By Michael Hopkin
Chimpanzees are closely related to humans,
and similar in behaviours and genetic makeup. But how human are they?
Promiscuity rules
Chimpanzees are far more promiscuous than humans, a fact demonstrated
by the impressive size of their testes. Relative to body size, a male
chimpanzee's testes are ten times larger than a man's, with a prolific
sperm-production capacity to match. And they need it - female chimpanzees
don't stand by their man; they have multiple partners, which means that
male chimpanzees need to produce as much sperm as possible to give themselves
the best chance of fathering offspring during the ensuing 'sperm war'.
And female chimpanzees, besides maximizing their chances of reproducing
with a genetically superior male, also benefit from the confusion over
paternity - a male is less likely to kill a young chimp if there's a
chance he fathered it.
Tools of the trade
Chimpanzees show an amazing capacity for learning new skills from one
another, particularly when it comes to catching dinner. In a recent
experiment1 carried out at Yerkes National Primate Research Center in
Atlanta, Georgia, female chimpanzees that were taught one of two methods
for getting food out of a novel feeding apparatus quickly passed on
their expertise to fellow group members, showing that skills can be
passed rapidly from chimp to chimp without direct human guidance. What's
more, the groups tended to persist with whatever method their female
had been taught, even when neighbouring chimp groups devised a simpler,
more efficient technique. This indicates they have a certain cultural
attachment to their practices, and, like humans, can be social conformists.
The passing on of new knowledge or skills through
social interaction is an important part of life for a chimpanzee. But
they're a little slower than people are at picking up new tricks. Their
famous method of 'termite fishing' with sticks takes around five years
to master, and complex group hunting techniques, such as those used
by elder males to catch monkeys, can require decades to perfect.
Ref.
1. Whiten, A., Horner, V. & de Waal, F. B. M.
Nature doi:10.1038/nature04047 (2005)
Brain power
Humans are the undisputed world champions of brain power, but how far
behind are chimps? Studies of both captive and wild chimps have shown
that they can pick up both maths and language. A captive chimp called
Ai managed to learn the numbers one to nine, and could arrange a selection
of three single-figure numbers in order - as long as they weren't too
close together in value. Even in the wild, numbers play a simple role
in chimp behaviour. A chimpanzee will usually only attack a solitary
male from another group if accompanied by two accomplices or more. And
a male chimp's response to rival calls will vary depending on the size
of the group he is travelling in. Chimpanzees have also shown great
facility for language. Some can understand and follow verbal instructions
such as "Take the keys and put them in the refrigerator", whereas others
have mastered an impressive vocabulary using sign language. (See 'Apeing
our language')
The numbers game
Chimps are almost identical to us, genetically speaking. But perhaps
not quite as similar as we thought. It is often said that the two
species are 98.5% the same, in terms of single-letter changes in the
DNA code. But mapping of the chimpanzee genome shows there are further
differences in how DNA chunks are duplicated and rearranged. These
differences cause our genome sequences to diverge by 2.7%, while single-letter
substitutions add about another 1% of difference.
One way in which we are alike is that both
our Y chromosomes, the hallmark of maleness, seem to be heading into
dangerous evolutionary territory.
To keep up in the race to fertilize promiscuous
females, male chimps need to create a lot of sperm, making the Y chromosome's
genes for sperm production very important. As a result, natural selection
tends not to do anything to disrupt the Y, so harmful mutations can
also hitch a ride. The end result? Chimp Y chromosomes are accumulating
harmful mutations even faster than human ones.
Animal strength
Chimpanzees are different from humans in several obvious ways, one
of which is their sheer physical strength. But why are they so much
stronger than us?
The answer isn't just sheer muscle bulk. It's
also to do with that fact that their muscles work around five to seven
times more efficiently than ours. Studies of human and other primates'
jaw muscles show that our muscle fibres are far smaller and weaker than
those of our cousins - roughly an eighth the size of those seen in macaques,
for example.
The reasons for this remain poorly understood,
but one contributing factor is the genes that encode myosin, the protein
fibres from which muscles are made. Comparison of human and ape sequences
for a myosin gene called MYH16 show that all humans have a
mutant version of this gene.
Some have even credited the more diminutive
muscles in human jaws for our larger intelligence. One theory says that
these smaller muscles gave our skulls the room to grow rounder, allowing
for a bigger brain cavity1.
Ref.
1. Stedman, H. et al.
Nature 428, 415 - 418 (2004).
