A pair of zebra finches. Credit: Jim Bendon

When it comes to extreme heat, zebra finches should theoretically have it handled. The birds are native to Australia’s hot, arid interior, which experiences intense summer temperatures. But new research suggests heat waves take a toll on the birds’ reproductive health, misshaping male finches’ sperm and rendering them unviable. It’s a discovery that has worrying implications for the future of birds in a warming world.

High temperatures were already known to reduce the quality of mammalian sperm, but the relationship between heat and male fertility in birds isn’t well-established. However, zebra finches (and other birds in interior Australia) ramp down their breeding activity during the hottest months of the year, suggesting that heat imposes a reproductive cost—one that the birds avoid.

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To confirm that reproductive cost and gain additional insight, a team of researchers from Australia and Norway took male zebra finches and kept them in climate-controlled conditions in the lab for two weeks, exposing them to one of two daytime heat treatments: a typical 30 degrees Celsius (86 degrees Fahrenheit), or a toasty, heat wave scenario of 40 degrees Celsius (104 degrees Fahrenheit). The researchers extracted finch semen samples, and counted how many sperm were mobile, or were abnormal in shape (indicating developmental problems). They also recorded the temperature of the males’ cloaca—birds’ catchall orifice that includes the genitals and the ends of the digestive and urinary tracts. This revealed the temperatures the testes were being exposed to. After a twelve day recovery period at cooler temperatures, the team repeated the experiment, but switched the birds between the 30 and 40 degree treatments.

Their results—published today in Proceedings B of the Royal Society—show that sperm suffers when the mercury rises. In heat wave conditions, the proportion of normal, undamaged sperm fell over the two week period, dropping by more than 30 percent in the birds that got the 40 degree Celsius treatment first. Even when the birds had a chance to chill out after baking, their sperm quality didn’t recover to pre-heat wave levels.

The persistence of this reproductive ruination weeks afterwards may have to do with the temperature of the males’ cloacas, which rose by 2 degrees Celsius during the experiment, and stayed there through the recovery period. While the heat had less of an impact on sperm swimming speed, the deformation of sperm was clear and long-lasting.

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Malformed sperm aren’t great at navigating to the females’ eggs, so anything that reduces the number of competent sperm also decreases fertility. Heat-related sperm degradation causing infertility had been seen before in chickens, but by tracking sperm quality over realistically fluctuating temperatures, the new research places this effect in the context of naturally-occuring heat waves.

If this vulnerability of sperm to extreme heat is widespread among birds, Earth’s future climate may be hard on them. By the end of the century, brutal heat waves are predicted to be much more commonplace. This may be exceptionally true in Australia, which is already feeling the burn, and just this season endured nightmarish, bat-frying temperatures. It’s not yet known if heat deforms other species’ sperm, but since zebra finches are already adapted to hot environments with regular heat waves, yet still show sperm vulnerability to heat, the authors note that it’s possible that birds in temperate or cold regions have even less ability to deal with future heat waves. Perhaps they’ll find a way to avoid cooking their sperm, like modifying their breeding schedule—perhaps not.

Either way, heat-induced infertility would add to a long, accumulating list of disruptions and challenges climate change has in store for birds, including habitat loss, shifting ranges towards the poles or higher in elevation, and more frequent and intense wildfires, which destroy and fragment habitat and food resources. For some birds, there are even reproductive consequences of warming climate outside of sperm damage. For example, fowl-like, mound-nesting megapode birds in Australasia are the only birds whose sex is determined by egg-stage environmental temperature rather than genetics; increasing temperatures could disastrously skew sex ratios, as they are currently doing with sea turtles, which use the same sex determination system.

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In a world where male birds have less viable sperm for more parts of the year, bird populations may be less able to cope with the sundry other obstacles of the Anthropocene, which may translate into dire conservation consequences.

Jake Buehler is a Seattle area science writer with an adoration for the Tree of Life’s weird, wild, and unsung—follow him on Twitter or at his blog.