and female rats and (ii) uncovering the underlying molecular features, focusing on the cortical endocannabinoid (eCB) and glucocorticoid systems. To this aim, we applied a protocol of environmental manipulation based on early postnatal socially enriched or impoverished conditions. Social enrichment was realized through communal nesting (CN). Conversely, an early social isolation (ESI) protocol was applied (post-natal days 14-21) to mimic an adverse early social environment. The two forms of social manipulation resulted in specific behavioral and molecular outcomes in both male and female rat offspring. Despite the combination of CN and ESI did not affect emotional reactivity in both sexes, the molecular results reveal that the preventive exposure to CN differently altered mRNA and protein expression of the main components of the glucocorticoid and eCB systems in male and female rats. In particular, adolescent females exposed to the combination of CN and ESI showed increased corticosterone levels, unaltered genomic glucocorticoid receptor, reduced cannabinoid receptor type-1 and fatty acid amide hydrolase protein levels, suggesting that the CN condition evokes different reorganization of these systems in males and females.

Communal nesting differentially attenuates the impact of pre-weaning social isolation on behavior in male and female rats during adolescence and adulthood.

Early social isolation (ESI) disrupts neurodevelopmental processes, potentially leading to long-lasting emotional and cognitive changes in adulthood. Communal nesting (CN), i.e., the sharing of parental responsibilities between multiple individuals in a nest, creates a socially enriching environment known to impact social and anxiety-related behaviors. This study examines the effects of (i) the CN condition and of (ii) ESI during the 3rd week of life (i.e., pre-weaning ESI) on motor, cognitive, and emotional domains during adolescence and adulthood in male and female rats reared in the two different housing conditions, as well as (iii) the potential of CN to mitigate the impact of ESI on offspring. We found that in a spontaneous locomotor activity test, females exhibited higher activity levels compared to males. In female groups, adolescents reared in standard housing (SH) condition spent less time in the center of the arena, suggestive of increased anxiety levels, while the CN condition increased the time spent in the center during adolescence, but not adulthood, independently from ESI. The prepulse inhibition (PPI) test showed a reduced PPI in ESI adolescent animals of both sexes and in adult males (but not in adult females), with CN restoring PPI in males, but not in adolescent females. Further, in the marble burying test SH-ESI adolescent males exhibited higher marble burying behavior than all other groups, suggestive of obsessive-compulsive traits. CN completely reversed this stress-induced effect. Interestingly, ESI and CN did not have a significant impact on burying behavior in adult animals of both sexes. Overall, our findings (i) assess the effects of ESI on locomotion, sensorimotor gating, and compulsive-like behaviors, (ii) reveal distinct vulnerabilities of males and females within these domains, and (iii) show how early-life social enrichment may successfully counteract some of the behavioral alterations induced by early-life social stress in a sex-dependent manner. This study strengthens the notion that social experiences during early-life can shape emotional and cognitive outcomes in adulthood, and points to the importance of social enrichment interventions for mitigating the negative effects of early social stress on neurodevelopment.

Earthen mounds (heuweltjies) of South Africa and their termite occupants: applicability of concepts of the extended phenotype, ecosystem engineering and niche construction.

Heuweltjies are earthen mounds found throughout the Succulent Karoo of South Africa and are inhabited by the termite Microhodotermes viator. Many have assumed that heuweltjies are constructed by the occupying termites. Consequently, heuweltjies have been used as an example of several important concepts in ecology and evolution: the extended phenotype, ecosystem engineering and niche construction. However, recent findings demonstrate that M. viator does not directly construct heuweltjies. Rather, termite colonies enrich the soil around their nests with plant nutrients, which promotes development of widely separated patches of denser vegetation. Eventual formation of heuweltjies represents a response of the physical environment to the windbreak effect of the denser vegetation patches (localized reduction of wind velocity and resultant deposition and accumulation of airborne sediment). Other structures constructed by the termites are justifiably regarded as extended phenotypes. Identification and investigation of a complex cascade of processes are required to more precisely assess the manner in which this termite species functions as an ecosystem engineer or niche constructor, thereby significantly influencing the availability of resources within local ecosystems. Environmental alterations that are either directly or indirectly generated by social animals that construct large, communal nests represent ecological processes that contribute significantly to local biodiversity. This article is part of the theme issue 'The evolutionary ecology of nests: a cross-taxon approach'.

The evolutionary ecology of nests: a cross-taxon approach.

Nests, including the enormous structures housing colonies of eusocial insects and the elaborately built nests of some fishes, have long fascinated scientists, yet our understanding of the evolutionary ecology of nests has lagged behind our understanding of subsequent reproductive stages. There has, however, been a burgeoning amount of interest in nests over the past decade, and this special issue on 'The evolutionary ecology of nests: a cross-taxon approach' outlines our understanding of the form and function of nests in diverse animal lineages. Papers in 'The function of nests: mechanisms and adaptive benefits' theme examine the various functions of nests, while papers in 'The evolution of nest characteristics' theme examine the evolution of nesting behaviours. Meanwhile, papers in the 'Large communal nests in harsh environments' theme examine how the enormous structures constructed by eusocial insects and social birds enable them to inhabit harsh arid environments, whereas papers in the 'Nests in the Anthropocene' theme examine how adaptive shifts in nest architecture allow animals to adapt to breed in the age of accelerating global human impacts. Finally, the synthesis outlines how the mixture of ideas and approaches from researchers studying different taxa will advance our understanding of this exciting field of research. This article is part of the theme issue 'The evolutionary ecology of nests: a cross-taxon approach'.

Abundance of Non-Native Birds in the City: Spatial Variation and Relationship with Socioeconomics in a South American City.

Cities commonly support a high abundance of non-native species that can affect both wildlife and human health; however, their distribution across the urban environment and their relationship with socioeconomics are not well documented. Here, we map the abundance of three non-native birds in a Latin American city-domestic pigeon (Columba livia f. domestica), house sparrow (Passer domesticus), and monk parakeet (Myiopsitta monachus)-and investigate the effect of socioeconomics on their abundance. We found that C. livia f. domestica exhibited a random distribution of abundance across the city but reached its greatest abundance in low-income areas. P. domesticus exhibited an aggregated distribution of abundance, being most abundant in the southern and western areas of the city and in low-income areas. M. monachus exhibited an aggregated distribution of abundance, being most abundant in the northeastern part of the city and reaching its greatest abundance in high-income areas. Low-income areas likely provide high abundance of food, shelter, and nesting sites for both C. livia f. domestica and P. domesticus, whereas high income areas have greater tree cover and larger trees in which M. monachus can build communal nests. Our study finds that the abundance of non-native birds varies across the city and between socioeconomic groups; therefore, targeted management is needed in different city zones to limit negative effects on native species and prevent zoonotic diseases.

Communal nesting behaviour of Siberian flying squirrels during the non‐winter season

AbstractCommunal nesting behaviour of small mammals is thought to function mainly as a method of thermoregulation. However, this behaviour is also observed during the non‐winter season when there are no obvious thermoregulatory benefits. Siberian flying squirrels (Pteromys volans) nest communally during the non‐winter season. This may facilitate mating, but other functions such as thermoregulation have not been sufficiently studied. Therefore, we examined whether communal nesting of P. volans during the non‐winter season plays a role in thermoregulation in addition to mating. In the event that communal nesting in the non‐winter season has any function other than mating, we expected an increase in non‐mating groups consisting of only‐males during the nurturing period (duration of mother squirrel and cubs living together) because females rearing their young do not participate in communal nesting. In Obihiro, Hokkaido, Japan, during the non‐winter season (May–October) between 2019 and 2021, we captured P. volans in 252 nests by using nest boxes. The occurrence of male‐only groups showed a tendency to increase during the nurturing period. Moreover, the number of mixed‐sex groups increased between August and October each year; these changes in communal nesting during the non‐winter season were unrelated to the variation in air temperature or precipitation. These results suggest that during the non‐winter season, communal nesting of P. volans is related to reproductive behaviour. Additional studies are needed to determine how communal nesting during the non‐winter season enhances the reproductive success of male and female P. volans.

Notes on an interspecific interaction between distant taxa: Observation of active cohabitation and communal nesting resources between Gehyra mutilata (Wiegmann, 1834) and two vespid wasp species

AbstractTropical ecosystems are biodiverse and ecologically complex, supporting nearly half of the global herpetofauna. Ecological interactions play a role in the maintenance of species diversity with a range of ecological interactions occurring within and among species in tropical ecosystems. Incidental observations of ecological interactions provide an important contribution to the understanding of the ecology and natural history of species. Here, we report an interspecific interaction between distant taxa, the Four‐Clawed Gecko (Gehyra mutilata) utilizing an active communal nesting site and shared nesting resources with two vespid wasp species (Eumeninae and Polistinae), where previous observations documented species utilizing abandoned nesting resources. This is the first known record of three species actively cohabitating in a shared nesting resource. This observation, with one or all species positively benefiting from the ecological interaction, likely indicates a mutualistic or commensal ecological interaction. Incidental observations such as the present study are likely to reveal that interspecific ecological interactions including communal nesting are more widespread than previously thought and the co‐utilization of existing nesting resources presents an advantage for taxa. Observations such as these provide the basis for understanding the complex network of ecological interactions occurring in tropical rainforests.

Nest-sharing behavior of captive Broad-snouted caimans (Caiman latirostris): cooperation or exploitation?

Abstract Communal egg-laying is not frequently observed in crocodilians that exhibit nest attendance. However, it has been reported for both wild and captive Caiman latirostris. To understand the proximate causes of communal nesting, we aim to describe the nesting behavior of caimans during communal egg-laying in a captive population. Video cameras were placed at nests and the behavior of females was monitored throughout three incubation periods. We documented and analyzed social behavioral data. Nest-sharing behavior consisted of several females congregating and performing nest construction, nest vigilance and nest maintenance. It was observed in all nests previous to egg-laying. After the first laying, one female prevented others from approaching. The second egg-laying female negatively affected the previous clutch. Working together at the same nest would appear to be an exploitative rather than a cooperative activity, and communal nesting could be a competitive interaction, due to limited suitable nesting sites or overcrowding due to captivity.

Communal nesting is the optimal strategy for heat conservation in a social marsupial: lessons from biophysical models.

Endothermy, understood as the maintenance of continuous and high body temperatures owing to the combination of metabolic heat production and an insulative cover, is severely challenged in small endotherms inhabiting cold environments. As a response, social clustering combined with nest use (=communal nesting) is a common strategy for heat conservation. To quantify the actual amount of energy that is saved by this strategy, we studied the social marsupial Dromiciops gliroides (monito del monte), an endemic species of the cold forests of southern South America. It is hypothesized that sociability in this marsupial was driven by cold conditions, but evidence supporting this hypothesis is unclear. Here, we used taxidermic models ('mannequins') to experimentally test the energetic benefits of clustering combined with nest use. To do this, we fitted and compared cooling curves of solitary and grouped mannequins, within and outside of a nest, at the typical winter ambient temperatures of their habitat (5°C). We found that the strategy that minimized euthermic cost of maintenance was the combination of nest use and clustering, thus supporting communal nesting as a social adaptation to cope with the cold. Considering the basal metabolic rate of monitos, our estimates suggest that the savings represents almost half of energy consumption per day (in resting conditions). This study shows how simple biophysical models could help to evaluate bioenergetic hypotheses for social behavior in cold-adapted endotherms.

Monk Parakeet’s (Myiopsitta monachus) Ecological Parameters after Five Decades of Invasion in Santiago Metropolis, Chile

Monk Parakeets are considered one of the most invasive bird species given its unique capacity among psittacines to build their own communal nests. Originally introduced as pets in houses from where they escaped or were released, they are currently considered invasive in more than 20 countries worldwide. This is the case in Chile, where Monk Parakeets were introduced during the 1970s. Between 2016 and 2019 we searched Monk Parakeets’ nests structures in the Santiago metropolis region. We identified 1458 Monk Parakeets’ communal nests on 546 trees belonging to 34 tree species. Ninety-one percent of the occupied trees were also introduced. Paraná pine and cedar of Lebanon were the tree species with highest abundance of nests, averaging more than four nests/tree/species, with 23 and 18 maximum number of nests, respectively. Tasmanian blue gum and black locust were selected by parakeets more often than expected, based on availability. From all trees, 24.6% denoted health problems and 47.3% were pruned. The average nest height was 14.2 m and nests were observed mainly in secondary branches (59.3%). The occupancy rate was 89.7% and was associated to nest height and type of branch. During two reproductive seasons we quantified eggs and nestlings in chambers averaging 4.5 and 4.2, respectively. We provide a rough population size estimate and the characteristics of Monk Parakeets nest and tree selectivity, aiming to characterize several decades of a neglected urban invasion to warrant strategies for improved management measures.

Cheek‐to‐cheek: Communal nesting in an ephemeral pool‐breeding frog

AbstractCommunal nesting is a behaviour exhibited by some oviparous species, the adults of which deposit their eggs over the same time period in a common area and possibly in direct physical contact. While this may occur inadvertently, it is proposed to be an adaptive trait in some species. Herein, we describe both solitary and communal nesting behaviours in the sandpaper frog (Lechriodus fletcheri). Field observations over two consecutive breeding seasons revealed that adults of this species frequently deposit their eggs in frothed nests alongside those of other mating pairs to form floating communal rafts or “masses” on the surface of the water. The presence of communal masses despite; (i) the relatively small number of nests deposited within each breeding episode and (ii) the space available for nests to be deposited separately within a pool, both indicate that some adults are deliberately choosing to lay their nests together. We propose that adults are thus engaging in a form of social behaviour and that communal nesting may be highly advantageous for improving offspring survival by complimenting the anti‐predator properties of the froth nest. This is because the communal mass further isolates embryos from the external environment during development prior to hatching, given a reduction in the surface area relative to volume of each nest that composes the mass. While future research is required to determine why both solitary and communal nesting behaviours are exhibited in this species, it could suggest that there are certain costs associated with communality that occasionally outweigh the benefits.

Consequences of Oviposition Site Choice for Geckos in Changing Environments.

Simple SummaryMost lizards lay eggs inside nests where developing embryos experience large temperature fluctuations. As females do not incubate eggs, the embryos can experience lethally high temperatures during heatwaves. Thus, future changes in the frequency and intensity of summer heatwaves may threaten lizard populations. However, variation in female nest site choice might buffer the embryos in some nests from high temperatures. In this study, we incubated eggs of the velvet gecko under two fluctuating temperature regimes to mimic the temperatures experienced inside currently used sun-exposed (“warm”: mean = 25.4 °C; range = 16.5–35.5 °C) and shaded (“cold”: mean = 23.3 °C; 17.5–30.5 °C) communal nest sites. We found that warm-incubated hatchlings hatched 15 days earlier, on average, and were smaller than their cold-incubated clutch mates. We released the hatchlings to the wild, and monitored their survival over six months. Egg incubation treatment did not influence the survival of hatchlings. This result is reassuring, because even if air temperatures increase by 2 °C in future, some currently used shaded nests will provide thermal regimes that are suitable for embryonic development. Variation in female nest site choice may therefore allow some populations of velvet geckos to persist in changing environments.Most lizards lay eggs inside nests where embryos experience daily fluctuations in temperature. As embryos are sensitive to exposure to high temperatures, increases in nest temperatures may pose a risk to lizards. In the velvet gecko Amalosia lesueurii, nest temperatures are positively correlated with air temperatures, so nests may get hotter in future. However, maternal variation in oviposition site choice might buffer populations from future warming. To evaluate the consequences of oviposition site choice, we incubated eggs under two fluctuating temperature regimes that mimicked temperatures experienced inside sun-exposed (“warm”: mean = 25.4 °C; range = 16.5–35.5 °C) and shaded (“cold”: mean = 23.3 °C; 17.5–30.5 °C) communal nests. We measured the phenotypic traits of hatchlings, released them to the wild, and monitored their survival over 6 months. Warm-incubated hatchlings hatched 15 days earlier, on average, and were smaller than their cold-incubated clutch mates. Incubation treatment did not influence the apparent survival of hatchlings. Hence, even if air temperatures increase by 2 °C in future, thermal regimes inside some currently used shaded nests will be suitable for embryo development. Maternal variation in nest site choice may therefore allow southern populations of the velvet gecko to persist in changing environments.

Sunny-side up: ontogenetic variation in egg mass temperatures of the wood frog Rana sylvatica

The efficacy of most biological processes is temperature dependent and, within physiological limits, on average, warmer is better. This axiom of biology has led to a wide range of adaptations for dealing with temperatures that are outside of an organism’s preferred temperature. Many pond-breeding amphibians lay their eggs during early spring, when water temperatures are near freezing. Communal nest-site selection has been proposed as a mechanism to increase developmental temperatures, and temperatures near the center of egg-mass aggregations are elevated relative to egg-masses on the aggregation’s periphery. It is unclear whether this spatial variation in temperature is due to concentration of metabolic heat, absorption of solar radiation, or both. Here, we explore finer scale spatial variation within egg masses of the wood frog Rana sylvatica, one of the earliest amphibians to breed during the North American spring. We compared peripheral and core temperatures of egg masses that were exposed either to 1) ambient sunlight from above, or 2) sunlight reflected by a mirror from below. We found that differences between core and peripheral temperatures were higher in the control than in the mirror treatment, but core and peripheral temperatures were statistically indistinguishable in both cases. Moreover, the difference in peripheral and internal temperatures increased significantly over the course of development. However, these trends were only significant in ambient sunlight and actually decreased in the mirror group. Our results suggest that the benefits of communal nesting are also experienced by individual egg masses, albeit to a lesser extent. In addition, the lack of effect in shaded egg masses suggests that the thermal advantage is tied to sun exposure and not due to concentration of metabolic heat.

Communal nesting and clutch characterization of Lygodactylus klugei (Squamata: Gekkonidae) from Brazilian semiarid region

Communal nesting and clutch characterization of Lygodactylus klugei (Squamata: Gekkonidae) from Brazilian semiarid region

Yolk corticosterone and progesterone levels in Greater Rhea (Rhea americana) eggs vary in a changing social environment.

Maternal hormones in avian egg yolks may signal and prepare offspring for the prevailing conditions. However, this adjustment requires some degree of flexibility in regulating yolk hormone deposition. The Greater Rhea (Rhea americana) has a particular mating system that combines mixed polygyny and polyandry, communal nesting, and exclusive paternal care of chicks. In this species, we previously found that yolk hormone deposition varies among eggs of different captive populations and could influence chicks' physiology and behavior. However, it is still unknown whether females can modify yolk hormone deposition in a changing social environment. Using a captive population of Greater Rheas, in this study, we quantified yolk hormone levels before and after a reduction in the number of females present in the population. We found that females deposited on average higher yolk corticosterone and lower yolk progesterone after the change in their social environment. Since corticosterone deposited into the yolk comes exclusively from the female's plasma, our results suggest that females had, on average, higher plasma corticosterone levels. The change in the number of females may increase the events of male-male competitions, courtships, and matings, leading to an increase of corticosterone in the females' plasma and then into their eggs. Since we previously found that higher yolk corticosterone and lower yolk progesterone were associated with the production of chicks that have an attenuated stress response, the present study results suggest that yolk hormone deposition is mediated by flexible mechanisms that could adjust development to the prevailing conditions.