Why would two stubby-legged, blue-tongued Australian reptiles want to stay together not just for a mating season, but for decades? A 31-year study of the reptiles has suggested an answer. While newly formed couples are still getting to know each other, lizards in long-term relationships can start mating earlier in the season. And dispensing with the foreplay might give them a reproductive advantage over their casually dating neighbors. Tiliqua rugosa is a species of blue-tongued skink that's also called the shingleback, bobtail, pinecone, or sleepy lizard. People may have had time to give the lizards so many names because they don't move very quickly. In fact they seem prepackaged for predators, with their sausage bodies and tiny appendages. Aside from their armored skin, the only thing going for them is that their tails sort of look like their heads, which might confuse other animals. What's more remarkable about the sleepy lizard is its commitment to monogamy. For long stretches of the spring and summer, partners don't stray more than 12 inches from one another's scaly sides. (After mating in the fall, they give each other more space.) Staying with one mate is rare in lizards. When animals of any kind practice monogamy, evolutionary biologists look for a reason. What advantage do the animals get from sticking with one partner, rather than switching whenever they want? Maybe a long-term couple can better coordinate their reproductive cycles, for example, or their nest-building behaviors. Or maybe they forage for food more efficiently as a pair, or help each other evade predators. Tiliqua rugosa lizards can live for decades, so a long-term study of this species is no joke. Stephan Leu of Flinders University in South Australia and his coauthors gathered data on the skinks for 31 years. "From 1982 to 2012, we searched for lizards for 5–10 hours per day, along about 120 km of tracks in the study area, usually on 5 days each week," the authors write. Whenever they spotted a sleepy lizard, they grabbed it (again, they're not fast) and marked it for later identification. In total they recorded more than 53,000 encounters with almost 12,000 lizards. They kept track over the years of which lizards were couples and which had split up. Additionally, during one year of the study, the researchers captured 14 lizard couples and taped GPS devices to their backs. This let them record every movement of these lizards over the next six weeks. Half the couples had been seen together in previous years, and the other half were newly partnered. Over the whole study period, about 20 percent of the lizards the scientists captured were in couples. They tended to stick together; only a third of lizards changed partners over a five-year period. The researchers observed 110 lizard partnerships that lasted more than a decade, and 31 that lasted more than 15 years. One relationship had lasted 27 years and was still going strong when the experiment ended. Possible benefits of these long-term lizard marriages emerged from the GPS recordings. The researchers couldn't actually observe the lizards mating (they may be slow, but they're discreet). Instead, the researchers used the knowledge that sleepy lizards spend more and more time in close proximity until they mate, then get distant right after they do the deed. The GPS data showed that established lizard couples mated nearly two weeks earlier than lizards in new relationships. The scientists think this earlier mating may lead to higher reproductive success, like it does in some other species. Having your babies sooner might give them a better chance of survival. The scientists couldn't actually measure this—female sleepy lizards hide while they give birth to live young, which then scurry away to live on their own. But if it's true, it would give established lizard couples a reproductive edge. Why do lizards in long-term relationships mate so much earlier? "A female sleepy lizard only becomes receptive after prolonged male attention," the authors write, "priming her to reach mating readiness." Ahem. So males in established pairs might have an easier time getting their females in the mood for mating. While the new couples are still engaged in preliminaries, the old couples are getting down to business. It's also possible that familiar couples can forage or watch for predators more efficiently. If a female can spend less effort watching out for snakes, she might be able to save up her energy for mating and pregnancy. Either way, these lizards clearly know a thing or two about staying in a relationship. Once they figure out which end of their partner is which, that is.

Image: by Alan Couch (via Flickr)

Leu, S., Burzacott, D., Whiting, M., & Bull, C. (2015). Mate Familiarity Affects Pairing Behaviour in a Long-Term Monogamous Lizard: Evidence from Detailed Bio-Logging and a 31-Year Field Study Ethology DOI: 10.1111/eth.12390