Describing the sound producing behaviour of narwhals based on spatial and temporal covariates: Independence vs. dependence

Specialeforsvar ved Aleksander Søltoft-Jensen

Titel: Describing the sound producing behaviour of narwhals based on spatial and temporal covariates: Independence vs. dependence

Abstract: This thesis examines how the patterns of certain types of sounds produced by East Greenland narwhals relate to the spatial location of the whales and the time of day. Two categories of sounds are used as response variables: "buzzing" and "calling". Both serve a distinct purpose in the life of narwhals. Besides an initial exploratory analysis, the data is investigated using three different frameworks: a standard logistic regression setup, a point process model with a memory component, and a hidden Markov model. In all models, including the area, depth and time of day as covariates appears much more reasonable than ignoring them. Conditioning on these covariates, the first model assumes independence of observations while the others do not. For the whales that are investigated using all three methods, 10-fold cross validation reveals that the models with dependence fare better in terms of reducing the mean deviance of unseen data. In addition to comparing the performance of the models, the adequacy of the models themselves is also investigated through various diagnostics. Here, it is seen that for some whales, even the best performing model (in terms of cross-validation) still relies on assumptions that don't necessarily fit well with the data. Finally, the fitted models with dependence can, among other things, be interpreted as having the possibility of "conversations" between narwhals built into them. The validity of this interpretation should be judged by experts in the field, though.

Vejleder: Susanne Ditlevsen
Censor: Uffe Høgsbro Thygesen