1. 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
CorrectedFemaleHornLengthCorrectedMaleHornLength
MaleFaceMaskScore
Female:MaleHornLengthRatioFemaleFaceMaskScore
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