Approximate Bayesian inference for a "steps and turns" continuous-time random walk observed at regular time intervals
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The study of animal movement is challenging because it is a process modulated by many factors acting at different spatial and temporal scales. Several models have been proposed which differ primarily in the temporal conceptualization, namely continuous and discrete time formulations. Naturally, animal movement occurs in continuous time but we tend to observe it at fixed time intervals. To account for the temporal mismatch between observations and movement decisions, we used a state-space model where movement decisions (steps and turns) are made in continuous time. The movement process is then observed at regular time intervals. As the likelihood function of this state-space model turned out to be complex to calculate yet simulating data is straightforward, we conduct inference using a few variations of Approximate Bayesian Computation (ABC). We explore the applicability of these methods as a function of the discrepancy between the temporal scale of the observations and that of the movement process in a simulation study. We demonstrate the application of this model to a real trajectory of a sheep that was reconstructed in high resolution using information from magnetometer and GPS devices. Our results suggest that accurate estimates can be obtained when the observations are less than 5 times the average time between changes in movement direction. The state-space model used here allowed us to connect the scales of the observations and movement decisions in an intuitive and easy to interpret way. Our findings underscore the idea that the time scale at which animal movement decisions are made needs to be considered when designing data collection protocols, and that sometimes high-frequency data may not be necessary to have good estimates of certain movement processes.
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