Juvenile hormone, metabolic rate, body mass and longevity costs in parenting burying beetles

Levels of juvenile hormone (JH) are elevated during parental care in burying beetles (Nicrophorus) at a time when ovarian activity is suppressed, suggesting that JH plays an alternative role to its better known gonadotropic function. Because parental activity in burying beetles is time-intensive, it might be expected to be energetically stressful and to impart longevity costs. We predicted that the active (feeding) stage of care would be associated with elevated JH and greater energy expenditure (higher metabolic rate, lower body mass), and that this stress would result in shorter life span. Parents experimentally manipulated into providing more care for young had increased levels of JH. For females, there was a significant longevity cost associated with parental care but not with mating or egg production. During carcass preparation, females and males that would later regurgitate to larvae showed an initial increase in body mass followed by a significant decrease in mass during the intense period of parental provisioning of young (during and just after the JH peak). Males that provided little care (on small carcasses with a female partner) showed no such anticipatory weight gain. An indirect measure of metabolic rate (VCO2) was nearly twice as high in caregiving females compared to nonbreeding females. These results suggest that the energy demands and/or high JH levels during care extract a significant cost on longevity. We propose that JH has evolved to play a novel role in parental care in burying beetles associated with extreme energy demands during feeding of offspring.