Biology Reference
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experiments involving fights between two males of
the same size, those with larger horns were more
likely to win. On the other hand, horns reduced a
male's running speed and agility in the tunnels.
Therefore, horns are good for fighting while hornless
is good for sneaking (Moczek & Emlen, 2000). But
what would we expect the threshold size to be for
horn development?
Mart Gross (1996) suggested that a conditional
strategy's threshold for a switch in tactics should
occur at the point where high status individuals (in
this case major males) would benefit by investing in a
competitive phenotype (in this case horns). Fig.
5.15a explains his model. John Hunt and Leigh
Simmons (2001) tested this model for a population
of Onthophagus taurus in Western Australia. In
laboratory experiments they placed groups of ten
males and ten females together in buckets with moist
sand and cow dung. They then collected broods of
larvae and measured male fertilization success. By
varying male size they could test how size influenced
a male's competitive ability. On average, major males
had five times the reproductive success of minor
males, so fighting was clearly more profitable than
sneaking. For minor males, fertilization success did
not vary significantly with body size. However, for
major males there was a marked increase in success
above a pronotum width of 5 mm (Fig. 5.15b). This corresponded well with the threshold
for horn development in this population. Therefore, selection has led to a switch in
tactics from hornless to horns at the body size at which males gains increased fitness by
adopting a competitive tactic.
In theory we would expect this threshold switch in morphology to occur at different
body sizes in different populations, depending on how local conditions influence
competition for mates. Douglas Emlen (1996) showed by selection experiments that the
threshold has a genetic basis. He selected males that produced unusually long horns
and males that produced unusually short horns for their respective body sizes. After
seven generations of selection he had shifted the position of the sigmoid allometry
switch along the body size axis (Fig. 5.16). These results show that populations have the
potential to evolve in response to selection on horn length, through modifying the
threshold for horn development.
A study of the European earwig Forficula auricularia provides an excellent example of
how selection has shifted a threshold switch in morphology (Tomkins & Brown, 2004).
Just like the dung beetles, males come in two morphs. 'Macrolabic' males have long
abdominal forceps which they use in fights for females under stones and logs.
'Brachylabic' males have short forceps and they attempt to steal copulations. Again,
just like the dung beetles, there is a threshold body size for development of long forceps
dung
guarding male
sneaking male
female
egg
brood ball
Fig. 5.14 Alternative
mating tactics in
Onthophagus dung
beetles. Large, horned
males guard burrow
entrances and fight to
defend females. Small,
hornless males sneak
matings through side
tunnels. From Emlen
(1997).
Predicting the size
threshold for horn
development
Size thresholds
for morphological
switches are
subject to natural
selection
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