Evolution of Feeding Motions |

Collaborators: Matthew G. McGee; Samuel R. Borstein; Peter C. Wainwright

A major axis of feeding diversity in ray-finned fishes concerns a trade-off between force application and the speed at which the jaw can open and expand. Furthermore, a complex feeding apparatus, consisting of numerous mobile components, has allowed fishes to occupy a variety of trophic niches in aquatic systems. This research seeks to understand the diversity and evolution of feeding motions in a trophically diverse assemblage of African cichlids from lakes Malawi and Tanganyika.

sample trajectory — **Figure 1.** Principle components 1 and 2 for a single motion trajectory (solid blue line) in *Lamprologus lemairii*. A linear baseline between start and end shapes is also plotted (blue dotted line). Selected video frames are shown with landmarks (light blue dots) and sliding semilandmarks (yellow dots).

Few kinematic studies evaluate biological motions across more than a handful of species. We were interested in developing a framework for characterizing variation in motions that is amenable to comparative study. To do this, we used geometric morphometrics to summarize the complex features of fish feeding motions into a single object, a trajectory of shape change; as an organism completes a motion, it undergoes an ordered series of shape transformations that traces a trajectory through a multidimensional shape space (Fig. 1, solid blue line). Characteristics of these trajectories tell us about the nature of motions producing them. For example, the length of a trajectory provides information about the amount of shape change that a biological system has undergone during a motion, which is a metric of the magnitude of movement or kinesis. When trajectories are generated across a wide range of species, we start to get a picture of broad patterns of motion diversity (Fig. 2). We showed that there was a relationship between interspecific head shape and kinesis; species with more shallow, elongate heads (smaller PC 2 values in figure 2) had longer shape trajectories that produced greater kinesis.

motion PC — **Figure 2.** Principle components 1 and 2 for motion shapes from 326 suction feeding strikes in 56 species of African rift lake cichlid. PC 1 captures a common axis of kinesis, where PC 2 represents interspecific variation of cranial shape. Note that trajectory lengths increase as PC 2 values decrease.

When kinesis is reconstructed on a time-calibrated phylogeny (Fig. 3), we get a glimpse of the evolution of feeding motions. For example, low kineses feeding strikes have independently evolved at least 5 times across African rift lake cichlids (reds and oranges in figure 3). Most of these transitions are associated with shifts to algae-eating, in which species feed by scraping their teeth along hard benthic substrates. In contrast, species with high kinesis (blues in figure 3) tend to eat more evasive prey, like shrimps and fishes. Our results suggest that feeding diversity in African cichlids has evolved along an axis of prey evasiveness and is consistent with our understanding of a major trade-off in fish feeding systems relating to the transmission of force versus speed.

Fig 2 — **Figure 3.** Maximum likelihood (ML) phylogeny of African rift lake cichlids compared in this study, with the most heavily sampled tribes labeled, “A” Haplochromini and “B” Lamprologini. The magnitude of kinesis is mapped onto branches, based on ML ancestral reconstruction and assigned feeding categories are shown as colored dots. Video frames from the start and end of strikes are shown for selected species to represent cranial morphologies associated with kinematic diversity.

Future Directions:

Based on my work in African cichlids, I am interested in applying geometric-based motion analyses to different systems and research questions. For example, I am currently working on an application of these methods to quantify the outputs of biomechanical models, which allows for evaluation and visualization of models within the context of their form-function landscape. Additionally, I am interested in studying extreme motions, like high protrusion capacities in some Neotropical cichlids (Fig. 4).

image seq fast.gif — **Figure 4.** Neotropical cichlid, *Caquetaia myersi*, showing off its highly protrusible jaw while feeding on a black worm.

Relevant Publications:

Martinez CM, McGee MD, Borstein SR & Wainwright PC. 2018. Feeding Ecology Underlies the Evolution of Cichlid Jaw Mobility. Evolution. 72 (8), 1645-1655.

Funding Sources:

UC Davis Chancellor’s Postdoctoral Fellowship (UCD)