Age-dependent changes in dogs' (Canis familiaris) separation-related behaviours in a longitudinal study

Separation related problems (SRP) caused by distress associated with separation from the preferred member of the social group, can be characterised by their symptoms e.g., excessive vocalisation. In dogs' separation whines, nonlinear phenomena (NLP) (abrupt changes in the resonance of the vocal folds) might occur, which could be adaptive in communicating aroused inner states. Previously, using a separation test we found that more dogs that were classified as having SRP by their owner have NLP in their whines than nonaffected dogs and that NLP ratio increases with age, which suggests that separation stress might intensify with age. We repeated the separation test 21.19 +/- 9.37 months later with 32 dogs from the previous study to investigate longitudinally how separation behaviour and vocalisations change with age. Beside behaviour, we measured the acoustic structure of the whines (jitter - small-scale irregularity of the pitch, entropy - vocal harshness, call length, pitch (f0) related parameters, and the spectral components) and calculated the NLP ratio. We formed clusters based on the dogs' behaviour changes from the first test to the second, to see individual ageing patterns. Finally, we compared the dogs' behaviour and the acoustic structure of their whines in the two occasions. We found that dogs could be clustered by the changes in their separation behaviour. 41 % of the dogs were stable over time, 38 % improved, and 16 % showed an increase in their separation behaviours. 3 % switched from barking to whining. Interestingly, SRP dogs were stable, some of them even showed improvement in their separation behaviour. On the contrary, we also found that SRP dogs tended to have an increased NLP ratio with age from test 1 to test 2 (p = 0.09), showed less escape-related behaviour (p = 0.01), but tended to spend more time passively whining at the door through which the owner left the room (p = 0.05), than non-SRP dogs. The behavioural and vocal results suggest elevated stress levels in SRP dogs with age, although they did not decline but mostly stayed stable in their separation behaviour, confirming that there could be a connection between SRP status, age, and NLP. However, together with the results of the clustering that showed that there are different patterns in dogs' separation behaviour, we emphasise the importance of individual level longitudinal investigations in order to facilitate the early diagnosis of SRP and to provide a solid basis for the development of individualised treatment plan for SRP dogs.