Mother Wallabies Are Delaying Births Due to Bright Lights

Marsupials exposed to artificial light had their babies a month later than those that spent nights solely lit by the stars and moon

An adult tammar wallaby on Kangaroo Island, Australia. (FLPA/Bernd Rohrschneider/FLPA/Corbis)
smithsonian.com

An abundance of artificial light already confuses migrating birds, sends baby turtles marching to their doom and torments countless city-dwelling humans with insomnia and other ill effects. Now, it seems all that illumination also messes with wallabies’ baby-making habits.

Wallabies are cuddly, pint-sized kangaroo cousins that hop around the forests and plains of Australia and New Guinea (and one Irish island). The animals are normally quite particular when it comes to reproduction. They mate in October, but the female's body takes its cues from the sun, holding the embryo dormant until after the summer solstice, which in the Southern Hemisphere arrives in December.

Decreasing daylight then triggers the female's body to produce melatonin, a hormone that helps regulate cycles of sleep and wakefulness. That in turn ups the levels of progesterone, which activates early fetal structures called blastocysts. Babies are delivered in late January—exactly six weeks after the solstice, when temperature and day length are just right.  

Most studies that examine light pollution’s effect on mammals either take place in the lab or consist solely of behavioral observations in the field. The Australian and German authors of this new study decided to go a step further, however, watching animals in their natural habitat but also collecting biological measurements.

For five years each December to February, the team tuned in to the activities of two populations of tammar wallabies on Garden Island, a narrow spit of land near Perth. One of the wallaby populations lived in the rugged bush at the northern tip of the island, far from any hints of human light. The other group lived around a massive light-blasting naval base.

The researchers captured five females from each population and attached small collars, which took almost constant readings of light levels and GPS coordinates. The team also obtained blood samples from nearly 70 females, which they measured for melatonin levels. Finally, they monitored the birth schedules of nearly 300 babies delivered over the five-year period.

As they report today in Proceedings of the Royal Society B, light does seem to be messing with the wallabies’ birthing schedules. Those mothers exposed to the base’s constant glow had a tougher time responding to the shortening days, the researchers found. Missing this vital natural cue, their bodies produced significantly less melatonin than the mothers in the bush, which enjoyed the soothing darkness of natural night.

For the naval mothers, the biological changes had a significant effect not only on their bodies but also on their babies: On average, they delivered their young a full month later than mothers in the north.

Whether those changes will lead to lower rates of baby survival is unknown. During the study, the naval base watered and gardened a large area of greenery, which the wallabies often munched on.

However, the base recently decided to stop watering its lawn. The researchers suspect that, until now, constant access to grass likely buffered any problems caused by babies showing up late in the season. With the lawn now gone, the adults might begin facing a food shortage, which could impact infant survival.

This problem is not going away—for wallabies or for wildlife in general. Artificial light is one of the fastest growing types of pollution, the researchers write, increasing at a rate of about 6 percent each year.

How light pollution affects other species and populations around the globe will require further studies, but the authors suggest that “profound impacts” are likely already playing out around the world. 

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