Incisor Arcade Research Articles

Repeated selection of morphometric traits in the Soay sheep on St Kilda

1. Long‐term studies allow the outcomes of repeated selection events to be monitored. Here, we investigate phenotypic selection in successive winter mortality events in the Soay sheep of St Kilda, Scotland, between 1985 and 1996. Selection of three quantitative morphometric traits, body weight, hindleg length and incisor arcade breadth, was investigated in different sectors of the population. 2. Evidence from fitness differentials of positive directional selection for large size was repeatedly found in lambs and adult females. Selection in the opposing direction was only found in one year in lambs. 3. Selection gradients showed that in most years when significant selection occurred, body weight was the focus of direct selection, whereas selection of hindleg length and incisor breadth was indirect, arising from their correlation with body weight. 4. Selection was strongest in years of low over‐winter survival and almost absent in years when survival was high. Intensity of selection was greatest in lambs, emphasizing the differences in selection pressure experienced by different sectors of the population, in addition to the temporal variation in selection pressure due to population density and environmental conditions. 5. Despite repeated positive selection of body weight, no evidence of a change in the population mean was found over the course of the study.

Incisor and molar wear in the tammar wallaby (Macropus eugenii Desmarest)

The morphology of incisor and molar teeth of tammar wallabies (Macropus eugenii Desmarest) is similar to that of the archetypal grazing macropod (M. giganteus) but there are some resemblances in the wear pattern of molars to that of grazer/browsers. Incisor wear patterns show that cutting during biting is by scissor-like action of the elevated labial enamel edge of an attrition facet on each lower incisor being occluded with, and rotated supero-medially across, the buccal surface of the upper incisor arcade. With increase in age and body size, the cutting surface moves from anterior to lateral upper incisors, progressive wear on the inner surface of the lateral upper incisors permitting an increasing degree of incisor action coincident with medial molar movement in Phase 2 occlusion, which is similarly achieved by medial rotation of the jaw. Significant distal movement of the reference point for molar index, along the line of the upper jaw, with increase in body size, indicates that this index does not measure the absolute mesial movement of molars in the plane of occlusion. The estimated value of absolute mesial movement of the first upper premolar along the line of the jaw (2.45 mm year–1) is at the known limits of mesial drift. Studies of size-related changes in the linear dimensions of various bony landmarks on jawbone and skull indicate that the high rate of movement may result from deposition of bone in the rear of the tooth row, i.e. ‘mesial shift’, as well as mesial drift. However, mesial shift may not account for significant differences in rates of absolute mesial movement of upper molars with gender. With increase in body size, the caudal insertions of the masseter and temporalis and the cranial origin of the line of action of masseter all move distally along the plane of occlusion. However, a concurrent mesial movement in the cranial origin of the line of action of the temporalis may act to counter any distal movement of occlusive force along the jaw-line and to decrease the relative force of the retraction component that opposes Phase 1 occlusion.

Incisor size and shape: implications for feeding behaviors in saber-toothed “cats”

ABSTRACT Although the great size and procumbency of the incisor teeth of many saber-toothed carnivorans have long been recognized, the potential functional significance of carnivoran incisor size and shape remains largely unexplored. Here, we explore upper incisor design and incisor arcade shape in living felids, canids, and hyaenids. In addition, we compare upper incisors of saber-toothed carnivorans with those of modern carnivorans and use these data to infer aspects of saber-tooth killing and feeding behaviors. Results demonstrate that the shape of individual upper incisors of extant carnivorans is intricately related to the dental arcade in which the teeth are rooted. Canids and hyaenids have robust incisors in parabolic arcades, whereas felid incisors are weaker, particularly in mediolateral bending, and are located in transversely linear arcades. It is proposed that the strength of the medial incisors is in part a consequence of arcade shape. Incisors in linear arcades can be weaker because they are...

Sources of Variation in the Foraging Efficiency of Grazing Ruminants

1. The relationships between foraging behaviour, incisor arcade breadth and body mass were described for sheep and goats grazing from swards of sown grass at two sward surface heights. 2. variation in bite mass was related to incisor arcade breadth more closely than to body mass. 3. Bite area scaled as the square of the incisor arcade breadth and was smaller on the Tall sward height treatment. 4. Sheep had greater bite masses than did goats but goats had a higher bite rate than sheep and as a consequence there was little difference in short-term intake rate between the species. 5. Young animals of both species had smaller bite masses and intake rates than did mature animals. However, when expressed relative to body mass there were no significant differences in bite mass between maturity classes. 6. The results of the current study confirm that the size of the incisor arcade is a direct determinant of bite dimensions and a driving variable for intake rate in grazing ruminants.

Selection for Foraging Efficiency During a Population Crash in Soay Sheep

1. There are surprisingly few examples of the fitness consequences of variation in foraging traits, despite their importance for survival and reproduction. We studied the relationship between traits affecting foraging efficiency and individual fitness in Soay sheep on the island of St Kilda, Scotland. 2. Survival during an over-winter population die-off was related to variation in incisor arcade breadth, a morphometric trait that is known to influence food intake. Other morphometric measures, such as body dimensions and weight, were less well related to survival. 3. Individual survival over winter was also related to gastrointestinal parasite burden, estimated in faecal samples in the previous August. Incisor arcade breadth and parasite burden affect nutrient assimilation rate, energy balance and susceptibility to mortality through starvation. An energetic model developed to predict energy intake and expenditure from incisor arcade structure, body reserves and thermal balance was no better at predicting individual survival than incisor breadth alone. 4. Our results raise questions about stabilizing selection on incisor breadth and the effect of spatial heterogeneity in the vegetation on selection pressure in different home ranges. We discuss the way pleiotropic effects may operate through increased fitness of animals with narrow incisor arcades feeding more selectively when food is plentiful at low population density.

Odontoma in a juvenile boxer: Clinical, radiographic and pathological findings

ABSTRACTCompound odontoma was diagnosed in a seven‐month‐old boxer dog. The hamartoma presented as an expansile painless mass in the right rostral hemimandible, causing severe distortion of the incisor arcade. Radiographs revealed filling of the medullary cavity of the bone by soft tissue containing mineralised foci, with attendant cortical thinning. The lesion was excised. Histological evaluation demonstrated a connective tissue stroma containing multiple irregular foci of enamel and dentine resembling rudimentary teeth (denticles). These were typically bounded by an ameloblastic zone and contained a central area of odontoblasts and dental pulp. At nine months after surgery, no recurrence has been recognised.

Variation in Foraging Behaviour in Red Deer and the Consequences for Population Demography

(1) Individual differences in the predicted foraging efficiency of forest-dwelling red deer Cervus elaphus L. are used to assess the consequences for population demography. (2) Analysis of the ontogeny of the incisor arcade and its allometry with body weight shows that growing animals experience restricted incisor breadth relative to weight during the transition from juvenile to mature dentition. (3) A model of bite dimensions predicts that yearlings, male calves and stags will have the greatest reduction in foraging efficiency when feeding on depleted resources. This is corroborated by observed patterns of seasonal weight loss. (4) Sex differences in growth strategy combined with the ontogeny and allometry of the incisor arcade may cause differential mortality between age and sex classes. It is suggested that population ecology and life-history theory should take greater account of natural selection on the juvenile phase.

Incisor Arcade Structure and Diet Selection in Ruminants

The structure of the incisor arcade in ruminant ungulates is likely to affect the degree to which animals can be selective in choosing food items. Species of ruminant which feed predominantly on grasses have broader more flattened incisor arcades when compared at the same size with species which browse on woody dicotyledonous plants. This is likely to reflect the differential dispersion of food items on the food plants of these two different groups of ruminants.

Relative growth of incisor arcade breadth and eating rate in cattle and sheep

ABSTRACTMaximum eating rate, rmaxin kj metabolizable energy per min, at a given body weight W kg, can be predicted in normally growing cattle when adult body weight, A kg, is known, by the formula of Taylor and Murray (1987) as rmax= 31μ0·86A0·73whereu= W/A is degree of maturity in body weight. When the pattern of normal growth is disturbed by fluctuating levels of food intake, a better prediction can be obtained in terms of incisor arcade breadth. This paper gives the allometric relationship between degree of maturity in body weight,u, and degree of maturity in incisor arcade breadth uD, fo r both cattle and sheep, asuD=u0·29.Combining this allometric relationship with that of Illius and Gordon (1987) gives the following formula for incisor arcade breadth, Du(mm), in the rth species at age,t, in relation to degree of maturity in body weight,uit, and adult body weight,Aikg): Dit= 7·8 uit0·29Ai0·36. Substituting uDand adult arcade breadth, Dadult, foruand A in the formula of Taylor and Murray (1987) gives the following more robust formula for predicting maximum eating rate, namely, rmax= 0·53u3DD2adut.