An ecometric exploration of bovid horn core size in modern Africa

Ecometrics rely on the principle that morphological traits reflect a species’ interaction with its surroundings, both biotic and abiotic (Eronen et al., 2010a). For example, traits such as tooth height and hypsodonty reflect variation in climate in both the past and present (e.g. Eronen et al., 2010b; Fortelius et al., 2002). The development of ecometrics is facilitated by using traits that are ubiquitous in both the modern and fossil record. Horn cores are both durable and abundant in the fossil record and modern museum collections, yet they have not been utilized as an ecometric trait.

We propose to analyze horn core size within the family Bovidae as a proxy for climatic variability in both the past and present. Within lineages of bovids, horns are used a weapon to facilitate mating and are strongly affected by sexual selection (Bro-Jørgenson, 2007). As a result, we hypothesize that climates with higher seasonality and more competition for resources will be correlated with larger horn size within lineages where males use horns as weapons. However, abiotic factors may affect tropical lineages differently than those in temperate ecosystems since horns have been found to have thermoregulatory functions by releasing heat (Picard et al., 1996). Limited pilot data support the hypothesis stated above. Data from paleontological collections in Hadar Ethiopia, identified a trend toward larger horn core size in the extinct impala, Aepyceros sp. nov. between 3.42Ma and ~3.0Ma. There is also a trend toward larger tooth size in Australopithecus afarensis male specimens within the same period. Both lineages show the most dramatic size increases in the youngest sediments (3.0Ma), which were reconstructed as being the most arid and seasonal of the formation.

As intern at the Smithsonian (in 2009), I followed up on this trend and tested the hypothesis that among modern Aepyceros melampus (impala) horn size would be larger in specimens from geographic regions with high aridity. The Aepyceros melampus range spans across sub-Saharan Africa. Using numerous specimens from the Smithsonian, I found that basal horn size (APxML), length, and horn shape were significantly correlated with all rainfall variables (e.g. fig. 1). However, the strongest pattern was that the eastern African impala had larger basal horn size, while southern Africa impala had smaller horns. Seasonality as measured by the CV of yearly rainfall correlated most strongly with basal horn size (fig. 2). Molar row size, the geometric mean of cranial variables for males and females (proxies for body size), were not correlated with any rainfall variables using bivariate plots and linear correlation.

Relationship to the FuTRES mission: The scope of this paper will explore the potential of using horn core size as an ecometric trait using more sophisticated climate data and methods. We will also comment on the relationship between sexual dimorphism and climate by expanding the study to measure horn size and cranial or post cranial traits (as proxies for body size) in other geographically widespread bovid species (beyond impala) where both males and females have horns. Some of the data described here have already been contributed to the FuTRES database but additional trait data will also be included. I have collected more data from the horn cores from other bovid species (beyond Aepyceros melampus). If the Smithsonian Museum Support Center opens again soon, I plan to measure more specimens and species. Since the question of the project explores population level skeletal traits, specimen level data is imperative. This project is a prime example of the type of ecological questions that will be facilitated by the FuTRES mission.

Project workflow and completion: The scope of the project is open to discussion. Since it was conceived when I was an undergraduate, I’d be open to ideas about how to broaden the scope, refine the hypothesis, or perhaps expand the project into multiple papers. See the proposed workflow below.

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*Though there are team members already listed, we welcome anyone with knowledge, skills, or data that may be useful i developing this project