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Whose Kids Are Those Anyway?
|A cura di Ron Coleman, 1997.|
The convict cichlid, Archocentrus nigrofasciatus, is an excellent choice for experimenting with parental care in cichlids.
(This article was originally published in Cichlid News magazine, Aquatic promotions, Vol. 6. No. 2, April 1997; pp. 30-31. It is here reproduced with the permission of author Ron Coleman).
One of the most fascinating things about cichlids is that not only do they breed in aquaria right in front of our eyes, but they take care of their young as well. The diversity of ways that cichlids provide parental care continues to fascinate hobbyists and scientists alike. Parental care in cichlids ranges from biparental substrate spawners which fan and guard the eggs, then protect the wrigglers and free-swimming fry, all the way to maternal, paternal, or biparental mouthbrooders which take the eggs and fry into their mouths for extended periods of time.
Parental care is costly to the parents providing it. They risk injury from attackers, lost opportunities to feed themselves, and delays in producing subsequent broods. So why do some parents take care of young not their own?
We have known for decades that if you present fry to parent cichlids which either have their own young or have recently lost them, there is some chance that the parents will adopt the new fry. It doesn't work in every case and many researchers have tried to understand why and when it does. One of the criticisms of such work was always about its relevancy to natural settings: so what if you can get a pair of cichlids in a tank to raise somebody else's kids, does it really occur in the wild? The answer, increasingly, is yes.
In New World cichlids (e.g., the convict cichlid, Archocentrus nigrofasciatus), brood adoption might occur accidentally. In Costa Rica I have observed a set of parents with fry moving upstream only to encounter a second pair with fry moving downstream. As the two families meet amid substantial posturing and even fighting among the parents, the schools of fry can easily become intermingled. As the parents depart, it is not hard to see how fry from one school could end up staying in the "wrong" school. And what harm is there in that? This gets at the very nature of parental care.
Parental care in animals can take on many forms, but one way to look at this diversity is to ask whether it is "shareable" or "nonshareable." An example of shareable care is something like guarding. Imagine a parent cichlid protecting a pile of wrigglers lying in a shallow pit. It matters not to the parent how many wrigglers are present in terms of what the guarding "costs" the parent; it can guard 100 fry as easily as ten. And all wrigglers in the pile benefit equally from the vigilance of a parent chasing off a predator. Such is shareable care.
At the other extreme, nonshareable care is something like mouthbrooding larger fry. A mouth is only so big. When a school of mouthbred fry is startled and seeks refuge in the parent's mouth, there is a finite limit to the number of fry that can fit in. Twenty-five might fit in, but 35 might not. So, in a nonshareable form of parental care, each fry consumes a finite resource; adding additional fry may diminish the resource available to others or it might exclude one of the others entirely.
Balanced against the costs of taking care of adopted young are the costs of detecting and/or discriminating against them. A parent who tries to exclude a foreign fry might risk the safety of its own in the process.
In addition, there may be benefits of having other kids around. If a predator attacks, perhaps it will take the foreign fry. This is called the "brood dilution" hypothesis, and the advantages of it can be increased if a parent can somehow keep the foreign fry on the outer perimeter of its brood. And, if the foreign fry are smaller and therefore more vulnerable, so much the better to protect the parent's own offspring (the "differential predation" hypothesis).
Several recent publications shed more light on these processes in the wild. Detecting brood-mixing or brood adoption is not simple. The simplest means to detect foreign fry in a brood is to look for distinct size classes among the fry. True, as anyone who has raised a large group of fry will tell you, there can be a great range of sizes even within a parent's natural spawn. But, these will fall into a smooth, bell-shaped distribution from large to small. However, if in a school of fry ranging from 5-8 mm there are also ten fry of 15 mm swimming about, something suspicious is going on. Either fry from a previous brood are hanging around (which does happen in some species), or foreign fry are mixed in the brood.
Brian Wisenden and Miles Keenleyside (University of Western Ontario, Canada) studied convict cichlids in northeastern Costa Rica to solidify the costs and benefits of brood adoption (Wisenden and Keenleyside, 1994). These authors had previously found that brood adoption was commonplace in the wild, but that the foreign fry were either similar in size or smaller than the fry of the adopting parent. So, they set out to test the effect of those fry on the survivorship of the parents' natural offspring.
Observation is useful, but if you really want to know what's going on, scientists often need to use manipulative experiments. And that is exactly what these investigators did. They found "families" (two parents with fry) of convict cichlids, then captured the fry. They measured the fry very carefully and returned 40 individuals of a certain size (e.g., 8.0 mm standard length) to the parents. These were the control families. For an additional set of families they did the same thing, but also returned an additional 20 slightly smaller fry (captured from other families in the area). They then monitored the fate of the control versus the experimental families by recapturing the families at later dates.
Sure enough, within a few days an effect of the manipulation was obvious. The smaller, foreign fry in experimental broods suffered greater rates of predation than the larger fry (differential predation), and in some cases, the parents' own fry survived better than comparable fry in the control broods (brood dilution). Thus it seems, that in some cases, there could be advantages to adopting foreign fry.
Sometimes the fry adopted may not even be of the same species, as Haruki Ochi and Yasunobu Yanagisawa from Ehime University (Japan) found in their ongoing studies of Lake Tanganyika cichlids. These authors note that while the chance of observing the actual process of brood transfer in the wild is relatively small, you can see the effects by capturing and identifying the parents and free-swimming schools of fry (Ochi and Yanagisawa, 1996). Working at Nkumbula Island (Zambia) and Pemba (Zaire), they captured dozens of families of six different species. All families (with one exception) consisted of two parents and their fry. Some of these schools are huge. For example, the largest was a school of Lepidolamprologus elongatus which numbered more than 800 fry and spanned five meters in diameter. Capturing such a school in midwater is no simple task. But, what the authors found was truly startling.
Not only did many parents in some species have foreign young in their schools but the numbers of them were truly impressive. For example, L. elongatus is a piscivore with large schools of its own fry. But within such a school might be the fry of any of six different species (L. attenuatus, Xenotilapia spilopterus, Microdontochromis tenuidentatus, Perissodus microlepis, Haplotaxodon microlepis, and a Cyprichromis species). At the Zambia site, most of the L. elongatus and P. microlepis schools contained foreign fry, as did half the L. attenuatus schools. In Zambia, in those schools with mixed young, the school might contain up to 41% foreign fry. Any one school might contain up to five different species of foreign fry. As the parents' own fry increased in size, so too did the chances of the school containing foreign fry.
In general, adoptees were smaller than the host fry, just as with the convict cichlid examples described above; however, in Lake Tanganyika foreign fry were occasionally larger than the host fry. Ochi and Yanagisawa suggest that the act of chasing off large foreign fry may cause such a commotion (which in turn attracts predators) that parents may be better off not to try to exclude large foreigners. This is just one more new twist to the continuing saga of understanding whose fry are being guarded by whom.
© Copyright 1997 Ron Coleman, all rights reserved
Coleman, Ron. (Aprile 09, 1998). "Whose Kids Are Those Anyway?". The Cichlid Room Companion. URL consultato in data Maggio 19, 2013, da: http://www.cichlidae.com/article.php?id=88&lang=it.