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  • Review Article
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The consequences of rising temperatures for animal fertility

Nature Reviews Biodiversity (2026)Cite this article

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Abstract

Thermal stress reduces fertility and fecundity in animals at temperatures below lethal. Reproductive output is impaired across taxa under diverse heat-exposure regimes, with consequences for individual fitness, population persistence and ecosystem dynamics. This pattern holds across terrestrial and aquatic systems, with implications for conservation, livestock, aquaculture and human health. Yet these sublethal effects remain underrepresented in biodiversity forecasts. In this Review, we synthesize evidence for the biological mechanisms associated with thermally induced declines in fertility and fecundity, and assess how life history and exposure regime can shape thermal sensitivity. Fertility-based thermal limits can predict species distributions and extinction risk better than survival-based measures, albeit tested across a limited taxonomic range. Evolutionary responses to fertility loss under warming seem constrained but increased mutational variation, local adaptation and hybridization might increase fertility resilience. Key research priorities include broader taxonomic evaluation of both evolutionary potential and ecological outcomes under more sophisticated conditions, assessing how fertility is affected when different environmental stressors interact, and understanding how community and ecosystem dynamics will change if fertility-sensitive taxa either shift distributions or go extinct. Recognizing and addressing fertility-based vulnerability is essential for anticipating biodiversity change and designing more effective responses to climate impacts.

Key points

  • Elevated temperatures can decrease reproductive output at temperatures below those causing mortality; hence an understanding of this phenomenon is critical for predicting organismal responses to climate change, including downstream effects on humans

  • Variation in thermal vulnerability within and between species is shaped by how temperature affects the fertilization environment experienced by gametes, the interaction between the magnitude, duration and frequency of heat exposure, and by the relative sensitivity of each sex and life-history stage; additional complexity arises under multiple stressor conditions and from uncertainty as to whether phenotypic plasticity aids or exacerbates responses

  • Heat-impaired reproductive output alters predictions of climate change impacts on species distributions and population persistence; however, evidence for ecological and evolutionary consequences is taxonomically limited, and scenarios involving shifting species interactions and community-level variation in thermal fertility sensitivity are unexplored

  • The potential for adaptive increases in reproductive heat tolerance depends on heritable genetic variation, sex-specific genetic architecture and linked traits, and temperature-dependent DNA mutation; yet evidence for adaptive responses is sparse, limited by a small number of studies in few taxa

  • Recognizing fertility-based vulnerability is essential for assessing the effects of climate change on populations, species, communities and ecosystems, including subsequent ecosystem service effects; improved taxonomic coverage under more complex conditions is required to anticipate ecological and evolutionary consequences and responses across scales

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Fig. 1: Temperature effects on reproductive output are variable and taxonomically widespread.
Fig. 2: Generalized schematic of processes of animal reproduction, identifying the timing and mechanism of heat-stress effects, where evidence shows abnormal processes under heat stress compared to benign conditions.
Fig. 3: The ecological relevance of thermal fertility limits.

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Acknowledgements

The authors thank the European Society for Evolutionary Biology for funding the Special Topics Network (STN) on The Evolutionary Ecology of Thermal Fertility Limits and the members of the STN, whose work and collaboration on this topic we value immensely. The authors thank the Vetenskapsrådet (grants 2022-03116 and 2018-04598 to R.R.S.), the Natural Environment Research Council (grant NE/X011550/1 to L.D.), the Biotechnology and Biological Research Council (grant BB/W016753/1 to A.B. and R.R.S. and grant UKRI1927 to A.B., R.R.S. and C.F.), and a Heisenberg fellowship from the Deutsche Forschungsgemeinschaft (grant FR 2973/11-1 to C.F.) for funding support.

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Authors and Affiliations

  1. Department of Zoology, Stockholm University, Stockholm, Sweden

    Rhonda R. Snook

  2. Faculty of Biological Sciences, University of Leeds, Leeds, UK

    Amanda Bretman

  3. Department of Evolution, Ecology and Behaviour, University of Liverpool, Liverpool, UK

    Liam R. Dougherty

  4. Institute for Zoology, Martin-Luther University Halle-Wittenberg, Halle, Germany

    Claudia Fricke

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  1. Rhonda R. Snook
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  2. Amanda Bretman
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  3. Liam R. Dougherty
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  4. Claudia Fricke
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All authors contributed equally to all aspects of the manuscript.

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Correspondence to Rhonda R. Snook.

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Supplementary information

Glossary

Acclimation

Physiological adjustment to an increase in temperature, in particular when longer-term exposure to a mildly stressful temperature increases tolerance of a subsequent higher temperature.

Adaptive introgression

Where genes from outcrossing to other populations or species improve fitness under novel conditions.

Antagonistic pleiotropy

When a gene/allele has beneficial effects in one/some traits but negative effects for other traits.

Critical thermal limit

The upper (CTmax) or lower (CTmin) temperature at which critical biological function (often measured as motor control or coordinated movement) is lost, or death occurs.

Critical thermal maximum

The highest temperature at which a physiological function is lost (often measured as motor control or coordinated movement), or death occurs.

Cryptic genetic variation

Genetic variation that is normally masked, having little to no effect on the phenotype, but that is exposed under stressful conditions, thereby putatively available to fuel adaptation.

Evolutionary rescue

Selection for evolutionary change enabling individuals to survive under stressful or novel conditions, sufficiently rapidly to save a population from extinction.

Fecundity

Number of eggs or viable offspring produced by an individual. (We note that in the medical literature the definitions of fertility and fecundity can be switched in meaning compared to those used in most non-human contexts. When fecundity is measured as the number of eggs produced, then the effect of heat treatment on offspring survival is not conflated. However, in many taxa, only offspring number is reported. In cases in which a paper reports offspring number, we included it in our review only when the parents, and not offspring, were placed under heat stress. This limits the potential confounding effect of heat stress on survival of offspring that is separate from the effect on the parents).

Fertility

Ability to produce viable offspring. (We note that in the medical literature the definitions of fertility and fecundity can be switched in meaning compared to those used in most non-human contexts).

Hardening

A period of elevated temperature that improves performance under subsequent higher (extreme) temperatures.

Heat-shock proteins

A family of proteins found in virtually all living organisms, produced by cells in response to exposure to high or low temperatures, as part of the heat-shock response; many heat-shock proteins function as chaperone proteins, stabilizing the structure of other proteins that are sensitive to heat stress.

Heatwave (or heat wave)

A meteorological term for a sustained period of abnormally warm weather that lasts for multiple days, measured relative to the normal climate for a given location and time of year. (We note that there is no universally agreed definition for the duration and severity of a heatwave, but as a rough estimate, most definitions require three to five consecutive days of temperatures 5 °C or more above the seasonal average. Many studies attempt to mimic heatwaves to make their temperature manipulations more ecologically realistic).

Infertile

Inability to produce viable offspring; can specifically mean inability to produce gametes.

Lethal temperature

The temperature at which there is, for example, 80% (LT80) or 50% (LT50) mortality in a population or set of experimental organisms, often measured similarly to a lethal dose response.

Local adaptation

Divergent selection leading to local populations having higher relative fitness under local environmental conditions.

Microclimate

A local set of atmospheric conditions that differ from those in the surrounding areas, of interest to thermal biologists because of the potential to provide protection from temperature extremes; for example, animals may be able to shelter from high temperatures in burrows or shaded areas that remain cool.

Operational sex ratio

The ratio of males to females within the mating pool (those animals fertile and ready to mate).

Phenotypic plasticity

The ability of a genome to produce multiple phenotypes depending on the environment.

Polyspermy

Entrance of more than one sperm into the egg (typically lethal).

Reinforcement

Natural selection for reduced reproductive costs for hybrid reproduction increases reproductive isolation.

Reproductive interference

When reproductive isolation is incomplete and heterospecific individuals still engage in mating/reproductive activities, resulting in a fitness reduction for at least one of the interacting partners.

Reproductive output

A holistic term incorporating both fertility and fecundity.

Secondary contact

Populations diverge during a period of geographic isolation but then come back into contact, increasing the opportunity for gene flow.

Sterile

Inability to produce viable offspring; can specifically mean inability to produce gametes.

Thermal death time

The integration of stress intensity (temperature) and exposure time. (We note that through cumulative effects, organisms may suffer more from a mildly stressful high temperature experienced for a long time than from an extreme temperature experienced for a short time. Because the relationship is exponential, a small increase in temperature may lead to a large decrease in tolerance time).

Thermal fertility limit

The temperature at which individuals become (at least temporarily) sterile (scored as a binary 0/1 outcome); as for lethal temperature, it is measured as TFL80 or TFL50, the temperature at which either 80% or 50% of individuals in a population or set of experimental organisms are infertile.

Thermal performance curve

Graphical representation of the relationship between temperature and the value of a biological trait (for example, how activity, climbing ability or offspring production change over a range of temperatures).

Thermal safety margin

The difference between the thermal limit and the maximum ambient temperature experienced in the environment.

Thermal sensitivity of fertility

The relationship between temperature and reproductive output; the number of offspring produced or proxies thereof (such as sperm velocity, follicle number) are a measure of fertility rather than a binary score (0/1) of fertility.

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Snook, R.R., Bretman, A., Dougherty, L.R. et al. The consequences of rising temperatures for animal fertility. Nat. Rev. Biodivers. (2026). https://doi.org/10.1038/s44358-026-00142-4

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