Hot weather banjaxes bird bollocks
Australian research finds heatwave reduce fertility in zebra finches, signaling a long-term challenge to fitness. Jeff Glorfeld reports.
Across the globe, animals are increasingly being exposed to more frequent and intense heat waves. A team of scientists from Australia’s Macquarie University and University of Sydney, and the University of Oslo in Norway, is investigating how these rising temperatures may be influencing the reproductive success of birds.
In findings published this week in the journal Proceedings of the Royal Society B, the researchers note that heat stress and high environmental temperatures have been linked to male infertility and reduced sperm quality in domestic poultry, but the impact of extreme temperatures on avian sperm quality has not been previously considered.
Using the common zebra finch (Taeniopygia guttata), found across most of mainland Australia, as study subjects, they discovered that birds exposed to 40 degree Celsius temperatures had poor-quality sperm, and that the quality deteriorated the longer the birds were exposed to the heat.
The team, led by Laura Hurley from Macquarie’s department of biological sciences, say this could affect male birds' fertility, and that the problem may become worse as global temperatures rise and heat waves occur more often.
Environmental temperature variation affects organisms across all life stages, the report says, influencing physiology, behaviour and global distribution. In birds, ambient temperature can have an impact on breeding cycles, incubation behaviour and reproductive success.
In some taxa, temperature can also affect sperm function and fertilising ability. For example, mammals exposed to high environmental temperatures exhibit reductions in sperm mobility and an increase in sperm defects.
The researchers found the zebra finch to be a “model system for avian sperm biology and sperm competition”, noting that, “In the wild, they are opportunistic breeders, capable of breeding year round given suitable environmental conditions”.
Despite being adapted to the hot, arid interior of Australia, the report says, atmospheric temperatures of over 40 degrees are likely to be physiologically stressful for the finches and, while mild hyperthermia is tolerated, a body temperature of 46 degrees is lethal.
The study used temperature data collected at Fowlers Gap, in New South Wales, a research station about 100 kilometres north of Broken Hill, the site of the longest-running study of wild-breeding zebra finches, and where temperatures reach or exceed 40 degrees about 18 days a year, with a maximum recorded atmospheric temperature of 46.8.
Although heat waves have been defined in many ways, the report says, “we consider a heatwave to be any period when atmospheric temperature exceeds 40 on two or more consecutive days”.
Using domesticated zebra finches in a laboratory, the researchers investigated the impact of extreme environmental temperatures on avian sperm function. They also examined how the duration of heat exposure and prior exposure to high, but not extreme, temperatures might affect potential temperature-related changes in sperm function.
In the final stage, they examined whether sperm function was restored when birds were returned to milder ambient temperature conditions.
In the wild, finches would experience an average daily minimum temperature of 22.6 degrees,
an average maximum of 42.8, and a 24-hour average of 33.6. In this experiment the daily minimum was 23, the maximum 40, and the 24-hour average was 28.7.
“Thus,” the scientists explain, “our experimental conditions were ecologically relevant.”
The report found that “extreme environmental temperatures resulted in decreased sperm quality and increased cloacal (body cavity) temperature in male zebra finches”.
“We observed a strong decline in the proportion of sperm with normal morphology, and
these proportions declined with continued exposure to 40-degree conditions,” the report states.
It continues: “We were unable to determine whether sperm damage was the direct result of high temperatures or the indirect result of systemic physiological stress induced by high temperatures. Future studies are required to tease out these options and to further investigate both the long-term effects of heat on sperm and the potential for acclimation to elevated environmental temperatures.
“Nonetheless, our findings indicate that ecologically relevant, extreme temperatures have the potential to impact sperm quality and function.
“Our findings suggest extreme environmental temperatures may result in a limited supply of functional sperm, and thus may also impact male fertility in passerine birds.”
In a general sense, the researchers say, their results might help to explain the recent finding of reduced avian breeding activity across the hotter parts of Australia, during the hottest parts of the year, an outcome that has yet to be explored in other regions of the world.
“More importantly, functional infertility may be an important selective pressure in systems where females mate with multiple males,” the researchers write.
“Under conditions of sperm competition, sperm numbers are an important determinant of fertilisation success. Thus, males that are best able to mitigate sperm damage resulting from extreme temperatures are likely to be superior competitors for the fertilisation of ova during hot conditions.”
Thus, they conclude, “heatwaves may have profound ecological and evolutionary consequences for the reproductive biology and behaviour of birds. An important next step will be to determine if the negative effects of temperature have consequences for male fitness and whether similar results are observed in wild populations under natural heatwave conditions.
“We suggest that temperature-induced or temperature-associated reductions in sperm quality may be an important biological consequence of the anthropocene as global temperatures rise and the frequency of extreme heat events increase.”