A study has found that male black-widow spiders are able to reduce the risk of being devoured by their mating partner by detecting and courting well-fed females. By sensing if the female is satiated the male can achieve passing his genes and stay alive.
For several species mating means a significant investment risk and often a high cost. In some cases, mating may result in success spreading the genes, but at the risk of survival and sacrificing the opportunity to repeat the feat and further generation of progenies.
In the case of species of Latrodectus spiders (black-widow) the male often runs a high risk of encountering cannibalistic behaviour and being consumed by the female before or after mating.
The western black-widow spider (Latrodectus hesperus) is a highly venomous, sexually dimorphic arachnid found in the western regions of Canada, United States and Mexico. The female is black, about twice the size of the male, and has a red hour-glass shaped marking on the abdomen. Unlike other species of Lactrodectus where the female often cannibalize her male partner after mating, the western black-widow female rarely devours the male that inseminated her eggs.
Researcher James Chadwick Johnson from Arizona State University studied what influenced both the male and the female western spider’s behaviour which resulted in lesser lethal encounters. The results of the study published in the journal "Animal Behaviour" describe that well-fed female black widows are less likely to attack their mates and that males are able to detect the female’s appetite by sensing chemo-tactile cues on the cobweb, thereby determining if the potential partner is hungry or satiated and if is safe to mate with her.
The researchers used two groups of female black-widows; one group was fed crickets (one per week) for four weeks, while the other group was starved for the same period. Then they placed the males in different webs of females, either starved or satiated, to see how they reacted.
Males have chemo-sensitive glands in their feet and were able to tell the difference when walking on the web. Those that encounter satiated females conducted their courtship ritual more actively. Males that were faced with the option of mating with hungry females avoided approaching them.
“A non-significant courtship bias in favour of well-fed females was also evident when these females were transplanted to the webs of starved females. Male courtship biases in favour of well-fed females appear adaptive, as starved females were significantly more likely to attack males before mating.” says the researchers report.
The reproductive behaviour of black-widow spiders is complex. The mature male spins a web ball and deposits semen on it. He carries the sperm-sac with his front appendages (pedipalps) and, following a mating ritual that can take up to an hour, the male inserts the sperm in the female opening. Then the male escapes as quickly as possible to avoid becoming a meal in case the female is hungry.
Ventral view of Latrodectus hesperus male. The large disks on the end of the spider's pedipalps are used by the male to transfer the sperm to the female.