Mechanical Stimulation and Imposed Monogamy

Make Him a Monogamy Junkie

The Monogamy Method

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Roth's (1964b) demonstration that the suppression of female receptivity results from the physical insertion of the spermatophore into the bursa in N. cinerea has been interpreted as evidence that males force monandry on females during their first reproductive cycle. The bursa and the brood sac are in close physical proximity within the female genital tract. This serves as the basis for the argument that males are co-opting the physiological mechanism evolved to suppress female receptivity during pregnancy, and so females are precluded from evolving countermeasures to this manipulation (Harris and Moore, 2004; Montrose et al., 2004). Several points must be carefully considered before accepting this interpretation.

First, while the brood sac is spatially proximate to the genital papilla on which the spermatophore is secured, there is no evidence that the two structures share a mechanism for suppressing female receptivity. The highly distensible brood sac is situated at the anterior end of the vestibulum. It is separated from the genital papilla by the laterosternal shelf (McKittrick, 1965) (Fig. 6.14A). When the female is incubating an ootheca, the genital papilla is forced to stretch as the egg case projects into the vestibu-lum (Fig. 6.14B). Nonetheless, engaging the mechano-receptors in the brood sac of a virgin has little to no effect on her receptivity. When glass beads were inserted into the brood sac without applying pressure to the bursa, 72% of virgins subsequently mated. Some physiological change occurs after ovulation that makes females responsive to inhibitory stimuli from the stretched brood sac (Roth, 1964b, p. 925). The loss of receptivity after the first copulation of her adult life, and the loss of receptivity in response to an ootheca stretching the brood sac, then, do not have a shared control mechanism.

Second, the imposed monogamy scenario is predicated on the assumption that multiple copulations within the first reproductive cycle confer benefits on female N. cin-era. In many insects, females profit from multiple matings because they can increase fitness via increased egg production and fertility (Arnqvist and Nilsson, 2000). A male, on the other hand, benefits by rendering females sexually unreceptive after mating, thus increasing the probability that his sperm will fertilize the majority of the female's eggs (Cordero, 1995; Eberhard, 1996; Gillott,

Fig. 6.14 (A) Sagittal section of the female genitalia of Gromphadorhinaportentosa (Blaberidae). (B) Diagrammatic sagittal section of blaberid female genitalia with ootheca in brood sac. From McKittrick (1964).

2003). If multiple matings do increase female fitness, it follows that the control of female sexual receptivity is a source of conflict between the sexes, and females are expected to evolve resistance to the stimuli males use to induce receptivity loss (Arnqvist and Nilsson, 2000). That does not appear to be the case in N. cinerea. Copulation is known to confer numerous fitness benefits on female cockroaches (discussed below),but within the framework of cyclic receptivity typical of N. cinerea there is currently no evidence that more than one mate within the first reproductive cycle is advantageous. Moreover, morphological and experimental evidence suggests that sper-matophore placement and therefore loss of receptivity in N. cinerea is likely under female control, suggesting that there is no conflict of reproductive interest between the sexes on this issue. Not only do females have morphological features specialized for proper spermatophore placement and retention, these features are regulated by her nervous system. Receptivity in N. cinerea is suppressed only if the spermatophore is firmly placed and properly positioned (Roth, 1964b). While in some blaberids a large amount of glue-like secretion cements the spermato-phore into place, in Nauphoeta and several related genera the bursa is largely responsible for spermatophore retention (Graves, 1969). The bursa is deep, is extensively membranous, and almost completely wraps around the correspondingly elongated spermatophore. If the nerve cords are severed prior to mating in female R. maderae, another species with a deep, membranous bursa, 70% of males were not able to insert the spermatophore properly. They were placed elsewhere in the genital atrium or dropped by the male without being transferred. In many cases the male had pierced the wall of the brood sac and the spermatophore was in the female's body cavity. "It seems the female takes an active role in the proper positioning of the spermatophore in the bursa copulatrix, and an intact nerve cord is needed for proper muscular movements of the female genitalia" (Roth and Stay, 1962a).

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