The reproductive biology of Ptomascopus morio, a brood parasite of Nicrophorus

Abstract

Nicrophorine beetles (Nicrophorus and Ptomascopus spp.) use small carcasses as a food source for young, a breeding ecology distinct from other silphid beetles. While adaptations to the use of small carcasses are well known for Nicrophorus (emitting sex pheromone, burying, rounding and removing hair from carcasses, regulating brood size, regurgitating to young, and preventing predation), there is little information regarding its sister group, Ptomascopus. Like Nicrophorus, Ptomascopus morio males emit pheromone to attract females. In the absence of carrion competitors Ptomascopus morio parents were found to stay with a carcass and their brood for up to 10 days. We tested six hypotheses to examine whether young benefit from this long period of parent–offspring contact. (1) There was no evidence that parents buried or otherwise pre-empted carcasses to reduce competitive pressure. (2) We found no evidence that parents influenced the decomposition of the carcass. This was supported by experimental manipulations in which brood production (number of larvae and total brood mass) was no greater on carcasses on which parents were present than on carcasses not ‘prepared’ by parents. In addition, the carcass was not rounded and little hair was removed by the adults. (3) The presence of parents benefited the brood by reducing the negative effects of competition with carrion fly larvae. This likely resulted from predatory feeding by adult beetles. (4) Females adjusted clutch size to the size of the carcass. Parents, however, did not make a second adjustment in brood size after young reached the larval stage (filial cannibalism), as occurs in Nicrophorus. (5) Although parents were observed to open feeding holes in the carcass, this was not necessary for normal larval growth and survival. Parents were not observed to feed young directly by regurgitation. (6) Lastly, parents did not reduce predation on their brood when a conspecific intruder was present. These findings suggest that after the female parent adjusts clutch size to the size of the resource, the only parental benefit is clearing the carcass of fly larvae. Other differences with Nicrophorus include an extended period of oviposition (5 days) and less pronounced changes in ovarian mass and juvenile hormone titers in response to discovery of a carcass. In a field experiment in Kyoto, Japan, 17 of 21 broods of N. concolor during August contained larvae of P. morio. Mixed Nicrophorus–Ptomascopus broods were less common at other times of the year and when N. quadripunctatus occupied carcasses. In the laboratory, P. morio was able to parasitize 19 of 20 broods of N. concolor. The pattern of oviposition, the absence of explicit parental behaviours, and the interactions with N. concolor in the field suggest that Ptomascopus morio is a brood parasite of Nicrophorus.