Coming of Age in an Ant Colony: Cephalic Muscle Maturation Accompanies Behavioral Development in Pheidole Dentata
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Naturwissenschaften (2011) 98:783–793 DOI 10.1007/s00114-011-0828-6 ORIGINAL PAPER Coming of age in an ant colony: cephalic muscle maturation accompanies behavioral development in Pheidole dentata Mario L. Muscedere & James F. A. Traniello & Wulfila Gronenberg Received: 7 March 2011 /Revised: 13 June 2011 /Accepted: 13 July 2011 /Published online: 27 July 2011 # Springer-Verlag 2011 Abstract Although several neurobiological and genetic food intake, respectively, also increase significantly in correlates of aging and behavioral development have been thickness with age. However, these fibers are only slightly identified in social insect workers, little is known about thinner in newly eclosed workers and attain their maximum how other age-related physiological processes, such as thickness over a shorter time span in minors. The different muscle maturation, contribute to task performance. We growth rates of these functionally distinct muscles likely examined post-eclosion growth of three major muscles of have consequences for how adult P.dentataworkers, the head capsule in major and minor workers of the ant particularly minors, develop their full and diverse task Pheidole dentata using workers of different ages with repertoire as they age. Workers may be capable of feeding distinct task repertoires. Mandible closer muscle fibers, and interacting socially soon after eclosion, but require a which provide bite force and are thus critical for the use of longer period of development to effectively use their the mandibles for biting and load carrying, fill the posterio- mandibles, which enable the efficient performance of tasks lateral portions of the head capsule in mature, older workers ranging from nursing to foraging and defense. of both subcastes. Mandible closer fibers of newly eclosed workers, in contrast, are significantly thinner in both Keywords Ant . Caste evolution . Functional morphology. subcastes and grow during at least the next 6 days in minor Mandibles . Muscles . Temporal polyethism workers, suggesting this muscle has reduced functionality for a substantial period of adult life and thus constrains task performance capability. Fibers of the antennal muscles and Introduction the pharynx dilator, which control antennal movements and Molecular and physiological studies of social hymenopterans demonstrate that worker aging and behavioral maturation, and hence task performance, are accompanied by developmental Communicated by: Sven Thatje changes in adults involving brain anatomy and chemistry : M. L. Muscedere J. F. A. Traniello (Barron et al. 2007; Brown et al. 2004; Gronenberg and Department of Biology, Boston University, Riveros 2009; Jones et al. 2009; Kühn-Bühlmann and 5 Cummington Street, We h n e r 2006; Riveros and Gronenberg 2010;Schulzand Boston, MA 02215, USA Robinson 2001;Seidetal.2005, 2008; Seid and Traniello W. Gronenberg 2005; Stieb et al. 2010), gene expression (Smith et al. 2008), Department of Neuroscience, University of Arizona, and endocrine profiles (O’Donnell and Jeanne 1993; 611 Gould-Simpson Science Building, Robinson 1987; Withers et al. 1995). However, the roles of Tucson, AZ 85721, USA other structural and physiological developmental events in Present Address: the ontogeny of worker task competence, while likely M. L. Muscedere (*) important, are not as well established (Robinson 2009). Program in Neuroscience, Boston University, Muscular development in adult insects (Chapman 1998; 2 Cummington Street, Boston, MA 02215, USA Finlayson 1975) is poorly understood, although it is e-mail: [email protected] taxonomically widespread and could have important effects 784 Naturwissenschaften (2011) 98:783–793 on social insect worker task abilities. Most described species (Seid et al. 2005, 2008; Seid and Traniello 2005). examples of adult muscle development concern flight However, the role of other physiological factors, such as muscles, which can continue to grow after the terminal molt muscle development, remains unknown. Colony populations (Chapman 1998;deKort1990; Finlayson 1975;Marden in P. dentata are comprised mostly of small minor workers, 2000). In the social insects, adult honey bee workers do not which are numerically dominant and responsible for most forage until they have experienced two bouts of flight muscle colony activities such as brood rearing, nest construction, and development over several weeks, resulting in dramatic foraging. Minor workers acquire new and more diverse task changes in muscle metabolism and biochemistry that abilities as they age, adding outside-nest tasks such as foraging significantly increase power output (Roberts and Elekonich to their existing within-nest repertoire (Seid and Traniello 2005; Schippers et al. 2010). Non-flight skeletal muscles can 2006). Task efficiency changes with age as well: Muscedere et also grow substantially in adult insects; examples include the al. (2009) recently showed that young P. dentata minors rarely oviposition muscles of locusts (Locusta migratoria), which pick up and pile scattered eggs and microlarvae in standard- reach full size and strength only after adult females attain ized laboratory assays, while older workers do so very quickly. reproductive maturity (Rose 2004), and some skeletal When caring for older brood, young minor workers in the muscles, including mandibular muscles, in the beetle Ips same experiments made antennal contact with and licked sexdentatus (Termier 1970). However, descriptions of the larvae and pupae with their inner mouthparts, but unlike older development of muscles apart from those associated with workers, rarely manipulated or lifted brood with their flight in adult insects are rare, and examples of this type of mandibles (ML Muscedere, personal observation). Addition- muscle development in social insects are nonexistent. ally, major workers, which are defensive in function, comprise Ant workers construct nests, forage, care for developing 10–15% of P. dentata colonies and are characterized by brood, and defend their colonies from predators and compet- allometrically large heads equipped with strong jaws powered itors. These tasks require the use of the mandibles, which by well-developed mandibular muscles (Wilson 1976b). Major typically have a multi-purpose, shovel-like morphology that workers exhibit a pattern of age-related behavioral maturation reflects their involvement in most colony tasks (Hölldobler and apparently similar to that of minors: young majors do not Wilson 1990). While the mandibles open and close only in a leave the nest to participate in defensive activities (ML single plane, bite speed and force can be altered to achieve Muscedere and JFATraniello, personal observation). very different functional outcomes, from rapid and powerful These observations prompted us to question whether prey-subduing bites, to slow, precise movements required for young workers are unable to effectively use their mandibles nursing small and delicate soft-bodied larvae. Mandibular because their mandibular musculature is not fully devel- movements are controlled primarily by the mandible closer oped and to determine whether cephalic muscles develop at (MC) muscles; accordingly, the MCs are particularly large and differing rates, resulting in constraints on the use of the well-developed in ants, comprising up to two thirds of the total mandible muscles relative to other cephalic muscles. These volume of the head capsule and containing at least two distinct considerations are significant to achieving a comprehensive fiber types with different contractile properties (Gronenberg et understanding of age-related task performance and testing al. 1997;Paul2001; Paul and Gronenberg 1999). While the theories of division of labor (Beshers and Fewell 2001) MCs dominate the cephalic musculature of ants, many such as the repertoire expansion model (Seid and Traniello smaller muscles are also present. These include the pharynx 2006), which posits that neural and physiological develop- dilator (PD) muscles, which facilitate food ingestion by ment in adult workers underlies their age-related acquisition producing suction in the alimentary canal (Paul et al. 2002); of new tasks and increased task proficiencies. Therefore, we the antennal muscles (AM), which produce the rapid and examined how the sizes and morphologies of developing controlled movements of the antennal scape that allow ants muscle fibers of the MC, AM, and PD change with age in to effectively sample their sensory environment (Ehmer and adult P. dentata workers. We focused on minor workers due Gronenberg 1997;Pauletal.2002); and other muscles, to their central role in colony labor, but include comparative including the mandible opener muscles and muscle groups data on majors to determine if workers in both subcastes that effect movement of the maxillae, labium, and other inner undergo similar patterns of muscle maturation. mouthparts (Paul et al. 2002). The dimorphic ant Pheidole dentata has served as a model for research into the behavioral organization and ecology of Materials and methods age-related worker behavioral development in social insects (Calabi and Traniello 1989a,b; Muscedere et al. 2009;Seid Colony maintenance and Traniello 2006; Traniello 2010; Wilson 1976a). Recent work has begun to describe the neurochemical and neuroan- All workers were sampled from queenright colonies of P. atomical correlates of worker behavioral maturation in this dentata, a small, completely dimorphic ant native to the Naturwissenschaften (2011) 98:783–793 785 southeast