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First published online August 17, 2006
Journal of Experimental Biology 209, 3448-3456 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02396
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Isometric contractile properties of sexually dimorphic forelimb muscles in the marine toad Bufo marinus Linnaeus 1758: functional analysis and implications for amplexus

Douglas Lee Clark and Susan E. Peters*

Department of Biology, The University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, NC 28223, USA


Figure 1
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  		Fig. 1. Muscles of the right forelimb of a male Bufo marinus are seen from a medial perspective (A) with hand pronated, and from lateral view (B) with hand supinated. Flexor carpi radialis (FCR) originates along the medial surface of the distal half of the humerus and inserts on the radiale. It is a ventromedial flexor of the wrist, and can also flex the elbow. The abductor indicus longus (AIL) originates laterally from the distal humerus and the radioulnar border and inserts by a narrow tendon onto metacarpal I. It can extend the wrist and abduct the first digit. These are both used in amplexus and are sexually dimorphic by size (see text). The extensor carpi ulnaris (ECU) is a non-dimorphic muscle that originates superficial to AIL on the lateral humeral condyle and inserts on the ulnare. It laterally extends the wrist and is not used in amplexus. (Modified from Peters and Aulner, 2000Go.)

 

Figure 2
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  		Fig. 2. Force/frequency curves compare the amount of force (as a percentage of maximal force) produced at varying stimulus rates for male and female muscles (FCR, AIL, ECU) over a train duration of 670 ms. Stimulus frequencies ranged from 5 to 80 pulses s-1. No significant differences were found between sexes for a given muscle. Values are means ± s.e.m. Sample sizes for these and subsequent figures comparing male and female muscles are shown in Table 1.

 

Figure 3
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  		Fig. 3. Force/duration curves compare the amount of force (as a percentage of maximal force) produced by male and female muscles (FCR, AIL, ECU) at different stimulus durations when the stimulus rate was held constant at 30 pulses s-1. Stimulus durations ranged from 50 to 670 ms. The only significant difference (asterisks) occurred in the FCR, in which the female FCR produced more force than the male in the range of 50 to 300 ms (P<0.05, adjusted with sequential Bonferroni; k=7). Values are means ± s.e.m.

 

Figure 4
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  		Fig. 4. The time course of fatigue is shown for the three muscles (FCR, AIL, ECU) over 4 min. Force was averaged in 30 s intervals and expressed as a percentage of the maximal mean force. In both FCR and AIL, forces declined more significantly (indicated by asterisks) in females than in males (P<0.05, adjusted with sequential Bonferroni; k=8). Values are means ± s.e.m.

 

Figure 5
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  		Fig. 5. A representative series of force traces during the 4 min fatigue test is shown for a male FCR. (A) The first 24 s of the intermittent, sub-maximal tetani (1 train/2 s; 200 ms duration, 30 pulses s-1; sweep speed=2.5 s cm-1). Note that the tetanic traces fall back to baseline (arrows) during the 2 s intervals between tetanic trains. By 2 mins into the test (B), the rate of relaxation of the muscle has slowed so that the trace returns only part way to baseline, resulting in sustained force between stimulus trains. By the last 20 s of the fatigue test (C), sustained force comprises approximately 70% of the total force at maximum stimulation. Note that the trace had not returned to baseline within this panel even after more than 8 s following the last stimulus train. In most cases it took from 10-15 s for the force to relax completely to baseline. The tick marks on the lower trace indicate the points of stimulation once every 2 s.

 

Figure 6
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  		Fig. 6. Sustained force is compared between males and females in the three muscles (FCR, AIL, ECU). Sustained force was averaged over 30 s intervals and expressed as a percentage of the average peak force for the same time interval. In both FCR and AIL, males produced significantly more sustained force than did females; however, in AIL significance was reached only at the 240 s time interval. Male and female ECU both produced equally small amounts of sustained force (note the different force scale). Asterisks indicate significant differences (P<0.05, adjusted with sequential Bonferroni; k=8). Values are means ± s.e.m.

 

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© The Company of Biologists Ltd 2006