Article Summary from Practical Fishkeeping
http://www.practical...nt.php?sid=4209


Daniel Welsh and Rebecca Fuller conducted experiments to test whether female Bluefin killifish (Lucania goodei) prefer to lay eggs in spawning substrates that already contain eggs from other females.

The authors placed pairs of Bluefin killifish in aquariums containing four spawning mops. They manipulated the position of the mops (top vs. bottom) and the presence of eggs (empty vs. full), such that two top mops (one full and one empty) floated near the surface of the water and two bottom mops (one full and one empty) lay at the bottom of the tank; the full mops were created by having killifish held in other tanks spawn over them, with at least eight eggs in each full mop.

The experimental pair were allowed to spawn over the mops and the number of eggs laid in each mop counted. A total of 30 successful spawnings were carried out, and after the authors analysed the results statistically, they found that females preferred to lay eggs in mops that already contained the eggs of other females.

While such preferences have already been documented in other fishes, they have not been demonstrated in a species that lacks male parental care and does not build a nest. The authors also found the preference for females to lay eggs in areas already containing eggs to decrease as the total number of eggs increased.

The authors propose several hyoptheses to explain their findings. In the first hypothesis, they posit that the egg mortality in the field is high and that this leads to selection to disperse eggs widely over time and space. Consequently, eggs laid among other eggs may face a smaller risk of predation or cannibalism if some eggs should be eaten, because the risk is ‘spread out’ among the entire nest. This benefit is the greatest when a small number of eggs are added to a larger mass, and this may also explain the decreased female preference with increasing clutch size.

The second hypothesis is that the presence of older, developing eggs that have not been preyed upon may indicate that the present location is good at concealing eggs from predators, thus prompting females to lay their eggs near other eggs.

For more information, see the paper: Welsh, DP and RC Fuller (2011) Where to place your eggs: the effects of conspecific eggs and water depth on oviposition decisions in Bluefin killifish. Journal of Zoology 284, pp. 192–197.

That would be me! (I'm Daniel Welsh). If anyone has any questions or would like a copy of the paper, send me a message.

I do have a question -- Does the male bluefin have any influence in site selection (e.g. displaying more enthusiastically at some sites) or does the female ignore his "opinions" while looking for a spawning site. Also, it might be interesting to test whether the presence of other plant-spawning fish eggs (killies, darters, minnows) would affect her (their?) choices. Glad you're here Daniel!

That would be me! (I'm Daniel Welsh). If anyone has any questions or would like a copy of the paper, send me a message.

That would be me! (I'm Daniel Welsh). If anyone has any questions or would like a copy of the paper, send me a message.

I have a question ... doesn't (the way) Hypothesis Two (is worded) ... I recall it as being very close to the way Gerald describes it ... imply a certain active perceptive intelligence at work ?

I don't know much about animal behaviors, so I wonder about the basis or mechanisms behind such things.

The fish don't have to understand that reasoning to benefit from it. The sight or smell of live eggs may just be one of the triggers that tell her "do it here".

>> "The second hypothesis is that the presence of older, developing eggs that have not been preyed upon may indicate that the present location is good at concealing eggs from predators, thus prompting females to lay their eggs near other eggs."

I have a question ... doesn't (the way) Hypothesis Two (is worded) ... I recall it as being very close to the way Gerald describes it ... imply a certain active perceptive intelligence at work ? I don't know much about animal behaviors, so I wonder about the basis or mechanisms behind such things.

This kind of confusion held back the study of sexual selection in general, and female choice in particular, for almost a century after Darwin first proposed these mechanisms in the 1860s. Most (male) biologists assumed that a female of any species couldn't possibly be "smart" enough to make any kind of choice. In the case of bluefins this choice can work without conscious higher thought processes, as Gerald noted, if there are genetic mechanisms controlling the fish's behavior. Daniel's article is interesting because the work studies a species without any dramatic spawning behaviors such as nest-building or parental care, and they can still demonstrate a female choice mechanism appears to be present.

Well that was kinda what I was getting at, that an inborn (hard-wired/genetic/whatever) predisposition to do something was rewarded by a survival benefit, which over time then became a ubiquitous "behavior" for this species.

"The second hypothesis is that the presence of older, developing eggs that have not been preyed upon may indicate that the present location is good at concealing eggs from predators, thus prompting females to lay their eggs near other eggs."

The "thus prompting" is misleading, there is no such causation.

That is our suggested anthromorphic explanation.

Mother Nature's explanation is more direct: It works, so [survival of the fittest and all that] let's do more of it.

The "thus prompting" is misleading, there is no such causation.

That is our suggested anthromorphic explanation.

"Prompting" doesn't need to be anthropomorphic. It could be prompting from internal chemical signalling or anything else that promotes spawning. Similar to how temperature changes can prompt males of some species to color up - the individuals aren't necessarily thinking about it, "Prompting" just means "to promote", really. I don't think a mechanism is implied.