This paper concerns the response of foraging animals to variability in
rate of gain, or risk. Both the empirical and theoretical literatures
relevant to this issue are reviewed. The methodology and results from
fifty-nine studies in which animals are required to choose between fo
raging options differing in the variances in the rate of gain availabl
e are tabulated, We found that when risk is generated by variability i
n the amount of reward, animals are most frequently risk-averse and so
metimes indifferent to risk, although in some studies preference depen
ds on energy budget. In contrast, when variability is in delay to rewa
rd, animals are universally risk-prone. A range of functional, descrip
tive and mechanistic accounts for these findings is described, none of
which alone is capable of accommodating all aspects of the data. Risk
-sensitive foraging theory provides the only currently available expla
nation for why energy budget should affect preference. An information-
processing model that incorporates Weber's law provides the only gener
al explanation for why animals should be risk-averse with variability
in amount and risk-prone with delay. A theory based on the mechanisms
of associative learning explains quantitative aspects of risk-pronenes
s for delay; specifically why the delay between choice and reward shou
ld have a stronger impact on preference than delays between the reward
and subsequent choice. It also explains why animals should appear to
commit the ''fallacy of the average,'' maximising the expected ratio o
f amount of reward over delay to reward when computing rates rather th
an the ratio of expected amount over expected delay. We conclude that
only a fusion of functional and mechanistic thinking will lead to prog
ress in the understanding of animal decision making.