Page 1 of 4 | © 2005-2010 text by Stephanie Shorter, Ph.D.
After ejaculation sperm can still be ambushed from the presence of sperm from a rival male. This post-copulatory phenomenon, called sperm competition, clearly contributes to the male ability to control how many offspring he produces and is relatively well understood. In contrast, to what degree can females control paternity? Do they have a means of exerting control post-copulatory also? Examining patterns of paternity in an arthropod with multiple mates allows such questions to be addressed because female arthropods have multiple sperm storage organs (spermathecae).
If a female's behavior can be predicted by which spermathecae was inseminated then this would be a fascinating example of behavioral strategy and morphology interacting. One mechanism of females controlling the paternity of their offspring could be through the selective use of sperm at fertilization. To what degree can females Voluntarily control sperm release? An attempt was made to answer this question before, yet the results were not easily interpreted; it was impossible to disentangle the factors under male control and those under female control. Therefore, the goal of this study was to create conditions in which male sexual behavior is manipulated in order to isolate the influence of the female. The logic rests on separating ejaculates by controlling which spermathecae the male inseminates. No other spider paternity study has experimentally manipulated how insemination occurs. Latrodectus hasselti, the Australian redback spider, was the species chosen to study.
Paternity was revealed using the sterile male technique. Virgin females were each mated with two virgin males, one which had been made infertile by irradiation. Eggs fertilized by the irradiated male would fail to develop and, in this way, paternity could be tallied for each spiderling. Spider males have bilateral reproductive organs called pedipalps ("palps"). Experimenters removed the embolus, the apical segment of the palp, to force the males to copulate with his remaining palp. This palp was inserted into either the same or opposite spermathecae as a rival male. Interestingly, many second males resisted this researcher-induced scheme: many males in the same-side group attempted to use their malformed palp on the opposite (virgin) spermathecae side.
Paternity differed as a function of insemination pattern. When rival males were forced to inseminate the same spermathecae the first male had a distinct advantage. The key result of the experiment was paternal dominance for the first male. However, paternity was usually shared between both males. Sometimes the first male attempted to thwart subsequent suitors by depositing a sclerite sperm plug (hardened bit of the embolus) immediately after copulation. The only way a male could produce 100% of the offspring was if he successfully mates twice with the female, inseminating both spermathecae. This is a bit easier said than done though as females often prevent males from copulating again by aggression or cannibalism.
When rival males used opposite spermathecae, there was mixed paternity, consistent with the use of a combination of rival sperm. No evidence was seen of a differential release of sperm when the spermathecae emptied into a common duct at fertilization time. An equal amount of sperm was used from both storage organs.
Although not always successful, male behavior usually seems tuned in to the strategy that will maximize the number of offspring produced. For instance, many of the manipulated males showed a side preference when they were the second mate of the female. They tried to use their nonfunctional palp rather than the good palp that was on the same side as the previous male. Control males were allowed one copulation with unmanipulated palps. Consistent with the main results of the experiment, the second control male always went for the side that had not been inseminated. In almost all cases, males chose to copulate with a virgin tract, even if physically unable to do so.
There was no difference between the incidence of cannibalism of the first and second mates. Yet other trends arose that await explanation. Females that cannibalized spent significantly longer copulating with those males. There also was a curious relationship between male body size and time of cannibalism. Males that were eaten at the females' first mating were very slim. But at the females' second mating the cannibalized males were significantly larger than average.
This study presents some fascinating examples of reproductive behavior in Australian redback spiders. However, the critical reader may wonder where is the aspect of female control. Although it is true that different ratios of paternity resulted when the researchers manipulated the insemination patterns of these spiders, there seems to be no evidence of females behaving any differently in a way that would systematically change her reproductive outcomes. The ratio of paternity between rival males did not change over four egg sacs, which contained roughly equal numbers of eggs each. Female control is not ruled out by the results of just one experiment. For now, a more reserved and appropriate conclusion would be that multiple sperm storage organs increase genetic diversity.
For more information: www.pubs.royalsoc.as.uk