I want to go into a little more detail for my study so that I can relay an interesting occurrence that took place yesterday. A priority pattern study investigates the multiple mates of a female and their respective reproductive successes. Most insects have a second male advantage meaning that the second male with whom a female mates fathers more of the offspring than the first male. In spiders, however, there is usually a first male advantage. I am going to directly (I hope) test the priority pattern for Nesticodes rufipes. The design for the experiment includes controlling the number of intromissions (insertions) the male makes when mating, which linearly correlates with the amount of sperm that he releases. I will have two types of males: 1) Normal fertile ones and 2) Sterile ones. The sterilization process is an interesting one in itself. Basically we irradiate the male, which leaves his sperm intact, but scrambles the DNA of the sperm leading to infertility. I will have four groups. The first control group will involve mating a virgin female with two fertile males. Then the control for the irradiation process will involve mating a female with two sterile males. The last two groups involve alternating the mating order of one sterile and one fertile male. Then the amount of offspring produced in the experimental groups will help to elucidate the priority pattern. I might try varying some of the number of percentage of intromissions of the two males (ie: 50/50, 25/75 & 75/25) just to see the results.
I currently have been in the process of trying to foster a breeding colony for my spiders so that I can produce a large enough sample size for controls and manipulations. Nesticodes rufipes is a species that is generally found indoors of laboratories, prey rooms of zoos, and perhaps barns. E.O. Wilson had these babies in his lab when he was doing his ant research. Anywho, these spiders mate for a total of 3 hours!! Talk about stamina! What takes so long? Well, the sperm producing organs are found in the lower abdomen of the male, but his organ of copulation are palps on his antennae. He has to build a sperm web and then dab his palps in it (called induction) so that he can then inseminate the female. It is a curious process because during intromission, he inserts one palp and basically turns the female in a semicircle. Then he switches palps and turns her the other way. Imagine a much bigger female just calmly putting up with this turbulence! Anyways, I put in a male with my most aggressive female yesterday and came back three hours later. She had mated with him, and then decided she was hungry and spun him up in her web. This is not the most common behavior but it does happen on rare occasions. I guess I am glad to be breeding some hearty stocks, but males are hard to come by and I would rather my females not consume their mates. Thought you all would find this amusing!
that sounds like a great study! the only advice i could conceivably offer at this point is that -- if you're worried about sample sizes (i.e. getting enough data) -- introducing the additional variable of 'number of intromissions' may exacerbate that problem. just a thought.
ReplyDeleteit is hard not to anthropomorphize, and -- falling into that trap, myself -- spider sex is particularly creepy!
I really appreciate the advice. I might consider removing the number of intromissions as a variable, but I actually just learned why I would be doing that today. Apparently, this is more of test of determining the mechanism that determines the priority pattern. There are a couple of proposals for why the first male has the advantage. The first is the placement of the sperm. Many female spiders are thought to have a conduit spermathecal structure. Again, this is the organ used for sperm storage. The idea is that in a conduit, the point of entry for the sperm is nearest the female's fertilization duct. This would be different than the cul-de-sac structure seen in other insects with a last male advantage. Secondly, the number of sperm released may also determine reproductive success. In this case, the first male may be able to release more sperm than the second male because the female's storage may already be partially full. It would therefore be pretty interesting to see how manipulating the number of intromissions would affect paternal success. Is it based on number of sperm? Is there sperm mixing going on? Is the second male able to displace any of the first male's sperm?
ReplyDeleteinteresting...
ReplyDeletei may be misinterpreting, but it sounds, though, like you're addressing two different questions -- no?
have you talked with your advisor (is it terry?) about the analyses he is envisioning? i'm wondering if this is a multivariate design.