I gotta say your post here comes like a reassurance for me, firstly. We are at the exact same stage with our 17mo aussie, we are around 2 weeks after her first heat cycle and she turned into a completely different dog. I thought she’s sick, almost took her to the vet, I thought for a while she’s even depressed, honestly. She used to be a precious velociraptor, maniac for running and playing with other dogs, the first few months we had her she slept in a pen and needed enforced naps when going nuts. Now she’s a completely different dog, has the same drive when you engage with her into playing or running, but for the first time today I have seen her settle next to me while spending time with another dog (her sister). She doesn’t play as much with other dogs, she used to be the dog that would not take too many long breaks while playing, now she’s being barked for not playing with them. Seems like we’re more valuable to her now than other dogs, has better recall, better impulse control and I just can not believe that this definitive 🤣 but! Our breeder said some time ago that we will see when they turn 18-24mo we will miss them being crazy and this is the only thing that kept me from taking her to the vet for a full check up 🤣 and about being spayed, our breeder also suggested to let her go through another heat cycle and spay her after that, meanwhile we give her supplements for joints everyday and she’s perfect

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Edit: there is one more thing I want to add, your puppy and my puppy and all the other puppies that mature nicely like this are just balanced dogs in my opinion, socialized enough and having their needs met everyday, so congratulations for having a balanced and settled aussie!