The Functional Significance of Jaw-Muscle Fiber Architecture in Tree-Gouging Marmosets

  • Andrea B. Taylor
  • Carolyn M. Eng
  • Fred C. Anapol
  • Christopher J. Vinyard
Chapter
Part of the Developments in Primatology: Progress and Prospects book series (DIPR)

Abstract

Marmoset species are specialized gummivorous callitrichids that gouge holes in trees to stimulate the flow of exudate. Recent experimental studies suggest that when common marmosets (Callithrix jacchus) gouge trees in the wild, they do so with jaw gapes approaching their maximum structural capacity for jaw opening. Common marmosets also have relatively elongated mandibular condyles (extending the radius of curvature) and glenoid articular surfaces, and low mandibular condyles relative to the height of the tooth row, features that are advantageous in improving the capacity to generate wide jaw gapes.

To determine if tree-gouging marmosets have jaw muscles that are architecturally suited to improving muscle stretch, and hence wide jaw gapes, we evaluated fiber architecture of the masseter and temporalis muscles in two tree-gouging marmosets (Callithrix jacchus and Cebuella pygmaea) and one nongouging tamarin (Saguinus oedipus). Common and pygmy marmosets have relatively longer masseter and temporalis fibers. As fiber length is proportional to muscle excursion, these findings indicate that tree-gouging marmosets have jaw-closing muscles that are well suited to facilitating muscle stretch, and thus large jaw gapes. Marmosets also have a lower tendon to muscle fiber ratio, which may facilitate greater neural control over this specialized feeding behavior. The masseter and temporalis physiological cross-sectional areas (PCSAs) are relatively smaller in tree-gouging marmosets compared to the tamarin. PCSA is directly proportional to the maximum tetanic tension that a muscle can generate. Therefore, tree-gouging marmosets have relatively lower force generating abilities compared to S. oedipus. The relatively smaller PCSAs support previous evaluation of the bony masticatory apparatus, which found no morphological evidence to suggest that marmosets have increased force generation or load resistance abilities compared to tamarins. Relatively longer fibers, and their capacity to enhance muscle stretch, suggest that masseter and temporalis fiber architecture function to facilitate the production of wide jaw gapes during tree gouging in the animal’s natural environment.

Resumen

Las especies marmosets son gumnívoros calitricidos especializados que cavan oquedades en los árboles estimulados por corrientes exudadas. Estudios experimentales recientes sugieren que cuando los Callithris jacchus excavan árboles en la naturaleza, realizan ellos con las mandíbulas abiertas acercándose a su máxima capacidad estructural para la apertura de la mandíbula. Los Callithrix jacchus tienen también cóndilos mandibulares y superficies glenoides articulares relativamente alargadas, y cóndilos mandibulares inferiores relativamente altos en relación a la hilera de dientes, rasgos que generan ventajas al incrementar la capacidad de generar aperturas de mandíbula amplias.

Para determinar si los marmosets excavadores de árboles tienen músculos mandibulares que están arquitectónicamente colocados para incrementar la elasticidad muscular, y asociados a las aperturas amplias de mandíbula, evaluamos la arquitectura de fibra de los músculos masseter y temporales en dos marmosets (C. jacchus y Cebuella pygmea) y un tamarin no excavador (Saguinus oedipus). Los C. jacchus y C. pygmaea tienen fibras masseter y temporales relativamente más largos. Como el tamaño de la fibra es proporcional a la excursión muscular, los hallazgos indican que los marmosets excavadores de árboles tienen músculos para cerrar la mandíbula los cuales están bien situados para facilitar la elasticidad muscular, y por lo tanto aperturas de mandíbulas amplias. Asimismo, también muestran el tendón bajo para reforzar el radio de la fibra muscular, el cual puede facilitar un mayor control neutral sobre dicho comportamiento especializado de alimentación. Las áreas fisiológicas seccionales cruzadas PCSA de los masseter y los temporales son relativamente más pequeñas en los los C. jacchus y C. Cebuella comparados con el S. oedipus. El PCSA es directamente proporcional a la tensión máxima tetánica que un músculo puede generar. Por lo tanto, los C. jacchus y C. Cebuella muestran habilidades generadoras de fuerza relativamente baja comparadas con el S. oedipus. La PCSA relativamente más pequeña apoya evaluaciones previas relacionadas con el aparato de hueso masticatorio, el cual no encontró evidencia morfológica que sugiera que los marmosets han incrementado la generación de fuerza o habilidades de resistencia de carga comparada con los tamarins. Fibras relativamente más largas, y su capacidad para realzar la elasticidad muscular, sugiere que las fibras largas masseter y temporales funcionan para facilitar la producción de aperturas amplias de mandíbula durante la excavación de árboles en su ambiente natural.

Resumo

Micos são espécies de calitriquídeos especializadas em comer goma, raspando buracos nas árvores para estimular o fluxo de exsudados. Estudos experimentais recentes sugerem que quando os micos-estrelas (Callithrix jacchus) raspam as árvores no campo, eles fazem com aberturas da mandíbula aproximando o máximo da capacidade estrutural de abertura da mandíbula. Micos-estrela também têm côndilos mandibulares (extendendo o raio da curvatura) e superfícies articulares glenóides relativamente longos, e côndilos mandibulares baixos em relação à altura dos dentes, caracteristicas que são vantajosas em aumentar a capacidade de mordidas amplas da mandíbula.

Para determinar se os micos “goivadores” de árvores têm músculos mandibulares que são arquiteturalmente adaptados para melhorar o estiramento muscular, e portanto amplas aberturas mandibulares, nós avaliamos a arquitetura de fibras do masseter e do temporal em dois micos (Callithrix jacchus e Cebuella pygmaea) e um mico não-goivador (Saguinus oedipus). Micos-estrelas e sagüis-leãozinho têm fibras massetéricas e temporais relativamente longas. Como o comprimento da fibra é proporcional à excursão do músculo, estas descobertas indicam que os micos goivadores-de-árvores têm músculos adutores da mandíbula que são bem ajustados para facilitar a estiramento muscular, e assim grandes aberturas da boca. Os micos também têm uma baixa proporção de tendão para fibra muscular, o que facilita mais ainda o controle neural(neurológico) sobre este comportamento alimentar especializado. As áreas de seção-transversal fisiológica (PCSAs) do masseter e temporal são relativamente pequenas nos micos-goivadores quando comparadas aos sagüis. A PCSA é diretamente proporcional a tensão tetânica que um músculo pode gerar. Portanto, micos-goivadores têm relativamente menor capacidade de gerar forças quando comparados à S. oedipus. As PCSAs relativamente menores suportam as avaliações prévias do aparato masticatório ósseo, que não mostraram nenhuma evidência morfológica que indicasse que os micos têm uma geração de força maior ou resistência a cargas quando comparados aos sagüis (Saguinus). Fibras relativamente longas, e sua capacidade de potencializar a extensão múscular, sugerem que a arquitetura do masseter e do temporal funciona para facilitar a produção de grandes aberturas da boca durante a raspagem das árvores no ambiente natural.

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Copyright information

© Springer Science+Business Media, LLC 2009

Authors and Affiliations

  • Andrea B. Taylor
    • 1
  • Carolyn M. Eng
  • Fred C. Anapol
  • Christopher J. Vinyard
  1. 1.Departments of Community and Family Medicine and Evolutionary AnthropologyDuke University School of MedicineDurhamUSA

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