Abstract
Morphology can be adaptive through its effect on performance of an organism. The effect of performance may, however,
be modulated by behavior; an organism may choose a behavioral option that does not fully utilize its maximum
performance. Behavior may therefore be decoupled from morphology and performance. To gain insight into the
relationships between these levels of organization, we combined morphological data on defensive structures with measures
of defensive performance, and their utilization in defensive behavior. Scorpion species show significant variation in the
morphology and performance of their main defensive structures; their chelae (pincers) and the metasoma (‘‘tail’’) carrying
the stinger. Our data show that size-corrected pinch force varies to almost two orders of magnitude among species, and is
correlated with chela morphology. Chela and metasoma morphology are also correlated to the LD50 of the venom,
corroborating the anecdotal rule that dangerously venomous scorpions can be recognized by their chelae and metasoma.
Analyses of phylogenetic independent contrasts show that correlations between several aspects of chela and metasoma
morphology, performance and behavior are present. These correlations suggest co-evolution of behavior with morphology
and performance. Path analysis found a performance variable (pinch force) to partially mediate the relationship between
morphology (chela aspect ratio) and behavior (defensive stinger usage). We also found a correlation between two aspects of
morphology: pincer finger length correlates with the relative ‘‘thickness’’ (aspect ratio) of the metasoma. This suggests
scorpions show a trade-off between their two main weapon complexes: the metasoma carrying the stinger, and the
pedipalps carrying the chelae.
Abstract
Morphology can be adaptive through its effect on performance of an organism. The effect of performance may, however,
be modulated by behavior; an organism may choose a behavioral option that does not fully utilize its maximum
performance. Behavior may therefore be decoupled from morphology and performance. To gain insight into the
relationships between these levels of organization, we combined morphological data on defensive structures with measures
of defensive performance, and their utilization in defensive behavior. Scorpion species show significant variation in the
morphology and performance of their main defensive structures; their chelae (pincers) and the metasoma (‘‘tail’’) carrying
the stinger. Our data show that size-corrected pinch force varies to almost two orders of magnitude among species, and is
correlated with chela morphology. Chela and metasoma morphology are also correlated to the LD50 of the venom,
corroborating the anecdotal rule that dangerously venomous scorpions can be recognized by their chelae and metasoma.
Analyses of phylogenetic independent contrasts show that correlations between several aspects of chela and metasoma
morphology, performance and behavior are present. These correlations suggest co-evolution of behavior with morphology
and performance. Path analysis found a performance variable (pinch force) to partially mediate the relationship between
morphology (chela aspect ratio) and behavior (defensive stinger usage). We also found a correlation between two aspects of
morphology: pincer finger length correlates with the relative ‘‘thickness’’ (aspect ratio) of the metasoma. This suggests
scorpions show a trade-off between their two main weapon complexes: the metasoma carrying the stinger, and the
pedipalps carrying the chelae.
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