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Rigatuso et al. published a fascinating application of Arrhenius' Law to ant travel velocity (1). They measured ant velocity at various temperatures, and their Arrhenius plot showed a break at 16 °C, with E a = 40 kJ/mol above 16 °C, and E a = 85 kJ/mol below 16 °C. They speculate that 16 °C may represent the freezing point for the "trail pheromone" that these ants secrete and follow. Below 16 °C solid pheromone would have a lower vapor pressure and be harder to follow, leading to slower motion.There are several problems with this suggestion. First, as pointed out by Rigatuso et al., pheromone secretions are mixtures of several components; hence they are unlikely to have a single, distinct freezing point. Second, pheromones are generally highly volatile liquids at room temperature; it is unlikely that they would freeze at 16 °C, and if they did, it is unlikely that they could be secreted from the Dufour's gland.I present here an alternative hypothesis to explain the 16 °C transition temperature in the ant velocity Arrhenius plot. First, consider that locomotion is not a simple chemical reaction. Instead, it is the end result of a series of distinct phases. A partial list of these phases includes: (a) pheromone binding to and activation of receptor, (b) signal transduction and initiation of sensory nerve impulse, (c) processing of sensory nerve impulse leading to locomotion "decision", (d) initiation and transmission of motor nerve impulse, and finally, (e) muscle contraction causing locomotion.Each of these five phases could in turn be broken down into several distinct steps. For example, muscle contraction involves, among other things, (i) ATP hydrolysis on the myoLetters sin head, (ii) a power stroke driven by phosphate release, and (iii) myosin-actin crossbridge cycling driven by ATP-ADP exchange. Also, (iv) the ATP that drives muscle contraction must be supplied by the mitochondrion, which carries out oxidative phosphorylation.The activation energy for any complex reaction will be determined by its rate-determining step. A break point or transition temperature in an Arrhenius plot is often interpreted as a change in rate-determining step. Although in principle any of the five phases (a-e) enumerated above could be rate determining for locomotion, due to the high speed of signal transduction and nerve transmission, it seems likely that phase e, muscle contraction, is the slowest phase.Of the four steps in muscle contraction (i-iv), it is interesting to note that mitochondria have a well-known transition temperature at 15-18 °C. Below this transition temperature, mitochondrial membrane characteristics change dramatically, causing most mitochondrial metabolic reactions (including ATP synthesis) to occur much more slowly. My guess is that this influence of temperature on mitochondrial ATP synthesis causes the activation energy for ant locomotion to nearly double as temperature falls below 16 °C. Literature Cited1. Rigatuso, R.; Bertoluzzo, S. M. R.; Quattrin, F. E.; Bertoluzzo, M. G.
Rigatuso et al. published a fascinating application of Arrhenius' Law to ant travel velocity (1). They measured ant velocity at various temperatures, and their Arrhenius plot showed a break at 16 °C, with E a = 40 kJ/mol above 16 °C, and E a = 85 kJ/mol below 16 °C. They speculate that 16 °C may represent the freezing point for the "trail pheromone" that these ants secrete and follow. Below 16 °C solid pheromone would have a lower vapor pressure and be harder to follow, leading to slower motion.There are several problems with this suggestion. First, as pointed out by Rigatuso et al., pheromone secretions are mixtures of several components; hence they are unlikely to have a single, distinct freezing point. Second, pheromones are generally highly volatile liquids at room temperature; it is unlikely that they would freeze at 16 °C, and if they did, it is unlikely that they could be secreted from the Dufour's gland.I present here an alternative hypothesis to explain the 16 °C transition temperature in the ant velocity Arrhenius plot. First, consider that locomotion is not a simple chemical reaction. Instead, it is the end result of a series of distinct phases. A partial list of these phases includes: (a) pheromone binding to and activation of receptor, (b) signal transduction and initiation of sensory nerve impulse, (c) processing of sensory nerve impulse leading to locomotion "decision", (d) initiation and transmission of motor nerve impulse, and finally, (e) muscle contraction causing locomotion.Each of these five phases could in turn be broken down into several distinct steps. For example, muscle contraction involves, among other things, (i) ATP hydrolysis on the myoLetters sin head, (ii) a power stroke driven by phosphate release, and (iii) myosin-actin crossbridge cycling driven by ATP-ADP exchange. Also, (iv) the ATP that drives muscle contraction must be supplied by the mitochondrion, which carries out oxidative phosphorylation.The activation energy for any complex reaction will be determined by its rate-determining step. A break point or transition temperature in an Arrhenius plot is often interpreted as a change in rate-determining step. Although in principle any of the five phases (a-e) enumerated above could be rate determining for locomotion, due to the high speed of signal transduction and nerve transmission, it seems likely that phase e, muscle contraction, is the slowest phase.Of the four steps in muscle contraction (i-iv), it is interesting to note that mitochondria have a well-known transition temperature at 15-18 °C. Below this transition temperature, mitochondrial membrane characteristics change dramatically, causing most mitochondrial metabolic reactions (including ATP synthesis) to occur much more slowly. My guess is that this influence of temperature on mitochondrial ATP synthesis causes the activation energy for ant locomotion to nearly double as temperature falls below 16 °C. Literature Cited1. Rigatuso, R.; Bertoluzzo, S. M. R.; Quattrin, F. E.; Bertoluzzo, M. G.
. 1. Data were compiled from the literature and our own studies on 24 ant species to characterise the effects of body size and temperature on forager running speed.2. Running speed increases with temperature in a manner consistent with the effects of temperature on metabolic rate and the kinetic properties of muscles.3. The exponent of the body mass-running speed allometry ranged from 0.14 to 0.34 with a central tendency of approximately 0.25. This body mass scaling is consistent with both the model of elastic similarity, and a model combining dynamic similarity with available metabolic power.4. Even after controlling for body size or temperature, a substantial amount of interspecific variation in running speed remains. Species with certain lifestyles [e.g. nomadic group predators, species which forage at extreme (>60 °C) temperatures] may have been selected for faster running speeds.5. Although ants have a similar scaling exponent to mammals for the running speed allometry, they run slower than predicted compared with a hypothetical mammal of similar size. This may in part reflect physiological differences between invertebrates and vertebrates.
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