Victoria M. Luce1, Megan T. Wyman2, Theodore Stankowich1 1Department of Biological Sciences, California State University, Long Beach, Long Beach, CA 90840 2Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, Davis, CA 95616

Evolution of Horn Size in Female Ungulates

Background Artiodactyla is one of the most diverse Orders of terrestrial mammals, ranging in size from small antelope to domestic cattle and can be found globally in a variety of environments. Within this order, all male members of Family Bovidae (antelope, cows, goats) have horns that are used, primarily, in competition over mates, and horns present in females of some species are used primarily in defense against predators, as well as for defending territories against other conspecific females. It remains unclear, however, when females DO have horns, why the females of some species have horns that are nearly equal in size to the males of their species, while others have horns that are much smaller compared to the males of their species. Since males primarily use their horns for fencing and defense, there may also be a correlation between male and female horn lengths and their respective facial colorations. We used horn measurements collected from literature and museum specimens and natural history data collected from the literature to test three hypotheses:

1. Species that are under greater risk of predation by being more exposed in their environment should have longer horns than species of females that are at less risk.

2. Males in species that are polygamous will have longer horns than males in monogamous species.

3. Species with longer horns will also have stronger facemask coloration to accentuate and draw attention to their weaponry.

Stankowich & Caro 2009: Among Bovids, horns found on females of species that are EXPOSED in their environment (openness of habitat x body size).

MALE FEMALE MALE FEMALE

Methods We collected horn length data from Groves & Grubb (2011) and supplemented it by measuring the following on 1-6 skulls from each of 43 species at the LA Museum of Natural History: Greatest Skull Length (Figure I), Horn Length, Horn Span, and Horn Tip-to-Tip (Figure II). Following Stankowich & Caro (2009), we estimated Exposure for each species as the product of shoulder height (collected from the literature) and habitat openness. Openness of each possible habitat type was scored based upon the relative cover and how far a prey would have to be to be spotted by a potential predator, and then an average was taken for each species based on the types of habitats they live in. Facial markings were scored from 0(least) to 5(most) based upon the presence of contrasting colors and vertical markings that may accentuate the presence of horns. We also scored each species mating system type, horn shape and texture, from previously published literature. We corrected male and female horn length measures for body size using the greatest skull length, and we calculated the ratio of Female to Male Horn length as a measure of sexual dimorphism. A phylogenetic generalized least squares analysis was used to estimate the effects of risk and monogamy on morphology and to account for the relatedness of the species. A consensus phylogenetic tree of the Bovidae was taken from 10KTrees.

Figure I: Greatest Skull Length Measurements (grey) Figure II: Horn length (red), Horn span (blue), and Horn Tip-to-Tip Measurements (green)

Discussion The PGLS analysis of bovid species show that females tend to have longer horn lengths when the species lives in more exposed environments while males of polygamous species tend to have longer horn lengths. This suggests that the size of female horns is directly proportional to the risk of predation in their environment: horns are useful antipredator weapons and the more dangerous environment in which a species lives, the more energy females should invest in weaponry to defend themselves and offspring. Males who have a chance to woo multiple mates have longer horns to compete with rivals. Finally, males but not females showed stronger facemask coloration when they had longer horns, suggesting that facial markings are a visual signal to conspecifics to enhance the apparent size of the headgear and intimidate rivals. Thus, all three hypotheses were supported by the data. The next step is to collect more data on group size and territoriality in these species and test for effects of social behavior on horn length, particularly in females.

Acknowledgements We thank Jim Dines and Dave Janiger at the Los Angeles Museum of Natural History for access to specimens, Colin Groves for advice on skull measurements, and Rita Collins for assistance with measuring skulls.

Results The data supports our hypothesis that females who reside in more open environments and males that are polygamous will have greater horn lengths. We found a significant positive correlation between exposure and the female horn length (Fig III, P=0.006). We also found that females have horns of lengths similar more similar to males when they are more exposed in their environment (Fig IV, P=0.001). There was also a significant positive correlation between horn length in males and mating system, showing that males will tend to have longer horns if they are polygamous (Fig V, P=0.013). Finally, while we found no effect of horn length on facemask score in females (Fig VI, P=0.139), males with longer horns had significantly stronger facemasks that accentuated horn length (Fig VII, P=0.014).

Impala

Grant’s Gazelle

Connochaetes taurinus Kobus kob

Muntiacus crinifrons

Tragelaphus strepticeros

Nanger granti

Phylogenetic Tree of the Bovid species used in the analysis

Ammotragus lervia

0 1 2

3 4 5 Scoring key for face markings: 0 = no face markings, 5 = strong face markings accentuate horns

Examples of habitat types; each weighted 0 (closed) to 1 (open)

Desert

0.95

Savanna

0.7

Tropical Forest

0.1

Tundra

0.9

Temperate Grassland

0.8

Wetland

0.3

Corr

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d Fe

mal

e Ho

rn L

engt

h Co

rrec

ted

Mal

e Ho

rn L

engt

h

Mal

e Fa

ce M

ask

Scor

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Fem

ale:

Mal

e Ho

rn L

engt

h Ra

tio

Fem

ale

Face

Mas

k Sc

ore

Exposure Exposure

Monogamous? Yes No

Corrected Female Horn Length

Corrected Male Horn Length

Fig III t = 0.304 P = 0.006

Fig IV t = 3.534 P = 0.001

Fig V t = -2.643 P = 0.013

Fig VI t = 1.151 P = 0.139

Fig VII t = 2.584 P = 0.014