Fertility and paternity in the Eucalyptus snout-beetle Gonipterus scutellatus: females might benefit from sperm mixing

Abstract

Mating in Gonipterus scutellatus is very long and lasts on average 6.98 ± 0.49 hr. This could be interpreted as a mechanism of male guarding, but males neither increase copulation duration nor ejaculate volume when sperm competition risk increases. We studied paternity patterns using RAPDs, and the influence of multiple matings on fertility, fecundity and sperm supply. Using twicemated females, we compared the percentage of polymorphic bands shared between first male/offspring, second male/offspring, mother/offspring, unrelated adults (males). In agreement with a previous study on paternity with the sterilemale technique, we found that sperm mixing seems the prevalent sperm competition mechanism. In a second experiment, adults were randomly assigned to three treatments: females mated only once; females allowed to mate ad libitum with one male; and females that could mate with seven different males. Females of the last group showed a significant increase in fertility. Additionally, we compared the stored sperm volumes among four groups: female whose copulation was interrupted after 15 min, those that mated once, twice or three times. This experiment indicates that sperm supply increases with the number of matings, excluding the possibility of sperm removal. These facts and the occurrence of sperm mixing might explain the lack of male response to increased male density in previous studies.