The Evolution Of Echolocation For Predation

Symp. zool. Soc. Land. (1993) No. 65: 39-63

The evolution of echolocation for predation
J. R. SPEAKMAN
DepartmentofZoowgy University of Aberdeen Aberdeen AB9 2TN, UK

Synopsis
Active detection of food items by echolocation has some obvious advantages over passive detection, since it affords independence from ambient light and sound levels. For predatory animals, however, echolocationwould also appear to have a significant disadvanrage-i-the echolocation calls might alert prey to the predator's presence. Surprisingly, therefore, all but two of the different groups of vertebrates that have evolved echolocation are predatory. Despite the diversity of predatory taxa in which echolocation has evolved it is still a relatively uncommon form of perception. It has been suggested that amajor constraint on the evolution of echolocation is its high energy cost, due to rapid attenuation of sound in air. The cost of producing echolocation calls has been measured in insectivorous bats, whilst hanging at rest. These measures confirm that echolocation is extremely costly. However, bats normally echolocate in flight which also has a high cost. How bats cope with the high cost ofecholocation, when it is combined with flight, is therefore of extreme interest. Measures of the energy cost of flight of small echolocating bats ggest that the cost is no greater than that for non-echolocating birds and bats. The son for this apparent economy is that the same muscles which flap the wings also tilate the lungs, and produce the pulse of breath which generates the echolocation For a bat inflight, therefore, the additional cost of echolocating is very low, sr for a bat on the ground, and presumably other terrestrial vertebrates, the cost ry high. The release from the energetic constraint may explain the proliferation cholocation systems amongst flying predators and their paucity amongst trial predators. This model for the evolution of echolocation has some 'cant probiems-s-norablythe absence of echolocation amongst predatory birds fruit bats. Possible solutions to these problems include the lack of an advantage iurnal predators and phylogenetic constraints in both the ventilatory and tual systems of these animals.

~Jversity of echolocators

al systems can be divided into two fundamentally different
Copyrlghr © 1993 lhe Zoological SocietJ of London All r-ightsofreproduction in any form reserved

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evolution of echolocation for predation

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categories. There are passive systems, where the animal passively receive inputs of sensory information. This is the system that we use, and it is also used by most other animals. Alternatively, an animal may actively sense its environment. Using this latter type of system the animal does not wai passively untilthere is an input of sensory information, but rather create that input itself. Two different active sensory systems have evolved. The fir method involves the production of bio-electricity. By generating a potential difference between one end of its body and the other, the animal produces an electric field around itself. When objects, which differ in their resistivity to the environment, come intothis field, they affect the flow of current, and can therefore be actively detected by the animal. Since air is a good electrical insulator this method can only evolve in water, and it has been found i several species of fish (e.g. Gymnarchus spp.: Machin & Lissmann 196 which live mostly in turbid water. Probably the most widespread and w< known method of active perception, however, is the systemof produci sounds and perceiving the environment by interpreting the returnin echoes---ealled echolocation (Griffin 1958). Active systems of perception have one major advantage over passivi systems: they allow the animal a substantial degree of independence fro variations in the amount of passive information that is available from t environment. A useful analogy in this context is to consider...