Older male spiders shudder longer in face of cannibal females

New research has revealed that the mating behaviour of the St Andrew’s cross spider changes with age, with older males investing more time in courtship, possibly to avoid cannibalisation by more aggressive females.

The St Andrew’s cross spider is a colourful orb weaving spider that is best recognised by its banded abdomen and the characteristic X-shaped cross on its web. These spiders typically live for several months during spring and summer. Adult males are small, between 3 and 4 millimetres in length, while females are much larger at between 10 and 16 millimetres.

Female A.keyserlingi
Female Argiope keyserlingi – Photo by Anne Wignall

Before mating, males approach the female across her web using a ‘shuddering’ motion in which they bounce up and down in regular intervals.

“They almost jump on the web like a trampoline,” said researcher James O’Hanlon from University of New England. “They thrust their legs out underneath them, so they’re actually shaking the web.”

This shuddering behaviour decreases the chance a male is cannibalised by the female as it distinguishes their movements from the typical vibrations of prey.

“The males shudder in very characteristic short bursts of vibrations. It’s almost like knocking on the door and saying, ‘Hey, I’m here. Don’t eat me’.”

Prior studies suggested courtship behaviour remains constant as these spiders age. However, this new research reveals changes in the courtship behaviour over time. While younger spiders approach the female quickly and with a smaller number of shudders, older males will put more effort into the ritual by increasing the amount of shudders while taking longer in their approach.

“That might be because the females get more aggressive throughout the season, so the male is putting in more effort to quell that aggression.” 

However, more research is needed before any concrete conclusions can be made, he said.

This new information can be added to what is already known about the spider’s mating behaviour. Generally, it is up to both spiders to choose a mate, with the male initially deciding which female and which web to approach. However, it is the female who makes the final choice, O’Hanlon said.

“She’ll prefer particular types of vibrations and choose whether she’s going to be receptive and mate with that male, or whether she’ll reject him by either walking away or eating him.”

Males can only mate twice in their lifespan. They have two small ‘arms’ called pedipalps which they use to transfer sperm to the female. On each occasion, the head of one pedipalp is ripped off and left inside the female. The male is then either devoured by the female or allowed to leave.

While the study’s results unveiled some interesting trends, the next step will be to examine the female’s perspective, O’Hanlon said. Aspects such as whether females preferred older or younger spiders, the likelihood of her eating different ages of males, and whether this cannibalism is quelled by different types of shuddering are likely areas of study.

This research involved collaboration between scientists at Macquarie University, the University of New England, and Massey University. It has been published in the journal, The Science of Nature.

Featured Image: Male Argiope keyserlingi – Photo by Anne Wignall


Miklos

Article by Miklos Bolza

Miklos Bolza graduated with an honours degree in science from UNSW (majoring in maths and physics), now works as a freelance science journalist, and can’t wait until space tourism really ramps up.

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