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Global warming: Animals mate more in warmer weather

Do climate changes make it easier or harder for animals and plants to reproduce? There is no simple answer, but several studies suggest that animals mate more frequently in warmer weather. However, that doesn't necessarily mean they have more offspring, or that the offspring's chances of survival are good. How does that add up?

By Birgitte Svennevig, , 3/22/2024

Spring arrives earlier, days become long and mild. If you’re an animal, you need less energy to stay warm and find food than in cooler springs, leaving you with surplus energy for extra ... sex.

Perhaps that's the reason why animals mate more in warming climates. Biologists believe they have observed several examples of this, recently confirmed by a comprehensive meta-study from 2022. Researchers Natalie Pilakouta and Anais Baillet from the University of Aberdeen reviewed 53 studies covering 22 species, ranging from frogs, butterflies, and geckos to birds. Their conclusion: Global warming may not have as catastrophic an impact on animals' ability to reproduce as feared – quite the opposite.

However, researchers also emphasize that no one knows if increased mating intensity leads to more offspring, and if so, how viable those offspring might be.

The ambush bug, Phymata americana, lives in Central and North America, where it lies in wait for smaller insects in flowers. (©Copyright Robert Webster)

Heat stress can lead to development of fewer organs for storing sperm in dung fly females of the species Scathophaga stercoraria. (©LeCardinal, under licence CC-BY 2.0)

Less rainfall in the Spanish national park Doñana led to reduced mating activity in the park's populations of red deer (Cervus elaphus hispanicus). (©WWW.LUCNIX.BE)

The seagrass S. maritima grows along the southern and western coasts of Europe. Heat stress can prevent the male plant's reproductive organs from developing properly. (©Copyright Wikipedia)

When their ponds get colder, the calling of male spadefoot toads for females also slows down. (©Copyright JN Stewart)

Field pansies (viola arvensis), growing wild outside Paris, now produce 20% less nectar than 20 years ago. (©Copyright Wikipedia)

More mating also means more predators

- There are so many factors at play, and it can vary tremendously. Let's say warmth leads to increased mating intensity. But perhaps it also leads to more parasites, bringing more disease, says population biologist Owen Jones from the Department of Biology, SDU.

Another scenario could be that increased warmth leads to more mating and more offspring. Perhaps the warmer climate also provides more available food for the offspring, allowing them to thrive – or maybe the climate becomes too dry to support the required food for the new offspring.

Or, if animals don't need as much energy to stay warm and can use the saved energy for mating, it might apply to both prey and predators. In that case, the proportion of prey being eaten may remain unchanged.

The important time before the mating act

- I understand that people wish for us researchers to provide a simple explanation and some reliable forecasts for how easy or difficult it will be for animals and plants to reproduce in the face of climate change. But there is no simple answer. Everything interacts, everything is interconnected. However, even though there are many things we don't know about individual species and ecosystems, we know that, on an overarching level, things are changing, says Owen Jones.

While waiting for studies on how offspring born in a warmer world fare, there is growing insight into how both animals and plants react when it comes to reproduction - and, for animals, the initiation of it. In scientific terms, the prelude to the mating act is called mating latency, covering the period from when an animal spots a potential mate to when mating begins. During mating latency, animals spend a lot of energy on showcasing themselves and convincing the chosen one of their health and strength.

Some invest resources in producing colorful ornamental feathers or long horns or in croaking, roaring, singing, whistling, or stomping for the chosen one. Mating latency is time-consuming because it takes time for individual animals to send, receive, and assess messages and signals before deciding whether to attempt mating.

Heat makes beetle, deer and toad change behaviour prior to mating

Climate change can affect animals' mating latency. For example, the American beetle Phymata americana, which lurks in central and North American flowers waiting for other insects, spends less time on mating latency as the weather gets warmer, according to the meta-study by Pilakouta and Baillet. Perhaps –  researchers don’t know yet – it means that in warm weather, it's less choosy about selecting a mate.

A 25-year Spanish study tracking red deer (Cervus elaphus) in a Spanish national park found that less precipitation in the period led to poorer forage, resulting in delayed and less intense mating periods for the males.

A 22-year American study of spadefoot toads in the southwestern USA found that the ponds and waterholes where they mate have become colder over time. As a result, the males' calls have slowed down.

Plants also show new reproduction strategies 

Plant studies have shown that plant reproductive organs can react very quickly to warmer climates. For example, certain cereal crops experience a decline in the development of male plant reproductive organs during heatwaves, making them less effective in pollinating female flowers, according to a study from the University of Melbourne.

Similar findings have been observed in the salt marsh grass S. maritima, which grows along Europe's southern and western coasts. When exposed to heat stress, the male plant's reproductive organs develop poorly, and they don't protrude far enough from the flower head for effective pollination by wind.

However, some plants have swiftly adapted to new climatic conditions. A study from the French National Centre for Scientific Research examined field pansies (Viola arvensis) growing wild in fields outside Paris, finding that the plants on the same fields now produce 10% fewer flowers and 20% less nectar than 20-30 years ago. This coincides with them being visited by fewer insects than 20-30 years ago. According to the study's researchers, the field pansy is evolving towards self-pollination instead of relying on insects.

- On the one hand, it might look like a successful path to take. But the cost is that the plant population gets less genetic variation, and that can make it more vulnerable to changing environments and diseases, says Owen Jones.

Heat stress may lead to abnorm sperm

- Environmental change can sometimes drive rapid evolutionary changes in wildlife. This study is a reminder of the surprising effects climate change can have on species, says Owen Jones.

Temperature itself can also directly impact an animal's reproductive organs. For example, some fish and insect species produce more abnormally shaped and slower sperm when exposed to heatwave-like conditions in the laboratory. Females of the common dung fly, Scathophaga stercoraria, produce fewer of the organs used to store sperm. In banana flies, heat stress leads to reduced fertility in males but not in females.

Earth's living organisms have an outstanding ability to adapt to changing environments, especially concerning the most crucial aspect of survival as a species: the ability to reproduce. Only time will tell if they can adapt in a way that not only allows them to reproduce in a rapidly changing environment but also produce offspring viable enough to perpetuate the species.

Meet the researcher

Owen Jones is a population biologist and associate professor at Department of Biology. He is also affiliated with Center on Population Dynamics and SDU Climate Cluster.

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Editing was completed: 22.03.2024