Table 1.

List of hypotheses concerning the energetic cost of brain evolution and predictions following from each hypothesis.

Hypothesis	Variables tested	Predicted link to brain size	Motivations for predictions
(1) Direct metabolic constraints	Depth as a proxy of BMR	Negative	Larger brains require higher BMR (Martin 1981; Isler and van Schaik 2006b)
(2) Expensive tissue	Gut length	Negative	The trade-off between investment into different energetically expensive organs (Aiello and Wheeler 1995; Kaufman et al. 2003)
(3) Energy trade-off	Egg size	Positive	Species with larger brains invest more into each egg to fulfill increased energetic requirements for offspring with larger brains (Isler and van Schaik 2009; Barton and Capellini 2011)
	Clutch size	Negative	Species with larger brains compensate for increased investment into a larger brain by reducing egg number per brood (Isler and van Schaik 2009; Weisbecker and Goswami 2010)
	Care duration	Positive	Species with larger brains prolong care duration because of longer developmental time in offspring with larger brains (Foley and Lee 1991)