How Tube-Lipped Nectar Bats Extend Their Tongues Past Their Bodies

Anoura fistulata is a small leaf-nosed bat from the cloud forests of the Ecuadorian Andes. It weighs about ten grams as an adult and is about six centimeters long from nose to tail. It feeds on nectar, like many of its close relatives in the genus Anoura, and it pollinates several specific flowering plant species in its range.

The unusual feature, formally described by Nathan Muchhala in a 2006 paper in Nature, is the tongue. The tongue of Anoura fistulata is about 8.5 centimeters long when fully extended. The bat itself is 6 centimeters long. The tongue is 150 percent of the body length, the highest tongue-to-body length ratio measured in any mammal.

The physical problem

A tongue longer than the body has a basic anatomical problem: where does it go when it is not extended? Most mammals with prominent tongues store them in the mouth or in a relatively short retracted state behind the jaw. A tongue 150 percent of body length cannot be stored anywhere in the head; the head is not large enough.

The solution Anoura fistulata has evolved is to store the tongue in the chest cavity. The base of the tongue passes through the mouth and continues backward through the neck, the chest, and ends in a specialized housing called a glossal tube that runs alongside the sternum and between the heart and the lungs. When the bat extends its tongue, the entire structure is pushed forward through the housing, out the mouth, and into the target flower. When the bat retracts the tongue, the structure pulls back through the same path and re-coils inside the chest.

The glossal tube is muscle-bound and elastic. It is not a passive storage container; it is an active organ that controls the protrusion and retraction of the tongue. The protrusion is driven by hydrostatic pressure: the tongue is filled with blood and lymph at high pressure, which lengthens it through the same mechanism that elongates an octopus arm or a chameleon tongue. The retraction is driven by muscle contraction in the glossal tube wall.

The plant partnership

Muchhala's 2006 paper identified the specific plant species that Anoura fistulata pollinates: Centropogon nigricans, a bell-shaped flower with a corolla tube 8 to 9 centimeters long. The flower's nectary is at the bottom of the corolla, accessible only to a pollinator with a tongue (or beak) at least 8 centimeters long. Most other nectar-feeding bats in the Anoura genus have tongues 3 to 5 centimeters long and cannot reach the Centropogon nigricans nectary. Hummingbirds in the area also cannot reach the nectary because their bills are too short.

The partnership appears to be specialized: Centropogon nigricans is the dominant food source for Anoura fistulata in the season when it is flowering, and Anoura fistulata is the primary pollinator for Centropogon nigricans across most of its range. Both species depend on each other. The morphological match between the corolla length and the tongue length is unusually precise: the tongue is just long enough to reach the nectary plus a margin, suggesting selection pressure that produces tight co-adaptation.

The partnership is also unusually exclusive. A flower with a 9-centimeter corolla cannot be pollinated by a bat with a 5-centimeter tongue; the bat cannot reach the nectar and either gives up or pollinates a different flower. A bat with a 9-centimeter tongue is overcommitted to flowers with long corollas; it cannot easily switch to shorter-corolla species without significant inefficiency. The mutual specialization makes both species vulnerable to environmental changes that disrupt either partner.

The evolutionary trajectory

The Anoura genus contains roughly ten species, distributed across the Neotropics from Mexico to Bolivia. Most of the species have tongues in the typical mammalian range (10-40 percent of body length). Anoura fistulata is the only species with a tongue exceeding body length.

The phylogenetic analysis suggests that the extreme tongue length is a relatively recent specialization within the genus. The ancestral Anoura probably had a tongue similar to the typical nectar-feeding bat range. The chest-cavity housing for the glossal tube is a derived feature that requires multiple coordinated changes in skeletal and muscular anatomy. The fact that no other Anoura species shares the trait suggests that the chest-cavity housing evolved once, in the lineage leading to Anoura fistulata, in association with the partnership with long-corolla Centropogon flowers.

The Centropogon nigricans corolla length similarly appears to be a derived feature within its genus. The genus contains species with corolla lengths ranging from 1 to 9 centimeters. The 9-centimeter corolla of Centropogon nigricans is the longest in the genus and corresponds to the longest tongue in its pollinator. The co-evolution appears to be reciprocal: as the bat's tongue lengthened, the flower's corolla lengthened in response, with each adaptation creating selection pressure for the other.

The functional questions that remain open

The glossal tube anatomy was characterized by Muchhala's 2006 paper using dissection and dye-injection studies on preserved specimens. The functional dynamics of extension and retraction (the speed of the movements, the muscle activation patterns, the hydrostatic pressure profile) have not been characterized in detail. The bat is small, lives in inaccessible cloud-forest habitat, and is challenging to keep in captivity, which limits the experimental approaches available.

The energetic cost of maintaining the chest-cavity glossal tube apparatus is also not well-characterized. The tube takes up space that other organs (heart, lungs, digestive system) might otherwise use. The presence of a large muscular tube in the chest cavity changes the mechanics of breathing and circulation in ways that may have measurable costs. Whether the energetic cost is small enough to be easily offset by the nectar reward, or whether it requires the bat to spend a substantial fraction of its time foraging, is an open question.

The development of the glossal tube during the bat's growth is also unstudied. The juvenile bats presumably have a smaller tongue and a smaller chest cavity; how the coordinated growth of the two structures is genetically controlled, and what happens if one outgrows the other, are interesting developmental questions that have not been addressed.

The broader pattern

Anoura fistulata is one example of an unusual specialization that emerges from a plant-pollinator co-evolutionary relationship. The pattern recurs across multiple lineages and continents. The Madagascan star orchid Angraecum sesquipedale and its hawkmoth pollinator Xanthopan morganii have similar coadapted tongue-and-corolla lengths (a 30-centimeter corolla and a 25-centimeter proboscis), famously predicted by Darwin before the pollinator was discovered. Various long-tongued bee species in the Andes have similar relationships with long-corolla flowers.

The structural feature that makes these specializations possible is the hydraulic-extension mechanism that biological tongues and proboscises use. A muscle-driven mechanism is bounded by the length of the contracting muscle and the mechanical advantage of the muscle-and-bone arrangement. A hydraulic-extension mechanism is bounded only by the volume of the extending tissue and the geometry of the storage compartment. Hydraulic extension can produce tongue-to-body ratios that muscle-driven extension cannot match.

The deeper observation is that the inventory of mammalian morphological possibilities is wider than the canonical textbook examples suggest. Mammals are not, in general, a body plan that supports unusual morphological specialization compared to insects or some fish lineages. But specific mammal species at the edges of the inventory show that the constraints are not absolute. The chest-cavity glossal tube of Anoura fistulata is one such edge case. The 6-meter ileum of the giant anteater, the 100-kilogram tongue of the blue whale, and the 15-meter wing of the wandering albatross are others. Each represents a specific solution to a specific problem at the edge of what mammalian developmental machinery can produce.

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