Exploring Evolutionary Causes for Different Types of Animal Weaponry
Molly Brazil
In order to survive in this world, every living thing, from plants to lizards to
bison, has some form of defense. This can be in the form of a chemical defense such
as the puffer fish’s neurotoxin; a physical defense like the porcupine’s quills; or even
a behavioral defense such as the gazelle’s speed or the prairie dog’s group living
style. Whatever the case, the defense is imperative for survival and persists in the
species due to the increase in fitness that it provides. The goal of this paper is to
research three different types of animal weaponry, antlers, horns and tusks, and try
to determine what selective pressures led to the development of these specific
defenses.
For each type of ornamentation, I selected three sample species as
representatives for that particular form of defense. I then assessed each species
based on the following factors: body size, weapon size, group size, habitat, and
territoriality. Once I compiled all of the data for these factors, I looked for trends
within each group of three species that could give insight into the evolutionary
development of the weaponry.
Antlers
Starting with antlers, I researched the species Alces americanus, Pudu puda
and Odocoileus virginianus. Antlers are restricted to the males of the family Cervidae
with the exception of female caribou.
The first species I researched was A. americanus, the American moose. The
moose is the largest cervid and second largest land animal in North America at 270-
700 kg, approximately 2.3 m tall and 2.4-3.2 m long. (Goss, 1983) Their antlers are
broad and flat with a maximum span of 2 m from tip to tip. They are used primarily
to fight other males and attract females during the mating season; males with bigger
antlers have greater reproductive success. Although moose are in close proximity
during the mating season, they are solitary during the rest of year. When their
ranges overlap and they come into contact with others, moose do not exhibit any
territoriality. They have large home ranges of 5-10 square kilometers in Alaska,
Canada and the northeastern United States. Generally these ranges are in forested
areas with
seasonal snow cover even though the snow increases their vulnerability to
predation. (Bartalucci and Weinstein, 2000)
The southern pudu, P. puda, lies at the other end of the spectrum in terms of
range and size. Pudu live in southern Chile and southwestern Argentina and are one
of the smallest cervids at 5-15 kg, 85 cm long and 33-45 cm tall. Their antlers are
also very small, ranging from 60-90 mm long. Instead of being broad and wide like
the moose’s antlers, the antlers of the southern pudu are spiked and point
backwards; as with moose, antler size is a very important factor for females during
mate selection. Though they feed in groups of up to three and come together during
the mating season, pudu are generally solitary and highly territorial, forming
dominance hierarchies based on body size. Males engage in head butting and are
known to fight to the death. Their home ranges are variable, from 2-100 hectares,
generally in humid or temperate rainforests with a thick understory of small trees
and low-lying vegetation. (Robidoux, 2014)
The last cervid I researched was the white-tailed deer, O. virginianus. The
white-tailed deer lies in between the moose and pudu at 57-137 kg, 1.5-2m long and
up to 1 m tall. Their antlers can spread anywhere between 8 to 64 cm. (Goss, 1983)
Like moose and pudu, they use their antlers to attract mates and fight off other
males. White-tailed deer are also considered solitary, with small home ranges
typically less than 1 square km, but females have been observed to form herds of
hundreds of does. Although their home ranges are very defined and most whitetailed deer will stay within the same range their entire life, white-tailed deer are not
very territorial. Additionally, while white-tailed deer prefer to live in dense thickets
with edges for finding food, they are able to survive in a variety of habitats from
forests to grasslands to deserts. (Dewey, 2003)
Within the cervid group, the only things that were true of all three sample
species, were the function of the antlers and the solitariness of males. Otherwise,
there were varying degrees of territoriality, group size and habitat. While all three
species prefer to live in forested areas, white-tailed deer are also found in open
grasslands and deserts. This information does not point to any common selective
pressures for antlers in these species.
Horns
Horns were the second kind of animal weaponry that I researched. Found in
both sexes of the Antilocapridae family, as well as all males and some females of
species in the Bovidae family, the three sample species I chose were: Ova canadensis,
Tragelaphus imberbis, and Bos mutus, all of which are bovids.
The first bovid I looked at was O. canadensis, the bighorn sheep. These sheep
are 1.5 to 1.8 m long, about 1 m tall, and weight between 57 and 127 kg. Their large,
curving horns can reach weights of 14 kg and lengths of 76 cm. Horn size
determines dominance in males, with the older, large-horned males having greater
reproductive success in a given mating season. Their large horns are also used to
their advantage as they participate in intense head butting battles over females.
Although they readily battle for a mate, bighorn sheep do not defend territories.
Typically they live in herds of 8-10 sheep in the alpine meadows and grassy slopes
of the Rocky Mountains. The close proximity of open grazing land to rugged, rocky
cliffs allows the sheep a quick escape route from predators. (Ballenger, 1999)
The lesser kudu, T. imberbis, lives in a very different environment from the
bighorn sheep, preferring densely thicketed and woodland areas in East Africa; they
are rarely observed in open areas. In terms of size, the two species are fairly similar;
their bodies are between 1.1-1.75 m long, .9-1.05 m tall and weigh between 60 to
105 kg. Their horns, however, are very different, ranging from 48 to 91 cm long,
pointing straight back with 2.5 twists. Just as the bighorn sheep, their horns size
attracts females and determines dominance. However, fighting is rare and they do
not exhibit any kind of territorial behavior. Until the mating season, males remain
solitary while females form groups of 2 to 3. (Paschka, 2000)
The last bovid I included as a sample species was B. mutus, the yak. At 3001000 kg, around 3.25 m long and 2 m tall, the yak is the largest of the bovids I
researched. (Leslie, 2009) Its upward curving horns can reach lengths of 99 cm in
males and 64 cm in females. Like all bovids, horn size determines dominance and
males use them during their battles over access to females. Preferring to live in large
areas of grassland such as the alpine meadows of the Tibetan Plateau, female yak
congregate in large herds ranging from 20 to 200. Males typically form groups of 10
or 12. Although yaks are not territorial, they have been known to attack and kill
their domestic counterparts. (Oliphant, 2003)
Besides functioning to attract mates and establish dominance, horns in these
three species do not exhibit any clear trends in the factors I examined. Horn size is
not correlated with body size, group size varies dramatically, and habitats range
from open to very concealed. However, it should be noted that all three of the
sample species, despite their differences, share a lack of territoriality. Again, the
data found does not point to any factors that would favor the development of horns.
Tusks
Tusks were the last type of animal weaponry I chose to research. They are
present in both males and females of species found in a wide variety of families
through out the class Mammalia. The species I chose as representatives were:
Loxodonta africana, Odobenus rosmarus and Hylochoerus meinertzhageni.
The African bush elephant, L. africana, is the largest land animal at 36006000 kg and 2.5-4.5 m tall. (Moss, 2009) Equally impressive in size are their tusks,
derived from the upper incisors, which can grow up to about 250 cm long and weigh
around 60 kg each. (Moss, 2009) Elephants use their tusks for digging, defense and
offense, sexual displays and lifting trees. These animals live primarily in the
grasslands of central and southern Africa in non-territorial herds of up to 200
elephants; within each herd there are clans of 9-11 elephants led by a matriarch.
Males leave the herd at maturity and either wander alone or in a bachelor herd.
(Norwood, 2002)
The tusks of the walrus, O. rosmarus, are much smaller than elephant tusks at
only 50 cm long and are derived from the upper canines instead of the incisors.
Walruses use their tusks to break through ice, help them get out of the water, defend
themselves from predators, establish dominance and compete for females. They live
in packs that range in size from hundreds to thousands and within these packs there
is a hierarchal system based on age, size and tusk length. On average, walruses
weigh 1000 kg and are about 3 m long. Walruses are also known for being extremely
territorial. (Baker, 2013)
The last tusked mammal I looked at was the giant forest hog, H.
meinertzhageni. The smallest of the species I researched, H. meinertzhageni stands
about 1 m tall and 1.9 m long and weighs between 180 and 275 kg. Both sexes have
small tusks derived from their upper canines that flare outward and can reach about
30 cm long; they are primarily used for digging for roots and minerals. These hogs
live in groups of 6-14 in forests with a thick understory or in dense thickets and
bushes of Africa, and they are known for being very intolerant of other hogs.
(Lundrigan and Bidlingmeyer, 2000)
Since the tusks are derived from different teeth types in multiple orders of
mammals, it is likely that they are the result of convergent evolution. Of the three
sample species, all of them used their tusks for very utilitarian purposes such as
digging, getting out of the water and defending themselves. Besides increasing in
length with body size, there are no other clear trends in the factors I examined; they
Tusks
Horns
Antlers
live in a wide variety of habitats with very different group sizes.
Species
Weapon size:Body size
A. americanus
0.625
P. puda
0.088
O. virginianus
0.053-0.320
O. canadensis
0.422
T. imberbis
0.52
B. mutus
0.305
L. africana
Weapon function
Attracting female;
Fighting
males/Asserting
dominance
Attracting female;
Fighting
males/Asserting
dominance
Attracting female;
Fighting
males/Asserting
dominance
Habitat
Group size
Territoriality
Forested areas
near water
Solitary
None
Dense understory
of rainforests
Solitary
High
Variable: forests,
grasslands, desert
Males
solitary;
Related
females form
groups
Low
Attracting female;
Fighting
males/Asserting
dominance
Attracting female;
Fighting
males/Asserting
dominance
Attracting female;
Fighting
males/Asserting
dominance
Alpine meadows
close to rocky
cliffs
8-10, but can
be over 100
None
Densely thicketed
areas/woodlands
Males:
solitary;
Females: 2-3
None
Alpine meadows
Males: 1012; Females:
20-200
None
0.556
Digging, sexual
displays, defense and
offense, lifting
Grasslands
Males:
solitary;
Females: Up
to 200
None
O. rosmarus
0.167
Breaking ice, getting
onto land, establishing
dominance,
competing for females
Arctic ocean and
areas with lots of
ice
100s to
1000s
High
H. meinertzhageni
0.158
Digging
Dense thickets
14-Jun
High
Table 1: Summary of morphological, ecological and behavioral factors showing the
relationships between species and types of weaponry. Weapon size and body size used for
this comparison were the maximum lengths found. Trends found were the group size of males in
the antler group as well as function of weaponry for both the horn and antler groups.
Unfortunately, as illustrated by Table 1, my research did not find any clear
selective pressures that could have been responsible for the evolution of horns,
antlers and tusks in these different animals. It is important to note that cervids and
bovids, whose antlers and horns are used for the same purpose, are both part of the
suborder Ruminantia. It is possible that divergent evolution, likely due to sexual
selection, acted on a common ancestor with some form of rudimentary
ornamentation that resulted in different structures serving the same purpose.
However, this is purely a speculation and more research needs to be done.
Otherwise, horns, antlers and tusks are present on animals that live in a wide
variety of habitats, show varying degrees of territoriality, cover a wide range of
body sizes, and are either solitary or are members of groups of very different sizes.
The only conclusion I was able to reach was that tusks likely developed via
convergent evolution. In order to find what selective pressures resulted in these
different types of animal weaponry, it is now clear that I will have to research
further. To pursue the idea of sexual selection, I think an appropriate next step is
looking at the types of mating systems in these species. Additionally, I think it is
necessary to look at the fossil record for the common ancestors and any transition
species because the extant species are too diversified to see any clear morphological
or ecological trends.
References
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November 6, 2014.
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Goss, R. J. (1983). Deer antlers: regeneration, function and evolution. Academic
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