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If you want to mate and successfully raise your young, it pays to be
big. At least among gorillas.
Larger male gorillas living in the rainforests of Congo are more
successful than smaller ones at attracting mates and even raising young,
suggests a new study from the Wildlife Conservation Society and the Max
Planck Institute for Evolutionary Anthropology.
The study—conducted over a 12-year period in Nouabalé-Ndoki National
Park in the Republic of Congo—helps to illuminate the selective
pressures that influence the evolution of great apes.
The study appears in a recent edition of Journal of Human Evolution.
The authors of the study include: Thomas Breuer of the Wildlife
Conservation Society and the Max Planck Institute for Evolutionary
Anthropology; and Andrew M. Robbins, Christophe Boesch, and Martha M.
Robbins of the Max Planck Institute for Evolutionary Anthropology.
In assessing the role of size in the reproductive success of
"silverback" gorillas, the researchers selected three physical factors
for measurement: overall body length; the size of the adult male's head
crest (also known as a sagittal crest which is absent in females); and
the size of an individual's gluteal muscles on the animal's posterior.
The researchers then compared data on individual size with information
on group dynamics to explore possible correlations between physical
characteristics of adult males, the number of female gorillas connected
to males, and the survival rates of an adult gorilla's offspring.
The results of the study revealed that all three characteristics were
positively correlated to an adult male's average number of mates. In
other words, the bigger the adult male, the more mates it had. An
unexpected finding was that only head-crest size and gluteal muscles
were strongly related to offspring survival (measured as infants that
survived to weaning age) and overall reproductive success, measured as
the number of surviving offspring.
"Our findings of correlations between physical traits and male
reproductive success could be considered evidence of a selection process
in gorillas, but it is not yet proof," said Breuer, the lead author of
the study. "More studies would be necessary to determine the links
between morphology and fitness in this and other long-lived species."
The research is the latest of several studies of gorillas made from the
ideal research conditions of Nouabalé-Ndoki National Park's Mbeli Bai, a
large, swampy forest clearing where gorillas (and scientists studying
them) gather for long periods. From 1995 until 2007, the team followed
the lives of 19 adult male western lowland gorillas and their family
groups from observation platforms with telescopes and cameras. Over that
time, the researchers were able to track the number of females each male
mated with, and the number and survival rate of offspring produced by
each adult male.
The gorilla group data was complemented by physical measurements of
adult male gorillas by using a novel, non-invasive method called digital
photogrammetry, which produces accurate measurements of individual
gorillas and their characteristics from digital images (converting pixel
size to actual lengths).
"By using non-invasive methods for measuring the size of individual
male gorillas and their features, we are gaining insights about the
factors that could be driving mate selection in our closest relatives,"
added Breuer.
big. At least among gorillas.
Larger male gorillas living in the rainforests of Congo are more
successful than smaller ones at attracting mates and even raising young,
suggests a new study from the Wildlife Conservation Society and the Max
Planck Institute for Evolutionary Anthropology.
The study—conducted over a 12-year period in Nouabalé-Ndoki National
Park in the Republic of Congo—helps to illuminate the selective
pressures that influence the evolution of great apes.
The study appears in a recent edition of Journal of Human Evolution.
The authors of the study include: Thomas Breuer of the Wildlife
Conservation Society and the Max Planck Institute for Evolutionary
Anthropology; and Andrew M. Robbins, Christophe Boesch, and Martha M.
Robbins of the Max Planck Institute for Evolutionary Anthropology.
In assessing the role of size in the reproductive success of
"silverback" gorillas, the researchers selected three physical factors
for measurement: overall body length; the size of the adult male's head
crest (also known as a sagittal crest which is absent in females); and
the size of an individual's gluteal muscles on the animal's posterior.
The researchers then compared data on individual size with information
on group dynamics to explore possible correlations between physical
characteristics of adult males, the number of female gorillas connected
to males, and the survival rates of an adult gorilla's offspring.
The results of the study revealed that all three characteristics were
positively correlated to an adult male's average number of mates. In
other words, the bigger the adult male, the more mates it had. An
unexpected finding was that only head-crest size and gluteal muscles
were strongly related to offspring survival (measured as infants that
survived to weaning age) and overall reproductive success, measured as
the number of surviving offspring.
"Our findings of correlations between physical traits and male
reproductive success could be considered evidence of a selection process
in gorillas, but it is not yet proof," said Breuer, the lead author of
the study. "More studies would be necessary to determine the links
between morphology and fitness in this and other long-lived species."
The research is the latest of several studies of gorillas made from the
ideal research conditions of Nouabalé-Ndoki National Park's Mbeli Bai, a
large, swampy forest clearing where gorillas (and scientists studying
them) gather for long periods. From 1995 until 2007, the team followed
the lives of 19 adult male western lowland gorillas and their family
groups from observation platforms with telescopes and cameras. Over that
time, the researchers were able to track the number of females each male
mated with, and the number and survival rate of offspring produced by
each adult male.
The gorilla group data was complemented by physical measurements of
adult male gorillas by using a novel, non-invasive method called digital
photogrammetry, which produces accurate measurements of individual
gorillas and their characteristics from digital images (converting pixel
size to actual lengths).
"By using non-invasive methods for measuring the size of individual
male gorillas and their features, we are gaining insights about the
factors that could be driving mate selection in our closest relatives,"
added Breuer.
