The Bogert effect and environmental heterogeneity

A classic question in evolutionary biology is whether behavioural
flexibility hastens or hinders evolutionary change. The latter idea, that
behavior reduces the number of environmental states experienced by an
organism and therefore buffers that organism against selection, has been
dubbed the “Bogert Effect” after Charles Bogert, the biologist who first
popularized the phenomenon using data from lizards. The Bogert Effect is
pervasive when traits like mean body temperature, which tend to be
invariant across space in species that behaviourally thermoregulate, are
considered. Nevertheless, behavioural thermoregulation decreases or stops
when spatial variation in operative temperature is low. We compared
environmental temperatures, thermoregulatory behavior, and a suite of
physiological and morphological traits between two populations of the
southern rock agama (Agama atra) in South Africa that experience different
climatic regimes. Individuals from both populations thermoregulated
efficiently, maintaining body temperatures within their preferred
temperature range throughout most of their activity cycle. Nevertheless,
they differed in the thermal sensitivity of resting metabolic rate at
cooler body temperatures, and in morphology.Our results support the common
assertion that thermoregulatory behaviour may prevent divergence in traits
like field-active body temperature, which are measured during periods of
high environmental heterogeneity. Nevertheless, we show that other traits
may be free to diverge if they are under selection during times when
environments are homogenous. We argue that the importance of the Bogert
Effect is critically dependent on the nature of environmental
heterogeneity and will therefore be relevant to some traits and irrelevant
to others in many populations.