Thursday, December 04, 2008

Male Redback Spiders Can Speed Up Their Sexual Development

Sexual selection is a subset of natural selection in which traits are evolving. We discussed several examples in class of sexual selection such as the iris elk antlers, manikins and their gaudy plumage, peacocks and their tail feathers, bower birds, and of course many more. All of these sexually selected traits decrease the survival of these animals. So for these sexually selected traits to evolve the advantage to the individual needs to be above and beyond the cost of survival. The males that evolve the sexually selected trait will reproduce much more then the males that do not evolve the trait.

In an article on Animal planet a study addressed male redback spiders speeding up their sexual development in order to beat their rivals to the females. The male and females exhibit extreme dimorphisms. The females are 100 to 200 times larger then the males. The female can live for up to two years and yet the males only live for only four to eight weeks! The males have to compete for the females, typically eight males will compete for one female. When the male mates with the female he leaves inside her a sperm plug so its important for the male to be the first to mate with her to pass his genes on. So there's a need for these male spiders to quickly mature so that they can mate first.

The males remain in the web where they're born until wind conditions allow them to leave and build their own web. At their own web they build up their fat reserves after molting since when he goes to find a female he spends all his time trying to locate her and no time eating. The spider needs to decide how much time to spend building fat reserves before trying to locate his mate. The males make their decision by smelling for the density of females around them. If there were many females in the area he wouldn't need to be well fed since he's not traveling far, but if there were few females he would need to store more fats to make the journey. If their are no females around the males will develop at a normal pace.

This study explains why there's a variation in the size of male redback spiders in the wild. Animal behaviorists typically would blame it on that the smaller males were not able to obtain enough resources but in this case the males are exchanging their body size to be the first to mate with the female to ensure his genetics are passed on.

Jennifer Smith (11)


At 8:17 PM, Anonymous Anonymous said...

So, the cue for the male spiders is the scent of the pheromones of a female spider. How can a male spider tell the density of females in an areas just by the smell?


At 1:49 PM, Anonymous Anonymous said...

this is interesting. Do any other insects do this? Allow themselves to have fat reserves to ensure a higher probability of mating with a female? Interesting

Mia DiFabbio

At 5:03 PM, Anonymous Anonymous said...

How is the speed of sexual development a sexually selected trait? It seems advantageous to the males to be able to control it based on whether or not there are female spiders around but the females don't really seem to be selecting for this. I am also curious as to why the females are so much bigger and live so much longer, especially considering they must put a lot more resources into making eggs than males do for sperm.

-Alex Jackson

At 7:48 PM, Blogger PWH said...

This is interesting. How is it actually known that the males have the ability to get as large as the females? Do the females actually display any kind of preference as to which male mates with her or is it just whom ever gets to her first is the one who mates? I know that you also mentioned that the females can live up to two years, can she reproduce more than once during this time?
Posted by: Lindsay Goodyear

At 10:42 PM, Anonymous Anonymous said...

How can the male spider speed up the development? If every males do, that's no longer speeding up the development. It will be just having faster development compared to females. In case of human beings, females usually have faster developments compared to males. do females speed up their development? No. I just wonder how they can do it, mechanisms not which parts gets developed faster.

-Yi, Jeongsang

At 11:30 PM, Blogger PWH said...

"We call it 'scent of a woman,'" said Andrade in the release. "They do this without contact — they smell how many females are around them." The spiders are smelling the females pheromones, it's like an odor fingerprint.

I can imagine others insects build fat reserves also, I mean it makes sense for them to doesn't it? It's not much different then a human baby growing inside a woman until it's time for birth-until they're bodies are matured enough and until they're large enough.

The males don't get as large as the females because they compromise their body size to be able to mate first.

Jennifer Smith (11)


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