A novel approach to quantifying mammal locomotor repertoires using scoring and cluster analysis

Describing behaviour using qualitative categories is a staple of studies on tetrapod functional morphology, ecology, and evolution. However, such categorisation has several issues, primarily subjectivity and the loss of important behavioural repertoire information. Here, we propose a novel method for quantifying behaviour, using mammal locomotion as a case study to demonstrate its utility and efficacy. Species are scored from 0-4 on their proficiency in five locomotor modes (swimming, climbing, digging, running and aerial movement), then Ward’s hierarchical clustering is used on the resulting data matrix to group species into biologically informative categories (the number of which can be optimised using clustering validation methods), thus producing a mathematically defined categorical variable. The method is demonstrated on a dataset of 250 mammal species, representing every extant mammal family. We show that this approach successfully quantifies mammal locomotion, producing both a data matrix that can be used as a set of covariates in multivariate analyses and a categorical variable. This method introduces a replicable technique for quantifying animal behaviour and subsequently deriving a categorical variable, which is highly versatile and can be tailored to analyse a variety of other behaviours and taxonomic groups for widespread use across evolutionary and ecological research.

Methods
1. Sample

A total of 250 extant mammal species were included in this study. The species were selected to represent one species per subfamily, or family where such divisions have not been defined. Decisions regarding which particular species within their subfamilies would be scored were largely based on the availability of locomotor information within the literature to avoid introducing missing data into the scoring matrix.

In the case of Chiroptera, not all subspecies are included, regardless of the presence of many subfamilies within the order, because the Chiroptera broadly represent a single behavioural repertoire which would be overrepresented if all subfamilies were included. Similarly, not all rodent subfamilies were included as it would risk overrepresenting this very large order. Note, however, that the reduced coverage still represents the whole locomotor variation within Rodentia.

2. Scoring

Each species was given a score from 0-4 in each of five locomotor modes: swimming, climbing, digging, running, and aerial movement. A score of 0 represents an animal being physically unable to engage in a behaviour (e.g. a whale cannot run), while a score of 4 represents an animal being able to perform this behaviour highly proficiently (e.g. whales are extremely good swimmers).

The criteria for scoring are given in Table 1 of the associated manuscript; these were determined prior to scoring and based on function rather than frequency (i.e., an animal’s ability to perform a behaviour informs its score in the column, not the frequency of use of that behaviour). Strict adherence to these criteria is essential, and thus the wording of the criteria is intended to give as little room for ambiguity as possible in order to reduce subjectivity, and example taxa are provided to further clarify and differentiate the scoring levels.

Scoring was based on information from literature, with full reference list provided in File 2.