Title
Using geometric probability to compare the random and actual mating success of the African
Giraffe, Giraffa Camelopardalis.
Author
Gabrielle Mitchell
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Abstract
The giraffe, giraffa camelopardalis, is an African even-toed ungulate mammal and the tallest
living terrestrial animal. Males inseminate the females using a dorsal-ventral method, the only
available method due to their height and lack of grasping appendages. The random geometric
probability of mating success was estimated to be about 3%. The probability of mating success is
increased by the males who roam and search for a fertile mate (available females secrete a
pheromone that the male tastes in her urine) before guarding a fertile giraffe cow for a short time.
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Introduction
The African giraffe is found in grasslands, savannahs, and open woodlands scattered
across the continent of Africa. A day-dweller, the giraffe will move with its herd during the day
before settling down at night in a circle of sorts with the most vulnerable giraffes on the inside
for protection. The giraffe’s primary source of food is acacia leaves which grow on trees and is
an appropriate food source given the giraffe’s extremely long neck and legs, which give it an
average height of 16-20 feet. Given the environment giraffes live in, they are mostly preyed upon
by lions, though, hyenas, leopards, Nile crocodiles, and wild dogs target the most vulnerable
giraffes as well.
Giraffes are a polygamous species in which males mate with as many fertile females as
possible. Because of this, male giraffes are not generally territorial and will roam about in search
of groups of females to mate with. Female giraffes with calves will generally join nursery herds
consisting of other mothers and calves, using each other as a support system for babysitting and
protection.
In this study, using a method of geometric probability, I compared the random mating
success with the actual mating success of the African giraffe.
Method
Random mating success was calculated based on four geometric figures (Fig. 1). The
surface area of the female’s dorsal area was represented by a circle for the hindquarters while the
triangles and rectangles covered the legs. The circle combined with the triangles and rectangles
represents the total surface area of the giraffe’s dorsal area. The square represents the female’s
vaginal opening where the male must successfully deposit sperm. Random mating success was
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calculated by the female’s vaginal opening divided by the female’s dorsal area.
Figure 1: This image of an African giraffe was obtained from Google Images. The large circle
combined with the triangles and rectangles represent the surface area of the female, as a male
would see her. The smaller square represents her vagina.
Results
Target surface area (circle) = 3.14 x 7/2 = 3.14 x 3.52 = 3.14 x 12.25 ≈ 38.5cm2
Target surface area (rectangles) = 10.2 x 1.1 ≈ 11.22cm2 x 2 = 22.44cm2
Target surface area (triangles) = .5 x 1 x 1.7 = .5 x 1.7 ≈ .85 x 2 = 1.7cm2
Total target surface area= 22.44 + 1.7 + 38.5 = 62.64cm2
Bull’s-eye surface area (square) = 1.3 x 1.3 ≈ 1.69cm2
Random probability of mating success (bull’s-eye/target): 1.69/62.64 = .027 x 100 =
2.7%
According to my geometric calculations, the surface area of the bull’s-eye is only 2.7% of
the total surface area of the target. Thus, by chance alone, only three out of 100 darts would hit
the bull’s-eye, meaning that only three of 100 matings would be successful.
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Discussion
In this study, it was found that there are only about three chances out of 100 that males
could find, mount, and successfully inseminate females with their eyes closed.
The male giraffe, giraffa camelopardalis, mates with the female in a ventro-dorsal
position. The random probability of mating success, 3% is increased by the male’s ability to find
a fertile female through smell and the taste of her urine, a process called the flehmen response.
The probability of success is also increased by the fact that females have an estrous cycle of two
weeks, increasing the chance to find an available mate through the taste of her urine. Giraffe
males continually look for new mates in an effort to boost species survival and strengthen the
gene pool.
Giraffe courtship can last up to one day and is usually initiated by the male, though
interested females have been found to follow males around, nudging them. When a male is
interested in a female he will follow her, nudging her along or wrapping his neck around hers.
Most often female giraffes do not consent quickly, and the male will unsuccessfully attempt to
mate a few times before the female consents. After courtship, intercourse follows, in which the
female giraffe will allow the male to come up to her rear and mount her by standing on his hind
legs with his front legs at her sides. At this point the female must stand still to prevent the male
from slipping, keeping her hind legs open and tail aside to aid entry, allowing the male to
inseminate. Though the probability of mating success is increased through prior methods, it is
not 100% even when a male finds a female. The giraffe body size and shape makes it difficult for
males to sustain penetration and the penis may only come into contact with the vagina in short
bursts, making it difficult to pass on sperm to a female. For this reason males tend to seek out
adult females with experience to increase the chance of mating success. After intercourse the
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male giraffe will guard the fertile female for a short time from other males before leaving in
search of another fertile female. The newly impregnated female will wait 14.5 months before
giving birth to her calf.
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References
Bercovitch, F. B., Bashaw, M. J., & del Castillo, S. M. (2006). Sociosexual behavior, male
mating tactics, and the reproductive cycle of giraffe giraffa camelopardalis. Hormones
and Behavior, 50(2), 314-321.
doi: 10.1016/j.yhbeh.2006.04.004
Kaleta, T., & Marczewska, S. (2007). The observations concerning reproduction and young
development in captive giraffe (giraffa camelopardalis) at Warsaw zoo. (Master’s thesis.)
Available from Annals of Warsaw University of Life Sciences (No. 4, 2007).
http://annals-wuls.sggw.pl/files/files/animal/asc2007no44art03.pdf
Pratt, D. M., & Anderson, V. H. (1985). Giraffe social behavior. Journal of Natural History,
19(4), 771-781.
doi: 10.1080/00222938500770471
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