This book focuses on sensing and the evolution of animals. Using the
five senses (visual, auditory, and olfactory perception, and taste and
touch), animals can receive environmental stimuli and respond to them.
Changes in these sensitivities might cause changes in aspects of
animals' lives such as habitat, activity timing, and diet--and vice
versa. Recent advances in genome and molecular analysis enable us to
investigate certain changes in the receptors or mechanisms involved in
sensing and provide clues for understanding the evolution of animals
related to those changes. The first chapter deals with the molecular
evolution of opsins. In addition to the well-known function of opsins as
visual receptors, opsins can be related to non-visual photoreception
such as photoentrainment of circadian rhythm, photoperiodism, and
background adaptation. Molecular phylogenic studies reveal that all
opsin genes have evolved from one ancient opsin gene. The evaluation of
the functions of each extant opsin protein based on the molecular
features enables us to predict the molecular evolution and
diversification of opsins during the evolution of animals. These studies
shed light on which amino-acid substitutions cause the functional
diversification of opsins and how they have influenced the evolution of
animals. The second chapter has to do with bitter taste perception, a
key detection mechanism against the ingestion of bioactive substances.
Genetic and behavioral evidence reveal the existence of "non-taster"
Japanese macaques for specific bitter compounds, which originated in a
restricted region of Japan. This finding might provide a clue for
elucidating the ecological, evolutionary, and neurobiological aspects of
bitter taste perception of primates. The third chapter presents an
extreme example of the evolution of olfaction, namely, that fully
aquatic amniotes have generally reduced their olfactory capacity
considerably compared to their terrestrial relatives. Interestingly, the
remaining olfactory abilities are quite different among three fully
aquatic amniotes investigated: toothed whales have no nervous system
structures that mediate olfaction, but baleen whales can smell in air,
and it has been suggested that sea snakes smell underwater.
