A number of years ago now I completed a PhD on the impact of human disturbance on the ecology of tammar wallabies on Garden Island. One of the more interesting results as I saw it (although one of my markers strongly disagreed and thought it was trivial and should be removed from my thesis) was that the median birth date of the tammars that lived on the HMAS Stirling naval base was approximately 1 month later than those that lived in the bushland on the island. This might not seem to be a big deal, but you need to understand a bit about tammar reproduction to get why it matters.
Tammar wallabies have the longest embryonic diapause of any animal on earth. When female tammars give birth in late January they come into oestrus about 12 hours later and will mate. The embryo that results from this mating then goes into diapause until one of 2 things happen. If the pouch young that the female has in her pouch stops suckling, ie it dies, before the winter solstice, then the embryo is reactivated and will be born. After the winter solstice the embryo remains in diapause until it is reactivated by the trigger of reducing day length after the summer solstice.
So how do tammar wallabies keep track of day length. Basically the same way as all mammals do, through the hormone melatonin. Melatonin is a hormone produced by the brain, but only when it’s dark. So the body is able to track day length by the levels of melatonin production in the brain. The problem is that when night isn’t dark then these levels get all messed around and the animals aren’t able to track day length properly.
So the hypothesis from my PhD was that the lighting around the naval base was impacting the tammars ability to sense the reduction in day length after the summer solstice which was then delaying their breeding. A lot of work, primarily driven by a Kylie Robert, has now shown this to be the case. Our paper which has just been published in the Proceedings of the Royal Society Part B: Biological Sciences shows that the tammars on the HMAS Stirling naval base are exposed to night time light levels an order of magnitude higher than those in the bush and the completely miss the cues of twilight due to the lighting on the base. This then results in higher melatonin levels during the critical few weeks after the summer solstice and a delay in the reactivation of embryos in diapause.
So why does this matter?
From a scientific pint of view this is the first time that anyone in the world has found a physiological impact of artificial light at night on wild mammals.
Secondly the delay in breeding has the potential to impact the survival of the young tammars. Tammars have this highly synchronised breeding system to time the period of highest lactation demand and the weaning of their young to occur when food availability is the highest from September through to November. By breeding a month later this may affect the survival of the juvenile wallabies over their first crucial summer. At the time of my PhD research this wasn’t as issue as their were lots of irrigated grasses on the naval base which gave the animals year round food. However, since then the road verges on the naval base are no longer irrigated and the ovals that the tammars fed on so much are now properly fenced to exclude them, so the impact of this breeding change is likely to be far greater.
You can find the paper which is entitled Artificial light at night desynchronizes strictly seasonal reproduction in a wild mammal, here. The paper is free to download until October 31, after that you’ll need a subscription or as always get in touch and I’ll be happy to send you a copy.