Tuesday, July 22, 2008

The Singing Bass: Kitschy or Insightful?

ResearchBlogging.org
If you were listening in on a discussion about the evolutionary origins of language, you might expect to hear theories bandied about concerning evidence for language-like processes in apes. You probably wouldn’t be too shocked to hear someone bring up an example of language in parrots. You might, however, be a little surprised if the conversation turned to the origins of human vocalization in toadfish.

Perhaps this isn’t that surprising, though, when one considers how much of our evolutionary beginnings are shared with fishes. While (of course) fish don’t have language in a human sense, some species do have the ability to make vocalizations in certain situations, like courtship or defense of territory. Although they lack an air tube leading to the mouth, and a larynx to create the vibrational variations more common to land animal utterances, some are able to make noises with an air sac used primarily for buoyancy control and secondary respiration, known as the gas bladder. Fish of the batrachoidid family in particular (i.e. the midshipman and toadfish) have a diverse group of vocalizations. They vary depending on the context, with specific calls for aggression, surprise, or mating (among others).

This leads to a couple of different hypotheses. One is that the ability to vocalize evolved independently a number of times throughout history: in fish, amphibians, reptiles, mammals, and birds. Another is that there is a common origin for the ability to vocalize that can be traced back millions of years to a piscine ancestor. A study published in this week’s Science explores the latter hypothesis by investigating the development of the neural circuitry for vocalization in larval fish.

Studying embryos or larvae is a method used in evolutionary developmental biology. Similarities in the embryonic development of two organisms are considered evidence of a common ancestor. This conclusion is based on the fact that evolution works by the alteration of existing structures. Thus, two related organisms will theoretically have similar embryonic development to a certain point, where it will then diverge in order to form the structures that make the two creatures taxonomically different. A commonly given example of this is the vestigial pharyngeal pouches (gill slits) that human embryos possess early in development.

The authors of the study in Science found that the vocal motor neurons in batrachoidid fish develop in a segmental region that spans the caudal hindbrain and rostral spinal cord. This is similar to the pattern of development found in other vertebrates like frogs and birds. Adult phenotypes seem to indicate a comparable developmental process in reptiles and mammals as well, although embryological studies here are lacking.

The authors conclude that these analogies in the distribution of vocal neurons indicate a conserved developmental pathway that involves Hox gene expression. They suggest this pathway predates the radiation of fish, originating over 400 million years ago. Thus, perhaps the Big Mouth Billy Bass is a more astutely-developed toy than it first appears to be…no, it’s still stupid.

Reference:

Bass, A.H., Gilland, E.H., Baker, R. (2008). Evolutionary Origins for Social Vocalization in a Vertebrate Hindbrain-Spinal Compartment. Science, 321(5887), 417-421. DOI: 10.1126/science.1157632

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