Male whiptail lizards adjust energetically costly mate guarding to male–male competition and female reproductive value

Simultaneous effects of mate guarding on a maleʹs energy intake and expenditure have not been measured. We tested whether guarding males of the western Mexican whiptail lizard, Aspidoscelis costata, reduce energy intake and increase expenditure of energy on male–male aggression. Also, we tested whether guarding males calibrate their aggressive behaviour and feeding to aggression from male rivals (a proxy of their femaleʹs probability of extrapair copulation) and body size of the guarded female (positively correlated with fecundity in this species). Observation during and after guarding showed that guarding males (1) ate 77% fewer prey/h and 54% smaller prey, (2) initiated 87% more agonistic interactions/h and (3) participated in 120% more escalated agonistic interactions/h than when alone. Also, guarding males initiated more aggressions when aggression from other males and female size were greater. These results indicate that mate guarding is costly for males because of simultaneous reduction in energy intake and increased expenditure on aggression, and that males incur higher guarding costs when competition with other males and female reproductive value are greater. These costs of mate guarding probably result in negative energy balances that could reduce male fitness through diminished survival. The evolutionary persistence of mate guarding by males can be explained if the net gain in fitness derived from guarding more than offsets its survival costs.