Global Warming is Spoiling Sex

Partnership, Health
Global warming means that female collard flycatchers are changing what they find attractive in males. • Illustration by Bea R. Vaquero

Global warming means that female collard flycatchers are changing what they find attractive in males.
• Illustration by Bea R. Vaquero

Words by Dr Claire Asher
Illustrations by Bea R. Vaquero
This story is brought to you by our partner Koala
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As if sinking cities, scorching heatwaves and hailstones the size of golf balls weren’t bad enough, it seems climate change has another unwanted gift for life on earth – the climate cockblock.

Across the natural world, the warming climate is leaving ladies without a mate, changing the rules of the playbook, putting partners out of sync and just generally spoiling the mood. Climate change may even be sterilising whole populations.

A shortage of males
Reptiles have a biological quirk that is putting them at risk of losing one sex entirely. This is because they use temperature to determine the sex of their offspring, rather than doing so genetically, like humans and other mammals. For instance, sea turtle eggs incubated at below 29°C tend to develop into males, whereas above 29°C they develop into females.

Jacques-Olivier Laloë, a research fellow in marine biology at Deakin University in Victoria, Australia and colleagues collected data from the last century and found that green (Chelonia mydas), hawksbill (Eretmochelys imbricata) and leatherback turtles (Dermochelys coriacea) in the North East Caribbean have shifted towards increasingly female-biased hatchlings and predicted that by 2030 green turtle clutches would contain just 2.4 percent males. “This suggests that complete feminization and future collapse of this important turtle population is possible”, said Laloë.

But sea turtles have some tricks up their sleeves to adapt to climate change. Females could lay their eggs in cooler locations or nest earlier in the year. For example, a study led by Jonathan Monsinjon at Université Paris-Sud in Paris, France showed that Loggerhead turtles (Caretta caretta) populations in Brazil are nesting approximately seven days earlier for every 1°C increase in temperature. However Monsinjon calculated that nesting earlier would only compensate for air temperature increases predicted by 2100 in one out of seven sea turtle populations they modelled. “The six other populations studied will experience high rates of incubation failure and will produce almost 100 percent of females”, he said.

In some reptiles, the embryos themselves may even be able to influence their sex by shifting to a cooler part of the egg, but it’s not clear yet whether sea turtles possess this remarkable ability.

Even so, sea turtles may also need a helping hand from humans. If they “cannot adapt to warming temperatures it may be time to implement hands-on conservation management of sea turtle populations world-wide”, said Laloë. For instance, shading nests with palm leaves, cooling them with sea water, or moving them to cooler beaches, could help prevent the world’s sea turtles from running out of males.

Sea turtles are facing potential population collapse due to complete feminisation from global warming.  • Illustration by Bea R. Vaquero

Sea turtles are facing potential population collapse due to complete feminisation from global warming.
• Illustration by Bea R. Vaquero


Changing fashion
Climate change also appears to be altering the rules of the mating game for some species. For instance, in collard flycatchers (Ficedula albicollis), males with large white head patches used to enjoy their pick of the ladies, but a 1.5°C rise in average spring temperatures over the course of the 30-year study reversed this preference, making females more attracted to males with smaller head patches. This change in fashion happened because the size of a bird’s patch used to be strongly related to genetic quality and survival, but at higher temperatures large-patched males had lower survival and were no longer the best choice of father.

Some species are also becoming less faithful as the climate changes. For example, rapidly changing environments tend to encourage infidelity in normally monogamous bird species, as females hedge their bets about which males have the best genes for the changing climate.

Aurélie Cohas, behavioural ecologist at Claude Bernard University Lyon and colleagues have found that female alpine marmots are more likely to be unfaithful when spring arrives earlier because the melting snow means they have more freedom to move around and meet new potential mates. However, less winter snow reduces the survival of pups, which actually increases fidelity. Marmot pups stick around after they’re fully grown to help raise the next year’s litter and Cohas has found that female marmots are more likely to be unfaithful when they have more helpers around at home. “Snowier winters, by increasing the number and survival of pups, lead to more subordinates in subsequent years, which in turn leads to more extra-pair litters”, said Cohas.

How these opposing forces will shape the future mating system of the marmots is difficult to predict. “I would not bet on what’s going to happen, whether it’s going to be bad or whether it’s going to be good, what I know is it’s going to be different”, she said.

But it’s not all bad news, lower rainfall due to climate change is forcing dominant male elephant seals out of their normal foraging areas, giving subordinate males more opportunities to mate, and fruit flies kept at higher temperatures harm their mates less during sex.

Ecosystems out of sync
Climate change is causing spring to arrive early in temperate regions, where many plants and animals time their seasonal activities based on temperature. As the climate warms, temperate plants are on average flowering 1.9 days earlier each decade. This might not sound like much of a problem, but it could damage the delicate ecological relationships that allow plants to reproduce.

Early snowmelt has led to a mismatch between flowering of a Japanese plant (Corydalis ambigua) and the emergence of bumblebees from hibernation, resulting in lower fertilisation success for the plant, and solitary bees are no longer emerging early enough to catch the first blooms of the pasque flower (Pulsatilla vulgaris) in Germany. Thankfully, there is evidence that some pollinators are managing to keep pace with the advances of spring flowering.

Marine plants are also reproducing earlier in the year as a result of climate change. Microscopic marine algae known as phytoplankton bloom in the spring as the warming waters trap nutrients at the top of the ocean where there is also light available to fuel photosynthesis, and as a result their blooms have been shifting earlier in the year. Using satellite data and climate modelling, Steph Henson at the National Oceanography Centre in the UK and her colleagues calculated n that phytoplankton blooms will happen on average six days earlier by the end of the century.


Sadly, for most it won’t be as simple as just turning the A/C on.

Marmots are becoming less monogamous due to climate change.  • Illustration by Bea R. Vaquero

Marmots are becoming less monogamous due to climate change.
• Illustration by Bea R. Vaquero


At higher temperatures, the warmer water forms a stronger barrier with the cool, nutrient-rich waters below, meaning that the phytoplankton run out of food sooner and the bloom is smaller. This is particularly important because phytoplankton release oxygen into the atmosphere and absorb carbon dioxide. “Without phytoplankton, atmospheric CO2 levels would be about 50 percent higher than they are now”, Henson remarked.

Going sterile
Perhaps the most extreme potential impact of climate change is complete sterility. It has been known for centuries that warmth damages sperm; “that’s the reason why testicles exist”, said Tom Price, an evolutionary biologist at the University of Liverpool, UK. “It’s a pretty bizarre idea keeping a major important organ hanging outside your body in a little sack”, he laughed, but testicles protect sperm from harmful body heat.

Despite this, very few studies have tested the effect of high temperatures on fertility in wild populations. In the laboratory, high temperatures have been found to decrease sperm count in fruit flies, beetles, solitary wasps, corals, chickens, pigs, and cows, and tomato (Solanum lycopersicum) plants cultivated at 32°C or hotter produce less viable pollen.

Zebra finches (Taeniopygia guttata) exposed to an artificial 14-day heatwave over 40°C – a temperature not uncommon in their arid central Australian environment – produced a higher proportion of damaged sperm. Sperm with physical abnormalities increased from less than 20 percent at the start of the experiment to over 40 percent. “If it happens in … high-temperature species like zebra finches it’s very likely it’s going to be affecting lots of other organisms as well”, warned Price.

Some researchers are concerned that the warming climate may even be hampering our own fertility. For instance, data from sperm banks shows that the quality of donations is affected by season - summer donations have a lower overall sperm count and more deformed sperm. Alan Barreca, an economist at the University of California, Los Angeles, compiled 80 years of birth records for the USA and found that unusually hot summers were followed by fewer births the following spring.

This may mean that in the future people have fewer children, or that a greater percentage of children are conceived in the winter months and born in the hottest part of the summer, which Barreca warns is “bad for prenatal and infant health”, as well as being extremely uncomfortable for the mothers. “People ought to have the opportunity to have the family size that's right for them”, said Barreca, advocating for low-emissions air conditioning units as a potential solution in the future.

Before you start making plans to move to a cooler country or upgrade your air conditioning unit, there isn’t enough evidence to be sure yet whether climate change will hamper human reproduction in a major way. Price agrees that there is reason to believe high temperatures harm human sperm, but on the other hand “there are lots of people living in hot countries, working outside at really high temperatures and they’re still having kids”.

Sadly, for most it won’t be as simple as just turning the A/C on.

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Dr Claire Asher is a freelance science communicator with a background in evolutionary biology. Her writing has been published in New Scientist, BBC Earth, Mongabay, Science and Nature, among others.