The faithful skink - a lizard that practices monogamy.

wikipedia

So called sleepy lizards, the Australian lizards that have large bodies and stubby legs, practice monogamy. When they first meet, they pair up and remain so close together that they are rarely more than 30 centimeters away from each other.
You might think this is just the male's way of keeping a female to himself, and stopping other males mating with her. But he even stays in contact with her after mating though not so closely.
Their pair bonds last better than many human relationships. Scientist spent 31 years studying these animals and found two thirds of them stayed faithful. 31 Partnerships lasted for more than 15 years and, 110 for more than ten years.
So why do they do it? It may be because going steady means a female mate is available, without the male having too much energetic seduction.  She's on tap, so to speak. And the earlier they mate, the better chance they have of having viable babies.
Mind you, not all the females are faithful. About 14% of the offspring are not those of the apparent mate. And it looks as though smaller and less healthy mates are more likely to  be jetisonned by the females.You can read more at

• 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

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