A new study conducted by researchers from the Department of Desert Ecology at Ben-Gurion University, under the direction of Prof. Yael Lubin, reveals that spiders of the brown widow species sacrifice their lives during mating, their partner submits to their courtship - and immediately eats them
To die for passion: spiders "lose their heads" during mating and invite their deadly partner to eliminate them. This behavior was observed during a study conducted by researchers from the Department of Desert Ecology at Ben-Gurion University, in collaboration with the Volcanic Institute. The research findings are published in the new issue of the university newspaper 'Avg'.
The researchers, Prof. Yael Lubin, Ruthi Arieli and Michal Segoli, found that the self-sacrifice of the male to the female occurred in 90 percent of the mating cases, and males that sacrificed themselves were in better initial physical condition than males that were not eaten.
Brown widow spiders (Latrodectus geometricus) have unusual sexual habits. The male courts the female by vibrating her silk webs, cutting webs and adding his own. After about an hour of vigorous courtship, the female responds to him. Then the male performs a surprising action: he raises his belly until it touches the female's mouthparts, an action that encourages the female to start eating him. A male that is released from the female will continue to court her until he reaches the mating position again and performs the "reversal" for the second time. At this stage the female usually wraps the male in webs and eats him completely, or leaves him to die in the webs.
Sexual cannibalism is defined as eating the partner during or immediately after courtship and mating. This is a rare phenomenon in the animal world, but it is known among spiders and mantises, in which the female is usually larger and stronger than the male. Sexual cannibalism can represent a conflict of interests between the spouses. According to this approach, males don't want to be eaten, but they can't avoid their deadly mate. The female eats the male for several reasons: mistakenly identifying the male as a prey item, deciding to use the male as food, or as a means of avoiding "tumultuous encounters" with unsuccessful males. But in some cases the male also benefits from sexual cannibalism, for example if he provides the female with nutrients that increase the number and quality of the offspring she will produce from his sperm, or if the cannibalism will increase the amount of the male's sperm that she will use relative to the sperm she received from other males. Therefore, the male does not prevent, and even encourages, the cannibalistic behavior of the female, as observed in brown widow spiders.
Self-sacrificing behavior was previously known only in one other species of spider - the Australian black widow (Latrodectus hasselti). In a study done by Canadian researcher Maydian Andrade, it was discovered that males that were eaten fertilized a greater number of female eggs than males that were not eaten. Contrary to their expectations, the researchers were surprised to discover that, based on recent phylogenetic studies, the brown widow and the Australian widow are not species with a common origin: on the phylogenetic tree there are species that are more "close" relatives of both and in which there is no self-sacrificing behavior. For example, in the species called the white widow found in the northern Negev, the rate of cannibalism was about 20 percent. The males that are eaten are smaller and weaker, which suggests the female's strategy to avoid mating with a less successful partner, rather than the male's mating strategy. The results of the present study suggest that the phenomenon of self-sacrifice has evolved more than once among widow spiders.
Environmentally friendly spiders
Researchers from the Departments of Desert Ecology and Life Sciences discovered the spider's insecticidal properties
Spiders have never been considered good friends of man. The virulence of some of them and their tendency to trap helpless organisms in their webs deterred many. Now their problematic image may change thanks to a new study that reveals their beneficial side - the ability of field spiders to destroy insects harmful to agriculture in desert areas. The findings of the research are published in 'Avej' - the newspaper of Ben-Gurion University in the Negev.
Doctoral student Efrat Gabish and Master's student Ron Rothkopf, from the Department of Life Sciences, carried out the research under the guidance of Prof. Yael Lubin, from the Department of Desert Ecology, and Dr. Moshe Kol, from the Department of Entomology at the Hebrew University of Jerusalem.
The findings of the study revealed that spiders from the Arsalani family destroyed a significant amount of aphids (more than fifty percent of aphids forced upon them by the company of the spiders, compared to lucky aphids that participated in the experiment without spiders). For the farmers whose crops are damaged by aphids, this is encouraging news, because until today they had to use chemical or biological pesticides that are not available in nature and cost a lot of money. Now the farmers will be able to use local spiders, mainly from the spider family, who do their work "voluntarily". This is the first comprehensive study done in desert areas; Previous studies were done in temperate regions, in the countries of North America, Europe and Asia.
The spiders are predators that feed on a variety of prey types - insects and other arthropods, such as other spiders and millipedes. In a study done in wheat fields in the Negev near the Yemen field, several species of spiders with the potential to suppress pests were found. The ability of these species to eliminate populations of the grain leaf aphid, which is a common pest of agricultural crops that transmits the yellow ninos virus of barley, was tested in a series of controlled experiments.
Biological control is based on "natural enemies" for pests, which in our view are environmentally friendly. In this way, it can contribute to maintaining the quality of the environment and the biological diversity of the natural environment. It is very important in preventing the establishment and growth of pest populations in field crops in desert areas where the pests have not yet established themselves.
The researchers hope that their research will encourage farmers to reduce the use of chemical pesticides and leave pit areas between the planted fields, which will result in increased activity of the spiders in the fields. "We are in contact with farmers in the Negev. We held lectures in which we emphasized the importance of spiders in the field of pest control. We were encouraged by the feedback we received from the farmers, and we hope that this will lead to the integration of spiders in pest control and, at the same time, to a reduction in the use of chemicals."
This research was supported in part by:
The American Arachnology Society (AAS) Research Fund; Mitrani Department of Desert Ecology research grant; FPVI EC Program – Taxonomic Facility Consortia Access Grant (SYNTHESYS) at the Royal Institute of Natural Sciences, Brussels, Belgium
