Friday, April 29, 2011

A spider Meta segmentata

I am an amateur naturalist trying to learn something about everything living in my garden.

Photo 1, taken late last summer, shows a spider about half-a-centimetre long. Spiders are surprisingly tricky to identify. You might think their often striking colour patterns would make things easy. Unfortunately for many species this is rather variable and the only really accurate approach to identification is to examine your arachnid's reproductive parts under a hand lens. I didn't subject my spider to the indignity of this. From the illustrations in my copy of Spiders (M.Roberts, publ. Collins) however I'm tolerably confident the species here is either Meta segmentata or Meta mengei. Both are common in Britain and look very similar. From the season (M. segmentata is more common later in the year and M. mengei earlier) and from the book illustrations which show M. mengei with a more distinctly hairy lower-leg ('metatarsus I and II' - I've marked these with the white line in photo 1) than my spider appears to possess, I'm going with the identification M. segmentata.

The spider here is a male as revealed by the presence of the two little 'boxing gloves' (the 'palps') emerging just in front of the head. You can see these most clearly in photo 2.

In the course of writing this blog I have learnt to expect that any creature I come across will have some complex and fascinating aspect to its lifestyle. As I discovered from browsing various online papers (1, 2, 3 - references below) M. segmentata is no exception. The mating strategy of male segmenta's is one example of the rich topics for exploration:

If you're a male M.segmentata wanting to mate with a female it does not pay to approach her directly. Do this and its likely she'll eat you! Why it benefits some lifeforms to routinely indulge in cannibalism and others (we human males might say, thankfully) not is a puzzle in itself. Anyway, regardless of the reason, the best chance a male M.segmentata has of mating with a female is to approach her at just the moment she has caught a nice juicy fly and is sufficiently pre-occupied not to eat her suitor. In detail, what males actually do is to approach the female on her web, drive her off her catch and cut the threads supporting the dead fly so that it dangles from a single 'nuptial' thread. The male then mates with the female whilst she attempts to recover the catch.

For the male to be present at just the moment a female catches a fly requires that he sits, sometimes for weeks, watching her web. This is known as 'mate guarding' (in my previous posting I discussed mate guarding amongst dragonflies). Mate guarding for male M.segmentata's is a high energy-expenditure task. Not only may his own food supply suffer, but he is also exposed to challenges from other males who want to usurp his position of guarding a receptive female. (It seems that males know receptive females by the presence of pheromones on her web). The whole question of why it pays males in nature to expend considerable energy to mate is a very deep and rich one in biology (I said something about it in my posting here). An illustration of how subtle things can become is afforded by the studies of Rubenstein and others above (ref.1) on M. segmentata:

Careful studies of colonies of M. segmenta (e.g. on an isolated bushes) show that over a season, a 'size hierarchy' develops with the biggest healthiest males taking over guarding the biggest healthiest females. Once a big male has succeeded in mating with a female, he will move on and find another big female to start guarding. He may need to battle with an already on-guard male to win the right to guard this new female, but being large he stands a decent chance of winning any such battles. In this way big males get to mate with lots of big females and enjoy high reproductive success.

Little males have no hope of winning at this 'roaming' lifestyle however. They would constantly lose the battle to guard large females to larger males. So instead, small males adopt an alternative strategy and choose monogamy, 'settling down' with a single small female. Being small, she herself will normally have been pushed out to an 'undesirable' part of the colony (low down on the bush, say) where prey is scarce. Her small size and poor diet will mean she is not likely to be very fertile (she will not produce a lot of eggs). However, in settling down with her the little male avoids the alternative of a series of fruitless battles over larger females.

So far so good, but now, the question arises: What of mid-sized males? Should they roam, continually seeking large, fertile females to guard but mostly losing them in battles with large males? Or do as small males do and 'settle down' with a single female, but at the cost their mate may have low fertility. In the studies of Rubenstein, 86% of medium males opted to roam. Precisely why the odds stack in this direction doesn't seem to be understood; a nice example of how subtle behaviours can be in the natural kingdom and how there are plenty of topics awaiting further study.

1. Alternative reproductive tactics in the spider Meta segmentata, D.I.Rubenstein, Behav. Ecol. Sociobiol. (1987) 20:229-237
2. Mate guarding, competition and variation in size in male orb-web spiders, Metellina segmentata: a field experiment. J. Prenter, R. W. Elwood, W.I. Montgomery, Animal Behaviour, 2003, 66, 1053–1058
3. The influence of prey size and female reproductive state on the courtship of the autumn spider, Metallina segmentata: a field experiment, J. Prenter, R. Elwood, S. Colgan, Anim. Behav. 1994, 47, 449-456.

1 comment: reviews said...

Oh sorry dear this post is kinda difficult for me as I have a phobia of spiders others might find it amazing but i think its so creepy.Anyhow I read a little,the post is good