Dalia A. Conde

The adult sex ratio (ASR; the proportion of males in the adult population) is a key demographic trait that varies widely across species and is linked to mating systems and sexually selected traits. Previous research on wild tetrapods showed that ASR is associated with genotypic sex determination system: species with male-heterogamety (XY sex chromosome system) tend to have more female-biased ASRs than species with female-heterogamety (ZW system). If genotypic mechanisms of sex determination are involved, these may also shape ASRs in controlled environments. Here, we test the latter proposition using demographic data from 1,192 zoo-housed reptile, bird, and mammal species. Across amniotes, ASRs in captive populations follow patterns similar to those observed in the wild: XY species tend to have more female-biased ASRs than ZW species. Captive mammals and birds showed similar ASR biases as found in wild populations, although in reptiles, the clade with diverse sex determination systems, the ASR did not differ significantly between XY and ZW species. Instead, reptiles with temperature-dependent sex determination (TSD) differed from reptiles of both genotypic sex determination groups. Together these results support that non-ecological processes linked to sex determination systems contribute to ASR patterns both in captive and wild animals.

Reproduction is hypothesized to constrain lifespan^1,2 and contribute to sex differences in ageing^3,4,5. Various sterilization and contraception methods inhibit reproduction, but predictions differ for how these influence survival, depending on sex⁵, how sex hormones are affected⁴ and species life history⁶. Here, using data from mammalian species housed in zoos and aquariums worldwide, we show that ongoing hormonal contraception and permanent surgical sterilization are associated with increased life expectancy. These effects occur in both males and females, although the sexes are differently protected from specific causes of death. Evidence of improved survival in males is also restricted to castration, with stronger effects occurring after pre-pubertal surgery. Complementary meta-analyses of published data reveal improved survival with sterilization across vertebrates and increased healthspan in gonadectomized rodents. Improved survival occurs in laboratory and wild environments, and with female sterilization approaches that either remove the ovaries or leave them intact. Reported increases in survival in castrated men^7,8,9 resemble the effects in other species, whereas survival of women is slightly decreased after permanent surgical sterilization. Thus the hormonal drive to reproduce constrains adult survival across vertebrates, regardless of the environment in which an animal resides.

Birth sex ratio biases can amplify extinction risks, especially in small, zoo-maintained populations which is of particular concern in species under threat of extinction. Thus, understanding the drivers of such biases is critical for conservation outcomes. We analysed birth records from 129 avian and 324 mammalian species in zoos worldwide between 1980 and 2021. Using Bayesian phylogenetic models, we found a phylogenetic signal in birth sex ratios (BSR), with substantial variation across clades. Penguins, falcons, and parrots showed slightly male-biased BSRs; ungulates showed female-biased BSR, and primates male-biased BSR. Across birds, variation in BSRs was predicted by sexual size dimorphism and clutch size, whereas in mammals, mating system was the main predictor of BSR. We identified 30 conservation flagship species with significantly biased BSRs, raising concern for the demographic sustainability of their captive populations. These results highlight the role of both evolutionary history and life-history traits in shaping sex ratio variation across taxa. Our findings underscore the importance of integrating phylogenetic and biological predictors into conservation planning and breeding program design. They also call for further research into the biological and management processes—that include sexual selection, parental investment, housing, and sexing practices—that may contribute to sex ratio variation in zoo populations.

Zoos and aquaria in the European Union (EU) can play a crucial role in the conservation of EU species, as they currently hold nearly half (49%) of EU terrestrial vertebrates. In this study, we analyzed the species composition and population sizes of EU zoos and developed a framework to prioritize recommendations for additional ex-situ and in-situ interventions for 277 at-risk EU species. Our results showed that EU zoos currently hold 39% of threatened EU species, 27% of EU endemic species, 62% of EU species vulnerable to climate change, 20% of EU species listed by the Alliance for Zero Extinction (AZE), 25% of Evolutionary Distinct and Globally Endangered (EDGE) EU species, while only 5% are subject to ex-situ conservation. Using our framework, we found that additional captive breeding was recommended for 60-61%% of species while expanding protected areas was recommended for only 2–22%, as 217 out of 277 species already met habitat protection targets. Both interventions were recommended for up to 20% of species, while the remaining 18% required no interventions because captive populations and habitat protection fully met targets. Our flexible framework can support more effective integrated conservation planning decisions for EU species and help identify target species for further in-depth assessment by the IUCN Ex-situ guidelines.

An intense public debate has fuelled governmental bans on marine mammals held in zoological institutions. The debate rests on the assumption that survival in zoological institutions has been and remains lower than in the wild, albeit the scientific evidence in support of this notion is equivocal. Here, we used statistical methods previously applied to assess historical improvements in human lifespan and data on 8864 individuals of four marine mammal species (harbour seal, Phoca vitulina ; California sea lion, Zalophus californianus ; polar bear, Ursus maritimus ; common bottlenose dolphin, Tursiops truncatus ) held in zoos from 1829 to 2020. We found that life expectancy increased up to 3.40 times, and first-year mortality declined up to 31%, during the last century in zoos. Moreover, the life expectancy of animals in zoos is currently 1.65–3.55 times longer than their wild counterparts. Like humans, these improvements have occurred concurrently with advances in management practices, crucial for population welfare. Science-based decisions will help effective legislative changes and ensure better implementation of animal care.

Overexploitation is a major threat to biodiversity and international trade in many species is regulated through the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). However, there is no established method to systematically determine which species are most at risk from international trade to inform potential trade measures under CITES. Here, we develop a mechanism using the International Union for Conservation of Nature’s Red List of Threatened Species to identify species that are likely to be threatened by international trade. Of 2,211 such species, CITES includes 59% (1,307 species), leaving two-fifths overlooked and in potential need of international trade regulation. Our results can inform deliberations on potential proposals to revise trade measures for species at CITES Conference of the Parties meetings. We also show that, for taxa with biological resource use documented as a threat, the number of species threatened by local and national use is four times greater than species likely threatened by international trade. To effectively address the overexploitation of species, interventions focused on achieving sustainability in international trade need to be complemented by commensurate measures to ensure that local and national use and trade of wildlife is well-regulated and sustainable.

Although cryobanking represents a powerful conservation tool, a lack of standardized information on the species represented in global cryobanks, and inconsistent prioritization of species for future sampling, hinder the conservation potential of cryobanking, resulting in missed conservation opportunities. We analyze the representation of amphibian, bird, mammal, and reptile species within the San Diego Zoo Wildlife Alliance Frozen Zoo® living cell collection (as of April 2019) and implement a qualitative framework for the prioritization of species for future sampling. We use global conservation assessment schemes (including the International Union for Conservation of Nature (IUCN) Red List of Threatened Species™, the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), the Alliance for Zero Extinction, the EDGE of Existence, and Climate Change Vulnerability), and opportunities for sample acquisition from the global zoo and aquarium community, to identify priority species for cryobanking. We show that 965 species, including 5% of all IUCN Red List “Threatened” amphibians, birds, mammals, and reptiles, were represented in the collection and that sampling from within existing zoo and aquarium collections could increase representation to 16.6% (by sampling an additional 707 “Threatened” species). High‐priority species for future cryobanking efforts include the whooping crane (Grus americana), crested ibis (Nipponia nippon), and Siberian crane (Leucogeranus leucogeranus). Each of these species are listed under every conservation assessment scheme and have ex situ populations available for sampling. We also provide species prioritizations based on subsets of these assessment schemes together with sampling opportunities from the global zoo and aquarium community. We highlight the difficulties in obtaining in situ samples, and encourage the formation of a global cryobanking database together with the establishment of new cryobanks in biodiversity‐rich regions.

Overexploitation is a major threat to biodiversity and international trade in many species is regulated through the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). However, there is no established method to systematically determine which species are most at risk from international trade to inform potential trade measures under CITES. Here we develop a mechanism using the International Union for Conservation of Nature Red List of Threatened Species to identify species that are likely threatened by international trade. Of 2,211 such species CITES includes 59% (1,307 species) leaving two-fifths overlooked and in potential need of international trade regulation. Our results can inform deliberations on potential proposals to revise trade measures for species at CITES Conference of the Parties meetings. We also show that for taxa with biological resource use documented as a threat, the number of species threatened by local and national use is four times greater than species likely threatened by international trade. To effectively address the overexploitation of species, interventions focused on achieving sustainability in international trade need to be complemented by commensurate measures to ensure that local and national use and trade of wildlife is well-regulated and sustainable.

Is senescence inevitable and universal for all living organisms, as evolutionary theories predict? Although evidence generally supports this hypothesis, it has been proposed that certain species, such as turtles and tortoises, may exhibit slow or even negligible senescence—i.e., avoiding the increasing risk of death from gradual deterioration with age. In an extensive comparative study of turtles and tortoises living in zoos and aquariums, we show that ~75% of 52 species exhibit slow or negligible senescence. For ~80% of species, aging rates are lower than those in modern humans. We find that body weight positively relates to adult life expectancy in both sexes, and sexual size dimorphism explains sex differences in longevity. Unlike humans and other species, we show that turtles and tortoises may reduce senescence in response to improvements in environmental conditions.