Understanding sexual parasitism and cannibalism

The mating ritual of the deep-sea anglerfish is one of the most bizarre in the animal kingdom. The female anglerfish are larger, reaching a length of around 10 centimeters, and the male fish are just a fraction of this and can be more than ten times smaller. These peculiar (read ugly) looking creatures live at a depth of 1 to 3 kilometers in the ocean.  In the darkest depths of the waters where food is scarce, finding a mate is problematic. It is estimated that 80% of the females never encounter a male in their lifetime (which is around 30 years). These fish have adopted a way as to resolve the issue of nutrient and mate acquisition through evolution.

Female deep-sea anglerfish with attached male. Photo: Dr. Theodore W. Pietsch, University of Washington
Female deep-sea anglerfish with fused male (circle). Photo: Dr. Theodore W. Pietsch, University of Washington

The larger female anglerfish release chemical factors known as pheromones into the water for the males to find them. Upon finding her, the male bites onto her skin (the ultimate love bite in the animal kingdom, if you ask me) and gets fused to her permanently! His internal organs degenerates – except for the gonads which are used to impregnate the female – and he now survives solely on the female’s blood vessels to acquire nutrients. He never has to hunt for food in his life. The female can fuse with multiple males (around 6 to 8) ensuring a fresh supply of sperm throughout her life! The males are reduced to a mere small lump of tissue on the female’s skin.

While the anglerfish’s mating ritual serves as a classic case of sexual parasitism, another interesting and even bizarre reproductive behavior is seen in Australian redback spiders, in which the males are consumed (yes, eaten!) by the females during copulation. A male redback spider performs a somersault behavior during sperm transfer and positions it’s abdomen on top of the female’s jaws. The female starts to feed on the male during the duration of copulation. One may ask, why would the male spider want to risk his life and sacrifice himself? The answer is simple: survival of the species through increased reproductive success.

A female Australian redback spider. Photo: Toby Hudson on Wiki Commons
A female Australian redback spider. Photo: Toby Hudson on Wiki Commons

Darwin in 1871, wrote The Descent of Man, and Selection in Relation to Sex, where he proposed sexual selection to explain how some traits evolve to give an advantage in the struggle for reproduction, but reduce the probability of survival. This is exactly what happens in Australian redback spiders. Sexual cannibalism is favored by selection because the cannibalized male spiders receive two paternity advantages. Through a series of behavioral experiments, Maydianne C. B. Andrade found that cannibalized males copulated for longer periods of time during which more eggs are fertilized (advantage #1). After consuming their first male, females reject subsequent males for copulation. This increases the paternity of the first male since the female only produces his off-springs and reduces the probability of her mating with a second male (advantage #2). This explains why sexual cannibalism is in fact, an adaptive strategy for male redback spiders since it increases their reproductive value.

These wild mating rituals of certain creatures may seem strange and even outrageous at first. However, one can appreciate these reproductive strategies when they’re understood in the light of evolution.