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Heat Waves Stress Sheep and Koala Fertility

Western Sydney University researchers have provided an insight into the physiological stress that summer heatwaves place on wildlife and livestock.

In a study published in PLoS ONE, Dr Edward Narayan analysed the physiological stress of Australian merino sheep during an artificial insemination breeding program in Dubbo, and found that the body temperature of the ewes was a significant factor behind reduced embryo survival.

Faecal samples and body temperature measurements were collected from 15 superovulating donor merino ewes during the 2015–16 summer, when ambient temperatures reached up to 40°C. “Ewes that had higher recorded temperatures had a significantly lower percentage of transferable embryos,” Narayan says.

These results suggest a plausible link between heat stress, physiological stress and reduced fertility in merino ewes. “Given that the Australian merino sheep industry is heavily reliant on the breeding efficacy of its ewes in order to maintain a profitable business, further research is required to determine the full extent that hot Australian climates could be impacting reproductive output.”

As part of his work in the Stress Lab, Narayan also investigates the key environmental stresses that are impacting on the mortality rate of Australia’s native fauna, in particular koalas. “Like the merino ewe, koalas also experience chronic stress as a result of extreme heat, and research indicates that it may also be affecting their ability to breed,” he says.

“In dry or high-temperature environments, koalas must relocate to habitats that contain free standing water and eucalypts with high levels of leaf moisture; otherwise they will suffer from dehydration.

“Once a koala climbs or falls down and is on the vegetation floor, it is vulnerable to more stressors – like dog attack or vehicle collision. Also, for an animal that usually spends 19–20 hours a day resting or sleeping, this activity also incurs a cost on the koala’s energy levels.”

When koalas are stressed they release the stress hormone cortisol. Narayan says high cortisol levels in koalas have been associated with diminished reproduction.

“Like the merino ewe, the breeding capacity of koalas is also impacted when we experience prolonged periods of higher than average temperatures,” he says. “When it comes to koala reproduction, extreme heat might not mean that Australia experiences the same losses in its agricultural sector – but it might experience an even greater loss – of one of its national treasures.”