Fossil Amber with Spider Cannibalism During Mating

An exceedingly rare occurence on foosil amber: spider mates with the female appearing poised to cannibalize the smaller male.

The spider mates are accompanied in their resin tomb by two Coleopterans (one large and one small) and a small Dipteran.

Recent research dispels a common misconception that the primary reason for spiders engaging in cannibalistic mating was that the male body provided nutrients for the female. Rather, the real reason for this practice has to do with sperm competition, whereby each male tries to ensure their sperm fertilizes as many of the female's eggs as possible, so as to pass on their genes to the most offspring. Female spiders may mate with many males and store the sperm, which they can use to fertilize egg sacs for up to two years. When the male is eaten during mating, however, more sperm is transferred since it continues to ejaculate while being eaten. This increases the time spent mating and the odds that their sperm will fertilize more eggs. Evolution predicts that the genes that encode this behavior will continue to be passed to progeny so long as it produces more offspring on average than males that avoid being eaten.