If you have one pet, you might be forgiven for thinking that bringing home another would make them both happy. Instead, many animals feel the opposite and may act aggressively towards each other. Two months ago, Sarah Chow adopted a 13-year old tomcat, Lucky, to join her 11-year old resident male, Puss. But, while the friendly Puss welcomed Lucky with open paws, the newcomer hardly reciprocated the love. For the first three days, Chow isolated Lucky. 'I kept him in a separate room so he could familiarise himself with my surroundings and different smells,' Chow says. 'Puss could smell the new cat and kept going up to the room, trying to communicate through the door.' Animal behavourist and veterinarian Cynthia Smillie says many people think they can bring another dog or cat home, but may be unaware of the need to establish the animals' relationships properly. 'Part of the problem comes when people are not aware of how dogs and cats behave naturally,' Smillie says. Cats are sociable animals, she says, but they are also solitary hunters that highly value their territory. 'If you are not sensitive to that, there is a strong risk of fighting between the new cat and the resident cat, and that may develop for a long time,' Smillie says. 'So it's important for [the introductions] to be [made] in the right way.' First, the new cat should be given its own room, litter box, food and water, the animal behavourist says. 'The new cat should be allowed to stay in the self-contained location,' says Smillie, who recently set up Animal Behaviour Veterinary Practice. 'This can take a few days and let the cat approach family members when it's calm and relaxed.' The pets also need a place to hide, possibly a high place that they can jump up to and see what is going on around them, Smillie says. Sprays or diffusers of the synthetic feline facial pheromone Feliway also help cats to relax and increase their level of security. Once your new furry friend seems content, the next step is scent introduction. Smillie advises owners to prepare two cloths labelled with each of the cats' names. Every day, the cloth should be rubbed over the face and flank of the cat. Then, when you feed or greet each cat, they should be presented with the opposing cat's cloth, allowing them to smell and investigate the scent. But remember not to mix-up the cloths at this stage. 'Repeat this process a few times a day, but it's very important that you don't force it,' Smillie explains. 'You want to reach the stage when the cat ignores the smell or reacts positively to it.' The next stage is Scent Swapping, when the cloths are mixed in a bag to combine the scents. Again, when you greet each cat, present them with the combined-scented cloth. When there is no reaction, the next stage involves rubbing the cloths over places the other cat frequents, such as the furniture and even yourself across the legs. 'Once they are happy with the mixed scent, you can allow the new cat out of its room to explore and run free, while the resident cat stays in another room. The new cat can find a good hiding place it can escape to without feeling vulnerable,' advises Smillie, the former deputy director of the Society for the Prevention of Cruelty to Animals. After exploring the home a couple of times a day for a few days, the cats are ready to be introduced. To avoid any risk of aggression, the first encounter could be made through an ajar door or via a baby gate with a mesh covering, Smillie says. Then, gradually feed them closer and closer on either side of the barrier by bringing their food bowls nearer to each other, to give them a positive association when they interact. Finally, when the animals don't show any aggression they can meet. Smillie says: 'The owner should be ready to intervene, and only give short introductions that will build up. For cats this can take a few weeks or a few months before there is a good relationship. 'But if there is one aversive event or incident, then it can create long-term damage that might never be resolved,' Smillie says. 'So it's better to go slowly.' For dogs, the approach starts outside the home. 'Dogs have a social hierarchy. So, to maintain order and conflict, and to establish territory against rivals, you need to introduce dogs in a neutral location,' Smillie says. 'Then the resident dog won't perceive the new dog is going into its territory, which could cause a lot of problems.' It's best for the two dogs to meet outside at a new place or park, she says. The location 'should be completely neutral, where the dog has never been before. This is one of the things that many people don't think about', says Smillie. At the neutral place, one person should handle one dog, and both animals should be on leads, she says. 'You want them to experience only good things, then, as they are sniffing each other, talk to them in an encouraging voice,' Smillie says. To associate positive interactions between the dogs, each handler should start a treat-giving process: obey a simple command, then repeat short interactions, followed by a treat. 'This starts the association of 'when this [other] dog is around me, then good things happen',' Smillie explains. 'If interactions are going well, then you can allow off-the-leash interaction, but be aware of their body language.' Signs of dogs' aggression, she says, are raised hair on the back of the neck, a hard stare, stiff body posture or growling. If this happens, intervene and distract them, then give them a reward. Short, positive interactions of a few minutes are better than longer periods that could turn sour, Smillie says. Once the dogs tolerate and accept each other, you can bring the newcomer home. Don't let your guard down through. The veterinarian suggests supervision for the first few days or a week to make sure they don't fight. It is easier to introduce a new puppy to an adult dog, but Smillie says the same gradual introduction process can be carried out. 'The puppy will pester the adult dog, and the adult won't play with a puppy. And the puppy might be too young to recognise the [aggressive] signals like growling or snarling,' she says. With an energetic puppy nipping at an adult's heels, Smillie says it's important for the adult to have some quiet time alone, while also getting plenty of attention from the owner. As for Puss and Lucky, newbie Lucky came out of his room for good after three weeks. 'Now, after seven weeks, they are not best buddies, [but] they respect each other's space,' Chow says. 'I wish they would be more friendly to each other, but it's quite obvious they would miss each other if the other one wasn't here.'