If you think size
matters, then you’ll love what I have in store for you......
LARGE MANDIBALLLLS!!!!
The Evolution of animal weapons
Douglas J. Emlen
This paper reviews the
empirical literature on the evolution of
enlarged male weapons due to sexual selection. Insects and arachnids are not
solely mentioned it also talks about other arthropod species such as
Trilobites and Crustacean. Vertebrates such as mammals and reptiles are also mentioned
in how they obtain there elaborate structures. So if you’re interested in how
such large structures have evolved and the behaviours which are associated with
them, give the paper a read !
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Figure 1: A visual description of many the types
of exaggerated structures within the
Arthropod Phylum: Arachnids and
Insects
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Sexual selection is a driving
force for large extravagant structures on animals. Sexual
selection is
preferences
by one sex for certain characteristics in individuals of the other sex. This
leads to mate success where these preferable characteristics are inherited to
the next generation (natural selection). This creates competition to occur between males to reproduce with the
females.
There
are two types of sexual selection. First is female choice where females are
looking for specific traits to describe the quality of the male, leading males
to obtain ornamental structures e.g. peacock tails. The second is male vs. male
competition where frequent
fighting can result in the evolution of large weaponry structures, such as
mandibles and horns, in order to fight.
Both types of
selection are generated by competition and can lead to enlarged and various
structures. In contrast the female is
usual absent in enlarged and elaborate structures; this is due to a lack of
completion between females in order to obtain mates. Due to this sexual
dimorphism occurs where females differ in size and/or colour compared to the
males.
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Figure 2: An illustration of an African tusked wasp.
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The African tusked wasp, Synagris, is an example of sexulay selected weapons. They have evolved long and sometimes branched facial
outgrowths (figure 2). These horns are used as visual
signals and weapons to compete for mud nest built by the female. Another example of this is in five
families of flies. They evolved long antler like extensions (figure 3). Within
these families the structures are various and divergent. Along with using these weapons in combat they are also a kind of visual signal to say “hey back off”
to potential competitors.
When a fight breaks out the male stands on his
hind legs and locks his antlers with the opponents.
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| Figure 3: Antler flies battling it out |
Weapon structures evolve in environments that have limited resources and is critical to defend them. Also being a male who can defend and provide
resources through using their large weaponry structures will
become favorable in females eyes and these structural traits will be pasted on to the next generation.
So with this said whenever the benefits of expressing weaponry outweigh the
costs of producing it, it will be selected for, keeping in mind that these
structures usually appear in limited resource environments. These structures can be used to avoid conflict. As combat can injure and may kill the insect there is a large cost associated with fighting. Therefore using structures as signals of comparing strength can avoid conflict. For example the antlers of flies (figure 3) are a function as these types of visual warnings/ tactical signals to evade combat. Fights will only escalate when a rival is of similar size. Due to this female choice can cross it to the male vs. male selection category as
females will prefer these traits to insure her offspring will be as successful.
If you'd like to read more bout these structures and the behaviors check out the paper by Douglas Emlem, its an interesting read!
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Random fact no. 1. The males of some members of Pselaphinae have a higher number of ommatidia (sub-units of compound eyes that conatin photoreceptor cells) than the females. This sexual dimorphism is supposed to make it easier for them to find the females.
ReplyDeleteMassive weapons seem like a great strategy to protect the resources and get the girl in a limiting environment! But I wonder how much resource would need to be put into creating and maintaining these elaborate structures? Then if there are pay-offs for having these weapons like prehaps easy to be preyed on by larger predators? Really cool stuff!!
ReplyDeleteThose dudes pack a punch. When does the sexual selection end? It doesnt look like those guys can get much bigger horns?!
ReplyDeleteWhen the selection pressures ends, these structures will become unfavorable and then unnecessary so will not be passed on as females will prefer other traits. And those traits that become favorable by the female will be selected and passed on and may lead to other intense structures, colours or behaviors. Even in male vs male competition if a better weapon evolves and that male is successful in mating then his structures will be passed on but if its not a successful structure it wont be passed on . The powers of selection can go a long way!!!
ReplyDeleteYeah good point, and I guess the only way for the selection pressure to end is if something changes - a changing environment, or a new mutation.
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