When Orb Weaver Spiders Make Love the Male Has to Be Careful

Oct 2, 2011
Albert Burchsted

Mating Male (L) and Female Garden Orb Weavers - Albert Burchsted

A female garden orb weaver spider often eats the male after or during mating. This seems irrational, but eating the male helps produce more and larger eggs.

Garden orb weavers (Araneus diadematus) live in Europe and northeastern North America. These spiders are found in backyards, gardens, and wooded areas. Females build webs up to 60 cm (two feet) or more in diameter, During the day, they move off the web and hide in foliage or squeeze into a crevice while holding one or two of the web's radial strands to detect prey that might be trapped in the web. At night, the female moves to the center of the web where she can move rapidly to inject venom into an ensnared insect.

These spiders mate in late summer when the males search for and enter the females' webs. Soon after a male enters, he begins plucking the web's spokes to announce his arrival. The female orients to and moves toward the male, while the male moves hesitantly toward the female making several rapid approaches and reversals.

The Mating Process

While approaching the much larger female, the male strokes the female's legs and abdomen with his legs. The female goes into a trance like state while her legs flush from a brownish color to reddish brown. The male alternates between plucking the web, and stroking her abdomen and legs – sometimes performing both actions at once.

During an observation I made of a mating pair, the female would twitch after approximately thirty seconds of the male's attentive stroking, after which the male rapidly retreated about two to three cm (about an inch) from her. Over the course of ten minutes, the male advanced and retreated eleven times. After each retreat, the male resumed plucking the web after a short wait and again moved forward to contact the female about thirteen seconds after breaking contact. On the male's sixth approach, the female exposed the genital cone on her abdomen. During each of the male's seventh and ninth through twelfth approaches, the male stroked her genital cone, apparently transferring sperm to the female. Yet, on the male's twelfth approach, she stabbed his thorax with her chelicerae and poisoned him.

Changes in Behaviors During Mating

On the male's first approach to the female, he moved slowly and remained next to the female for only 12 seconds before moving away for 49 seconds. On his last three approaches, the male remained next to the female for an average of 34.3 seconds, and moved away for an average of only ten seconds. Thus, the male spent three times as much time touching the female in the last three approaches than he did in his first approach; while spending only 1/5 the amount of time away from the female than he did in his first approach.

After several contacts, the female exposed her genital cone which the male touched several times with his front legs. While mating, male spiders gather sperm packages from a storage chamber under their abdomens and carry these packages using their front legs. Thus, touching the female's genital cone is either a prelude to or consummates the act of insemination of the female.

The female appeared to spend more time in a trance-like state during the first encounters than she did during the last several encounters. At the end of the male's last encounter, the female simply struck out with her fangs and stabbed the male in the thorax. Neither the male nor I detected any warning that this was about to happen.

After the Poisoning

The male went rigid immediately upon being stabbed. The female then grasped him tightly to her abdomen for the next 24 minutes – during which time, the female's legs flushed a much brighter red. Holding a prey item to the abdomen is an unusual behavior for a spider as they wrap most prey in silk to prevent them from escaping the web while digestive enzymes liquify the body contents.

When a lacewing became ensnared in the web 24 minutes after the female killed the male, she moved to the lacewing, still carrying the male's carcass, injected venom into this insect, and wrapped both the lacewing and the male's carcass in webbing to allow the insect to become digested. (The capture of the lacewing was an artifact of my observation of the spiders' behaviors. The light I used for focusing on the spiders attracted several flying insects of which the lacewing was one. The lacewing would probably not have become ensnared in the webbing otherwise.)

Why the Spider Eats Her Mate

Although it would seem counter-intuitive for the female to eat her mate, there are several reasons why this might not be so:

Studies have shown that well fed females are less likely to eat their mates than females who have had difficulty capturing insects. Thus, females who eat their mates would add a significant amount to the nutrients available to produce eggs.
By consuming the proteins and lipids in the male's body, the female can use these nutrients to produce more and larger eggs than if she did not obtain them. Thus, males who sacrifice themselves to their mates tend to leave more and stronger offspring than males that do not forfeit their lives.
Upon mating the male's energy reserves are mostly depleted from producing his spermatophores and it is possible he would die anyway before finding enough food to boost his energy for the opportunity to mate with another female.

Reference

J. Chadwick Johnson, et al. 2011. "Male black widows court well-fed females more than starved females: silken cues indicate sexual cannibalism risk" Animal Behaviour82(2): 383-390.