Artificial lighting at night delays wild tammar wallaby breeding, potentially pushing the nursing marsupial moms out of sync with their peak season for food.
Tammar wallabies (Macropus eugenii) that live on the well-lit landscape of Australia’s largest naval base muddle the timing of their natural breeding season. Births peak in February — a month later on average than normal — then dip only to surge again in April, says zoologist Kylie Robert of La Trobe University in Melbourne, Australia. By the time these late-arriving joeys have grown to their most demanding stage some 250 days in the pouch later, the best grazing greened by winter rains is fading. Nursing moms and their joeys once got through this season thanks to all the irrigated lawns on the base. New irrigation rules, however, now leave the wallabies facing food shortages.
In contrast, on the same island about six kilometers away, on land that’s still dark at night, wallaby births peak in January. Blood chemistry analyses show that wallabies in the unlit wilds are responding to darkness cues more than their naval neighbors do, Robert and colleagues report September 30 in Proceedings of the Royal Society B.
“We have reams and reams and reams of lab data about the effects of light on reproduction,” says urban ecologist Travis Longcore of the University of Southern California in Los Angeles. “The important thing here is [that the researchers are] showing that it’s happening in the wild with the animals going about their normal behavior.”
At least one study in birds has shown that exposure to artificial light can push forward reproductive timing in the wild, Robert notes.
Light pollution began to seem a plausible explanation with the wallabies, too. For other reasons, Robert and her colleagues were trapping the marsupials on Garden Island, where the naval base is, and noticed the lag in birth dates. The team put collars with light sensors on 10 tammar wallabies, five hopping around the bush and five on the naval base. The five in the bush experienced some night light around the time of full moons but not much otherwise. The naval base “was lit up like a Christmas tree, as you can imagine, for security reasons,” Robert says. There, the sensors indicated wallabies experiencing 10 times as much light intensity as bush animals.
That difference also showed up in blood levels of melatonin, a compound that builds up in darkness and helps the animals pick up seasonal cues and regulate body rhythms. In 67 wallabies tested, nighttime melatonin levels in bush-dwelling animals were more than twice as high on average compared with the naval base wallabies.
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Lights on, lights off
Tammar wallabies living out in the bush with just the moon for night light have higher concentrations of melatonin (left chart, dark blue bar) than wild wallabies roaming free at a well-lit naval base nearby (lighter blue bar). Births among the bush wallabies peak early in January (dark blue bars) but the light-polluted wallabies aren’t in sync with their environment.
Credit: K.A. Robert et al/Proceedings of the Royal Society B 2015
That difference fits with what biologists have learned about reproduction in tammar wallabies. The first time a tammar wallaby mates, the very early fertilized egg stops dividing for months. When nights start lengthening after the Australian summer solstice (around December 23), the increase in darkness boosts melatonin, and the embryo starts developing again. When the wallaby gives birth to a peanut-sized joey in the early months of the year, the mom mates again within 24 hours. This newly fertilized egg soon suspends cell division — until the waning days after the next solstice kick up melatonin again. The melatonin helps animals reproduce in sync with the usual seasons.
To learn more how this scenario plays out in the two wallaby populations, evolutionary ecologist John Swaddle of the College of William & Mary in Williamsburg, Va., would like to know what would happen if some naval wallabies swapped places with bush ones. Robert hasn’t tried that, but she says that tammar wallabies sent to North American zoos changed their reproductive timing by as much as six months to fit their new continent’s day lengths.
Editor’s note: This story was updated on October 14, 2015, to correct how much higher nighttime melatonin levels were in bush wallabies compared with naval base wallabies.