Humans And Leeches Aren't So Different After All...
Headed by Dr. Martin Burd (full article not available w/o subscription), the Aussie research team noticed that there was a bias in the amount of food that the baby leeches, which attach to their mother’s underside in groups of up to 60, got their ‘hands’ on based on aggressiveness. With the more common smaller-opening snails, only a certain number of baby leeches can fit their heads in at a given time. The more aggressive ones will crowd out the others and get the first servings every time. Dr. Burd and associates tracked this competition by weighing the leeches, with the most aggressive gaining more weight. What is interesting about this specific case of sibling rivalry is that there is no need for it. Baby leeches can spend as long as they want hanging on their mother without having to feed right after hatching, allowing for the unrealized possibility of staggered maturity and dispersal. Despite this, further research shows the competitive behavior to be wide spread amongst this species.
Another species that is more commonly studied for their sibling rivalry behavior is the Perisoreus infaustus, or Siberian jays. Birds, unlike the leeches, actually have a need for rivalry since they need to feed immediately after hatching. However, these birds have found a way around their particular predicament. The birds vary the ages at which the leave the nest leaving time for the weaker birds to catch up after the stronger have left. Therefore, the rivalry persists without interfering in the survival of all of the offspring.
All in all the leeches may have to take some pointers from the Siberian jays. Otherwise, this competition may hinder their ability to grow in population size…but, most campers and lake swimmers will agree, this may not be a bad thing.
Commented response added as last comment.
posted by PWH @ 3:31 PM
5 comments
5 Comments:
I found your post to be witty and entertaining, while still raising a few important questions. I would like to know more about how exactly the immature leeches go about feeding. I was a bit unclear about whether it was directly from a snail, or from their mother until they detach. Research should defiantly be conducted further into the “necessity” of receiving the first servings of food. Are these servings any more nutritious? As in some tribal communities, the more nutritious parts of any animal killed for food is given to the higher members of the unit. Finally, what do you think about scientists taking a closer look into the pattern of growth and development in relation to hormones within the leech? Is it possible that the leeches detach themselves from their mother as a physiological response? Overall, this is a great, thought provoking post!
Posted by JLB (3)
First of all thank you for finding an article about the phylum annelid, not the most glamorous group of animals. I was curious how the scientists deemed a newborn leech "aggresive". Could it just be a matter of positioning? Like the first commenter, I was confused how they feed. For both the leeches and the jays I am curious whether the authors report mortality amongst the siblings that are outcompeted. Even if there are survivors amongst the less aggressive siblings, it seems like some may just not be fit. After all, it is "survival of the fittest".
Posted by 4972 (3)
It is unclear to me why you say that the ‘possibility of staggered maturity’ is unrealized, or rather, why this competition would prevent a state of staggered maturity. Do the leeches that eat more mature sooner, or are their maturations independent of energy intake? If they matured sooner it would seem that this would create a staggered rate of maturation within the population of juveniles. It also seems that weighing more, i.e. having more energy reserves etc. would increase the fitness of those aggressive leeches. They would have more energy for reproductive investment. All in all I guess I’m asking you to elaborate on the idea that for leeches, there is no need for sibling rivalry.
A.McCandless
I think an important point here is that there is strong selection for the aggressiveness among siblings, even if they might make it by waiting.
By waiting they delay reproducing. In many species such a delay can mean a lower life-time reproductive output. In terms of natural selection this means lower fitness.
So, while waiting might still mean surviving, it is probably making the best of a bad deal.
Posted by PWH
I will post one longer comment in reply to the various comments and questions left to me...
The leeches feed while being carried with its siblings on its mother's underside. The mother presents its young with various sizes of snails to which the leech attaches in the same manner it would to any other food, including humans (although this species does not normally feed on human blood). So the feeding is directly from the snail but with a helping hand from mom. The servings aren't more nutritious, it is simply a matter of having enough to go around. The less aggressive leeches will simply not get any food or have to wait much longer when only smaller snails are available.
I think research into the hormonal relationship to the growth and development is a great idea. It is possible that there is a hormonal influence that results in the "aggressive" behavior giving some of the leeches the upper hand. It would also be interesting to see if there could be a hormonal trigger related to hunger that could start aggressive behavior.
The leeches detach themselves once they reach a certain size but they can survive quite happily staying small and on their mother. Like Prof. Houlihan said, they are delaying their ability to reproduce while still attached to their mother so it is beneficial to their reproductive fitness to detach as soon as possible.
The label of aggressiveness was simply applied to those that excluded their siblings in feeding. These "aggressive" leeches were simply the ones who got to feed first time after time. The competition is only observed with small-opening snails where there is a smaller area on which to feed so only a percentage of the young could attach at one time. With larger snail openings competition is greatly diminished as there is room for almost all of the young.
The article concerning the leeches did not report mortality; however, the jays do suffer mortality. This is most likely because the jays must feed immediately and it is optional for the leeches.
The leeches must eat to mature because their maturation to the point where they will detach is dependent on size. The reason I stated that there is no need for sibling rivalry is because the leeches do not live or die based on when they mature. You bring up a good point with the reproductive fitness in that it is bettered by early maturity. So in that sense, the sibling rivalry is a plus for those on the good side, but it is not necessarily needed.
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