Spiders are infamous for their deadly females, which often devour males before, after, or even during sex. Now scientists find that male dark fishing spiders (Dolomedes tenebrosus) apparently sacrifice themselves to females after mating to aid their offspring, as reported online October 6 in the journal Current Biology.
Male spiders often attempt to escape sexual cannibalism—for instance, male nursery web spiders (Pisaurina mira) tie up females with silk before and during sex. However, in 2013, study lead author Steven Schwartz, a behavioral ecologist now at Gonzaga University in Spokane, Washington, and his colleagues found that male dark fishing spiders appear to assist their own cannibalism by curling up and dying after sex with females.
The researchers speculated that such self-sacrifice on the part of male dark fishing spiders might help increase the number or enhance the fitness of their offspring. In the new study, they found that females that cannibalized their mates produced nearly twice as many spiderlings as females that did not. Furthermore, spiderlings grew up to 20 percent larger and survived roughly up to 63 percent longer if their mothers ate their mates.
The scientists did not see similar benefits when females were allowed to consume a cricket in lieu of a male after sex. “This suggests that there is something special about the male’s body, and the nutrients within, that is beneficial for offspring,” says study co-author Eileen Hebets, an evolutionary behavioral ecologist at the University of Nebraska at Lincoln.
This research helps address a longstanding paradox in sexual cannibalism, says behavioral ecologist Shawn Wilder at Oklahoma State University in Stillwater, who did not take part in this study. “Females are hypothesized to consume males as a meal, yet the highest rates of cannibalism occur in species where males are very tiny and likely provide very little energy and macronutrients,” Wilder says. This new research shows that despite the small size of these males, special, as yet unknown components of their composition can have significant effects on offspring size and survival, he adds.
These male sacrifices might seem pointless, given that the females will mate with multiple males, diluting the chance that the gift that dying males offer will aid their progeny. However, the researchers suggest that since the males are only 7 to 13% the mass of females, any attempts to resist cannibalization would likely be futile. As such, the males might simply be making the best of a bad situation by offering themselves in a way that might benefit their offspring, Schwartz says.
“Prior to this kind of work, you might characterize this self-sacrifice as evolutionarily stupid. These males should go out and mate multiply—why are they putting so much energy into this mate?” says ecosystems ecologist Ramesh Laungani at Doane University in Nebraska, who did not participate in this research. “These findings are bringing us closer to finding out the relationship between this seemingly dumb behavior of males and the benefits they gain. This helps us understand the larger picture of how and why different mating systems have evolved.”
Future research, says Wilder, might explore which chemicals in the male body help enhance the size and survival of the offspring of cannibalistic females, whether trace elements, vitamins, dietary essential amino or fatty acids, or other compounds. Hebets adds that they are also investigating what mechanisms trigger male death after mating.