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Simon B. Laughlin
considerable benefits. We will consider the relationship between cost and benefit
in sensory ecology, concentrating on the uptake and coding of sensory signals by
eyes. Here the benefits can be assessed by optical measures, resolution and
sensitivity, and by the quantity of information that is coded. The costs are
assessed in terms ofthe consumption of metabolic energy. Energy consumption is
determined by the size of a sense organ, and by the electrical current used in
receptors and neurons to encode and transmit information.
2 Adaptations Suggest Costs and Benefits
Adaptations of form to function are particularly prominent in well-developed
sensory systems. For example, the pinnae of bats' ears come in many shapes and
sizes (Aitringham 1996). The pinnae contribute to the sensitivity, directionality
and frequency tuning of hearing. Many interspecific variations in the frequency
content and temporal structure of bats' calls are adaptations that improve
echolocation in particular habitats (Neuweiler 1989). Thus the variety of pinnae
ultimately relate to sensory ecology. The benefits of larger and more highly
sculpted pinnae, higher sensitivity and better resolution, are balanced by costs,
such as the drag induced in flight. Costs will tend to reduce the size of pinnae.
Eyes vary widely in design, shape and size (Walls 1942). Anatomical resolving
power, defmed as the reciprocal of the angular separation of photoreceptors,
increases with the length or radius of the eye. Kirschfeld (1976) examined the
relationship between anatomical resolving power and body height. Taking 18
species with simple or compound eyes, ranging in size from Drosophila to
human, the anatomical resolving power increases by 3 orders of magnitude,
roughly in proportion to body height. A resolution proportional to body height
provides a constant subjective resolution because a given object on the ground
subtends the same number of photoreceptors. Deviations from this scaling
relationship, such as the enlarged eyes of jumping spiders and falcons and the
reduced eyes of bats, relate directly to sensory ecology (Kirschfeld 1976).
The scaling of eye to body height for constant subjective resolution is not seen
in better-defined taxonomic groups. Across the vertebrates, including birds, fish,
reptiles and mammals from shrew to whale, the ratio between eye length and
body length decreases as size increases (Hughes 1977).
The largest vertebrate eyes have axial lengths of 35 mm to 50 mm but occur in
animals of very different size, such as the ostrich, the eagle, the zebra, the giraffe,
the elephant and the baleen whale. Hughes ( 1977) observes "given the nature of
the elements common to its construction and the physical world to which it is
adapted, a globe from 35 to 50 mm diameter suffices to meet the most stringent
requirements of species over a 10 4 - fold range of body size".
Could Leuckart's Law, swifter-moving species have larger eyes, determine the
scaling of eye to body (Brooke et a!. 1999)? Faster moving animals need to
resolve objects further ahead and if resolution increased in proportion to speed,
Simon B. Laughlin
considerable benefits. We will consider the relationship between cost and benefit
in sensory ecology, concentrating on the uptake and coding of sensory signals by
eyes. Here the benefits can be assessed by optical measures, resolution and
sensitivity, and by the quantity of information that is coded. The costs are
assessed in terms ofthe consumption of metabolic energy. Energy consumption is
determined by the size of a sense organ, and by the electrical current used in
receptors and neurons to encode and transmit information.
2 Adaptations Suggest Costs and Benefits
Adaptations of form to function are particularly prominent in well-developed
sensory systems. For example, the pinnae of bats' ears come in many shapes and
sizes (Aitringham 1996). The pinnae contribute to the sensitivity, directionality
and frequency tuning of hearing. Many interspecific variations in the frequency
content and temporal structure of bats' calls are adaptations that improve
echolocation in particular habitats (Neuweiler 1989). Thus the variety of pinnae
ultimately relate to sensory ecology. The benefits of larger and more highly
sculpted pinnae, higher sensitivity and better resolution, are balanced by costs,
such as the drag induced in flight. Costs will tend to reduce the size of pinnae.
Eyes vary widely in design, shape and size (Walls 1942). Anatomical resolving
power, defmed as the reciprocal of the angular separation of photoreceptors,
increases with the length or radius of the eye. Kirschfeld (1976) examined the
relationship between anatomical resolving power and body height. Taking 18
species with simple or compound eyes, ranging in size from Drosophila to
human, the anatomical resolving power increases by 3 orders of magnitude,
roughly in proportion to body height. A resolution proportional to body height
provides a constant subjective resolution because a given object on the ground
subtends the same number of photoreceptors. Deviations from this scaling
relationship, such as the enlarged eyes of jumping spiders and falcons and the
reduced eyes of bats, relate directly to sensory ecology (Kirschfeld 1976).
The scaling of eye to body height for constant subjective resolution is not seen
in better-defined taxonomic groups. Across the vertebrates, including birds, fish,
reptiles and mammals from shrew to whale, the ratio between eye length and
body length decreases as size increases (Hughes 1977).
The largest vertebrate eyes have axial lengths of 35 mm to 50 mm but occur in
animals of very different size, such as the ostrich, the eagle, the zebra, the giraffe,
the elephant and the baleen whale. Hughes ( 1977) observes "given the nature of
the elements common to its construction and the physical world to which it is
adapted, a globe from 35 to 50 mm diameter suffices to meet the most stringent
requirements of species over a 10 4 - fold range of body size".
Could Leuckart's Law, swifter-moving species have larger eyes, determine the
scaling of eye to body (Brooke et a!. 1999)? Faster moving animals need to
resolve objects further ahead and if resolution increased in proportion to speed,