Female spiders tend to be larger than males. Large females are thought to be selected for as they can produce more eggs. The relative size differences between males and females depend on the type of competition between males, that is, on the importance of intra-sexual selection. When males do not compete much with one another they are smaller than females, whereas when there are male-male contests for females, then large males are selected for, and size differences with females become smaller. The spider, Metellina segmentata, is an orb spider common during September and October, when most mature individuals are found. Males are more powerfully built and have longer legs than females, suggesting that male-male contests occur in this spider. When young, males build their own webs to hunt insects, but upon reaching their final moult, they mature and start searching for female webs. Once they encounter a receptive female web - likely through detecting a pheromone - they adopt a curious sit and wait strategy. They have to wait, often days, until a large fly falls in the web. Only when the female starts eating the fly does he starts his courtship, taking advantage of the diminished cannibalistic tendencies of sated females. Of course, during this wait, other suitors might arrive to the female's web and when two males encounter each other fights follow. Males are often injured or killed in these fights, where the larger male has an advantage and this is what provides the selective pressure for powerfully built males.In addition, males are able to monitor several female webs when they are nearby and this results on males guarding webs becoming larger as the season progresses, and a pool of smaller wandering males.
The mature female Metellina on the top photo has hung her web on a tomato plant since at least mid september. I haven't noticed any males but I shall keep an eye for these in the next days.
Reference
Prenter, J., Elwood, R., & Montgomery, I. (2003). Mate guarding, competition and variation in size in male orb-web spiders, Metellina segmentata: a field experiment Animal Behaviour, 66 (6), 1053-1058 DOI: 10.1006/anbe.2003.2266
10 comments:
This blog has certainly gone international!
You're being spammed by what would appear to be a quite reputable small bug-killing company in Portland Oregon. Maybe they have identified Hull as the ideal base for their expansion into international markets.
It is fair to say that I think they are mistaken, and I have told them so.
Ray
Thank you Ray for pointing this out and contacting the company. These comments somehow never made it into my e-mail account. They have now been deleted.
At the beginning of your blog, you state that when male-male competition for females occur among spiders, male spiders are larger in size. Is this always the case?
Also, you seem to correlate these sexual dimorphic traits of the male being larger and with longer legs to advantages in male-male competition in acquiring a female however, can you explain how these traits aid the males?
Thank you for your comment margauxstanton. Male-male competition can take place in various forms and not always results in size differences. One way that can lead to larger males is when males guard the females, like in Linyphia triangularis , as the male will fight off any males coming into the females web. Other forms of male-male competition can result on sperm competition and not alter male size much, or the so called 'sneaky mating' in which a tactic used by small males may pay off. Check out this post:http://abugblog.blogspot.co.uk/2010/09/spider-chat-up-lines.html
Hi Africa,
Thanks for posting this! My name is James Dunbar. I am a macro photographer and film-maker based in Bristol. I made this short film about the fen raft spider
https://vimeo.com/57026161
The reason that I'm writing to you is because I managed to get some good footage of the behaviour that you describe above and I am in the process of editing it into a short 3 minute sequence about orb spiders. Would you like to see it once it's finished?
Thanks,
This is what I photographed yesterday and it appears to confirm all that you say about mating behaviour.
https://www.flickr.com/photos/67065881@N00/15397063886/
Thank you.
Margaret the Novice
and Africa, when you go onto flickr, have a look at this group: https://www.flickr.com/groups/arachtober/
It's annual opening was on October first, and features guess what! :)
Ray
I seem to have missed some comments, I don't know how. Thank you James, Margaret and Ray. It is probably a bit late to say I'd love to see your video James. Margaret, I love your photos, I should be so lucky to come across this behaviour. I'll have a look at the group Ray :-)
great info Africa - I have been watching these a lot recently - they are the commonest Orb weaver in my patch & this is very useful in making sense of what i see!
That's brilliant Conall, I always feel very lucky if I can see any sort of spider actions and watching any courtship is so fantastic.
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