Phenotypic Plasticity and Function of the Hard Palate in Growing Rabbits

Abstract

Morphological variation related to differential loading is well known for many craniomandibular elements. Yet, the function of the hard palate, and in particular the manner in which cortical and trabecular bone of the palate respond to masticatory loads, remains more ambiguous. Here, experimental data are presented that address the naturalistic influence of biomechanical loading on the postweaning development and structure of the hard palate. A rabbit model was used to test the hypothesis that variation in the morphology of the hard palate is linked to variation in masticatory stresses. Rabbit siblings were divided as weanlings into soft and hard/tough dietary treatment groups of 10 subjects each and were raised for 15 weeks until subadulthood. MicroCT analyses indicate that rabbits subjected to elevated masticatory loading developed hard palates with significantly greater bone area, greater cortical bone thickness along the oral lamina, and thicker anterior palates. Such diet-induced levels of palatal plasticity are comparable to those for other masticatory elements, which likely reflect osteogenic responses for maintaining the functional integrity of the palate vis-à -vis elevated stresses during unilateral mastication. These data support a role for mechanical loading in the determination of palatal morphology, especially its internal structure, in living and fossil mammals such as the hominin Paranthropus. Furthermore, these findings have potential implications for the evolution of the mammalian secondary hard palate as well as for clinical considerations of human oral pathologies.

Teaching an old jaw new tricks: Diet-induced plasticity in a model organism, from weaning to adulthood

“…Additionally, the experimental conditions mimicked the seasonal reliance of a species on foods that are usually avoided because they present more of a mechanical challenge to process than preferred foods, but which are critical with respect to survival during times of the year when preferred resources are scarce (Marshall and Wrangham, 2007). In accord with our predictions derived from previous plasticity studies in rabbits (Ravosa et al, 2007;Ravosa et al, 2008;Ravosa et al, 2010a;Menegaz et al, 2009;Scott et al, 2014) and current understanding of bone functional adaptation (e.g. Bouvier and Hylander, 1981;Lanyon and Rubin, 1985;Biewener, 1993;Pearson and Lieberman, 2004;Ruff et al, 2006;Ravosa et al, 2010b), we found that our experimental groups responded to mechanically challenging foods -and the increase in loading such foods are expected to engender -by increasing the cross-sectional areas of various jaw structures, thereby presumably reducing bone strain.…”

“…By contrast, preliminary observational data from a sample of 12 adults indicate that rabbits use approximately three times more chewing cycles per unit food mass when processing hay versus pellets (2.95 times more chews per g, 95% confidence interval: 2.58-3.35) (unpublished results), indicating that hay consumption does engender greater repetitive loading and correspondingly longer loading durations. Given the adaptive role of increased cyclical loading in bone formation (Bouvier and Hylander, 1981;Biewener et al, 1986), hay consumption should thus stimulate osteogenesis and result in larger jaw proportions in the two experimental groups in comparison with the control rabbits (Ravosa et al, 2007;Ravosa et al, 2008;Menegaz et al, 2009;Scott et al, 2014). With respect to differences between the experimental groups, we predicted an inverse relationship between age and the magnitude of diet-induced osteogenic responses; in other words: early rabbits at week 6>late rabbits at week 24>early rabbits at week 30>late rabbits at week 48.…”

“…Bouvier and Hylander, 1981;Burr, 1997;Westerlind et al, 1997;Bass et al, 1998;Lanyon and Skerry, 2001;Engelke et al, 2006) and its potential to inform reconstructions of behavior in past populations and extinct species (e.g. Menegaz et al, 2009;Holmes and Ruff, 2011;Scott et al, 2014;Standen et al, 2014). The way in which bone responds to loading over the course of an organism's lifetime is complex and multileveled (Bertram and Swartz, 1991;Hsieh et al, 2001;Pearson and Lieberman, 2004;Hamrick et al, 2006;Ruff et al, 2006;Ravosa et al, 2008;Ravosa et al, 2010b).…”

“…it is stiffer: pellets, E=29.2 MPa; wet hay, E=277.8 MPa; dry hay, E=3335.6 MPa) and it is probably tougher (Ravosa et al, 2007;Menegaz et al, 2009). Therefore, in comparison to the control rabbits, the early and late rabbits were expected to use highermagnitude bite forces or greater repetitive loading, or both, in order to process the hay-cube component of their diets.…”

Life history as a constraint on plasticity: developmental timing is correlated with phenotypic variation in birds

“…For example, fish reared on different diet types develop entirely different jaw morphologies (Meyer, 1987;Wainwright et al, 1991;Adams et al, 2003;Muschick et al, 2011), which adaptively affects their feeding performance on locally abundant resources (Bouton et al, 2002;Parsons and Robinson, 2007). Similarly, rabbits reared on different diets develop entirely different jaw, palate and cranial structures (Menegaz et al, 2009;Menegaz et al, 2010), whereas pigs reared in different locomotor environments differ in joint and bone structure (Hammond et al, 2010;Congdon et al, 2012). We focus on avian bills because variation in the length, depth and width of the bill has been tied to functional variation in avian foraging, both within and between species (Grant, 1979;Smith, 1987;Benkman et al, 2001;Herrel et al, 2005).…”