The bite force–gape relationship as an avenue of biomechanical adaptation to trophic niche in two salmonid fishes

ABSTRACTAll skeletal muscles produce their largest forces at a single optimal length, losing force when stretched or shortened. In vertebrate feeding systems, this fundamental force–length relationship translates to variation in bite force across gape, which affects the food types that can be eaten effectively. We measured the bite force–gape curves of two sympatric species: king salmon (Oncorhynchus tshawytscha) and pink salmon (Oncorhynchusgorbuscha). Cranial anatomical measurements were not significantly different between species; however, peak bite forces were produced at significantly different gapes. Maximum bite force was achieved at 67% of maximum gape for king salmon and 43% of maximum gape for pink salmon. This may allow king salmon to use greater force when eating large or elusive prey. In contrast, pink salmon do not require high forces at extreme gapes for filter feeding. Our results illustrate that the bite force–gape relationship is an important ecophysiological axis of variation.

Limits on egg predation by Richardson's ground squirrels

To test the inference, arising from circumstantial evidence, that Richardson's ground squirrels (Spermophilus richardsonii (Sabine, 1822)) frequently depredate eggs of greater sage-grouse (Centrocercus urophasianus (Bonaparte, 1827)), gape size was measured and the response of free-living squirrels to three sizes of eggs was observed. Maximum gape measured on carcasses was 26 mm and functional gape assessed from tooth imprints in artificial clay eggs was 17 mm. Squirrels left imprints in 46 of 110 clay eggs, but whether tested with domestic fowl (Gallus gallus (L., 1758)) or ring-necked pheasant (Phasianus colchicus L., 1758) eggs that approximated the maximum width of sage-grouse eggs or with much smaller Japanese quail (Coturnix japonica Temminck and Schlegel, 1849) eggs that approximated maximum gape, no squirrels (28 adults and at least 28 juveniles) spontaneously depredated eggs, even after multiple exposures. When re-tested with damaged eggs, 15 of 16 adult females scavenged contents, though usually not on their first exposure. After scavenging damaged eggs, 2 of 12 squirrels opened a few intact eggs, but only quail eggs and usually only if the shell was rough. Although Richardson's ground squirrels are potential scavengers of large damaged eggs and likely they could depredate small eggs, the inference from circumstantial evidence that they are major predators of greater sage-grouse eggs remains unsubstantiated.

The effects of temperature on prey-capture kinematics of the bluegill (Lepomis macrochirus): implications for feeding studies

Research with ectothermic organisms has demonstrated that temperature is positively correlated with an individual's power output during locomotion. This study investigates the effect of temperature on another aspect of power output, prey-capture kinematics, of the bluegill (Lepomis macrochirus Rafinesque, 1819). Feeding sequences for two treatments of four sunfish were filmed at three temperatures (18, 24, and 30 °C) with one treatment (A) experiencing an increasing range of temperatures and the other (B) experiencing a decreasing temperature range. Directional temperatures affected prey-capture kinematics. The time required to achieve maximum lower jaw depression and maximum gape, as well as the duration of maximum gape, time to close the mouth (from the point of maximum gape), and the total bite duration, increased as water temperature decreased. In addition, both the time to maximum gape and the time to maximum lower jaw depression were longer at 18 °C for individuals in treatment A than those in treatment B. These results indicate that water temperature can bias the results of feeding studies employing kinematics that do not control for its effects as well as those that make comparisons across such studies which utilize different temperatures and taxa.

Regulation of Masticatory Force During Cortically Induced Rhythmic Jaw Movements in the Anesthetized Rabbit

Hidaka, O., T. Morimoto, Y. Masuda, T. Kato, R. Matsuo, T. Inoue, M. Kobayashi, and K. Takada. Regulation of masticatory force during cortically induced rhythmic jaw movements in the anesthetized rabbit. J. Neurophysiol. 77: 3168–3179, 1997. To examine the relationships between masticatory force, electromyogram (EMG) of masticatory muscles, and jaw movement pattern, we quantitatively evaluated the effects of changing hardness of a chewing substance on these three variables. Cortically induced rhythmic jaw movements of a crescent-shaped pattern were induced by electrical stimulation of the cerebral cortical masticatory area in the anesthetized rabbit. The axially directed masticatory force was recorded with a small force-displacement transducer mounted on the ground surface of the lower molars. EMGs were recorded from the masseter and digastric muscles simultaneously with jaw movements. Five test strips of polyurethane foam of different hardness were prepared and inserted between the upper molar and the transducer during the movements. The peak, impulse, and buildup speed of the masticatory force increased with strip hardness, whereas duration of the exerted force did not vary with strip hardness. The integrated activity and duration of the masseteric EMG bursts also increased with strip hardness. The integrated EMG activity of the digastric bursts was weakly related to strip hardness, whereas the duration was not. The minimum gape increased with strip hardness, but the maximum gape did not. The horizontal excursion of the jaw did not vary in a hardness-dependent manner, although it was greater in the cycles with strip application than in the cycles without strip application. Deprivation of periodontal sensation by cutting the nerves to the teeth reduced the buildup speed of the force, maximum gape, net gape, and horizontal jaw movements. The denervation also elongated the force duration and that of masseteric EMG bursts. However, the rate of the hardness-dependent changes in the above parameters did not alter after denervation. The latency of the masseteric EMG response to strip application was evaluated before and after denervation. In both conditions, it was ⩾6 ms in ∼70% of the cycles and <6 ms in the remaining ∼30%, which cannot be explained by a simple reflex mechanism. On the basis of the analysis of correlation coefficients, the masseteric integrated EMG seemed to be a good indicator of the peak and impulse of the masticatory force both before and after denervation. We conclude that periodontal afferents would be responsible for a quick buildup of masticatory force and that afferents other than those from periodontal tissue would contribute to the hardness-dependent change of masticatory force during cortically induced rhythmic jaw movements.

The ontogeny of aquatic feeding behavior in Salamandra salamandra: stereotypy and isometry in feeding kinematics

To examine the extent to which aquatic prey-capture behavior in salamanders is stereotyped and how feeding kinematics scales with size, the ontogenetic variability of aquatic feeding behavior was examined in eight Salamandra salamandra. Feeding kinematics (seven duration and angular displacement variables), kinematic variance and capture performance were quantified and compared in the first several feedings after birth with a series of feedings 8 weeks later, just prior to metamorphosis. Analysis of variance revealed no statistically significant ontogenetic differences in the kinematic variables, and individual differences were found in only two variables (maximum gape angle and gape cycle time). A comparison of the relative kinematic variance within individuals revealed no significant differences in variance during ontogeny. In addition, capture success rate did not change. The strike is significantly faster than that of other salamanders. These results indicate that the initial prey-capture behavior remains unchanged throughout larval ontogeny. Thus, aquatic strike behavior in S. salamandra is developmentally fixed (innate) and does not appear to be influenced by learning or improvement in 'skill', supporting the hypothesis that aquatic salamander feeding is a highly stereotyped, unmodulated behavior. In addition, the lack of kinematic change through ontogeny indicates that feeding kinematics do not scale with body size, contrary to predictions that movements should be slower in larger animals.

Evolutionary attributes of headfirst prey manipulation and swallowing in piscivores

Headfirst swallowing of fish prey is a common attribute of gape-limited predators, conferring the presumed advantage of reduced esophageal abrasion. I evaluate swallowing orientation using cutthroat trout (Oncorhynchus clarki) as predator and three-spine stickleback (Gasterosteus aculeatus) as prey. Field data indicate that orientation is random when prey diameter is less than one-half the gape of the predator, but this increases to 90% headfirst orientation as prey diameter approaches and exceeds the maximum gape, consistent with the suspected reduction in abrasion. Experimental data show two additional advantages to headfirst orientation. Following capture, there is a 2- to 5-fold reduction in escape rate of the prey, and among prey that were swallowed, the manipulation period is substantially reduced (mean 29 vs. 81 s for headfirst and tailfirst orientation, respectively), the differences being most accentuated at large prey sizes.