K-mer analysis shows hybrid hummingbirds perform variable, transgressive courtship sequences

Abstract

Behaviours are challenging to describe. Here, we apply k-mer analysis to characterize complex courtship behaviours at four hierarchical levels: elements, displays, bouts and repertoires, on two species of hummingbirds and their hybrids. During courtship, male rufous hummingbirds, Selasphorus rufus, perform three types of displays: shuttles (S), half pendulums (H) and rufous dives (R), while Allen's hummingbirds, Selasphorus sasin, perform four displays: S, H, Allen's dives (A) and the pendulum (P). Here we explore the behavioural sequences expressed by their hybrids, on a sample of displays from 35 Allen's hummingbirds and 46 rufous hummingbirds sampled far from their hybrid zone, compared against 306 wild male birds from a hybrid zone in Oregon and California, U.S.A. Among Allen's hummingbirds, hybrids and rufous hummingbirds, there was almost no variation in the kinematic elements that comprised displays. By contrast, the displays themselves, display sequences and repertoires varied substantially among these three groups. Some hybrids performed transgressive display variants in which kinematic elements of typical displays were missing or duplicated. This transgressive segregation implies the displays are under oligogenic control. We analysed display sequences using k-mer analysis, in which we binned display sequences into 3-mers (sequences of three). Typical 3-mers for Allen's hummingbirds included sequences PPP, PPA and SHP while typical 3-mers for rufous hummingbirds included SHR and RRR. The most defining characteristic of certain hybrids was their sequence variability: the display repertoire of some hybrids encompassed the repertoire of both parent species (e.g. performed both PPA and RRR) as well as 3-mer sequences never observed in either parent such as PRA. Such high within-individual variability in hybrid phenotype differs from how modular morphological traits (such as flowers, feathers or hair) are expressed in hybrids. This within-individual variability may be unique to behaviour.