What is the function of a nuchal hump in cichlids? An adult male of the gold morph of the Midas cichlid, Amphilophus citrinellus, is shown swimming in one of the the 12,000 Central American cichlids display tanks of Jean Claude Nourissat in Toulón, France. Photo by Juan Miguel Artigas Azas.
In a small aquarium a cichlid may have a limited choice of potential spawning partners, and under such circumstances, as many cichlid keepers soon find out, a cichlid often will eventually try to spawn with whatever is available, even if the other fish isn't the same species (or even the opposite sex!). In an aquarium with several potential mates or in the wild, cichlids exhibit sophisticated mate choice. It is not enough to be the right sex and the right species, an individual must also be more appealing than any other potential mate.
Some of the most interesting aspects of cichlid biology are intimately involved with convincing a member of the opposite sex that they need look no further - the ideal mate is right in front of him/her! A mate may be chosen (or rejected) because of its physical traits (e.g., size, coloration, or finnage), its behavior (courtship performance or parenting skills), or based on the "real estate" it controls, such as a good nesting location or protected area. Frequently multiple factors will be involved in making a mate choice.
Because only those individuals that mate pass on their genes to subsequent generations, natural selection has driven organisms to unbelievable extremes in their quest to be the most desirable. This particular kind of natural selection is called sexual selection, and it can take two forms. Sometimes there is competition within a sex - called intrasexual selection - in which two individuals fight to determine who is better. In many animal groups intrasexual selection is most obvious among males, e.g. two mountain sheep ramming into each other. The other sex simply mates with the winner.
The other form of sexual selection is called intersexual selection. This term refers to mate choice of one sex by the other sex and often takes the form of elaborate displays by potential suitors. A male peacock displaying his lavish tail feathers is the epitome of this form of sexual selection.
The desire to mate and pass on genes is so strong that it may outweigh the value of self-preservation by the individual. Males will fight to the death over the opportunity to mate. Other males will do daring and crazy things or wear conspicuous colors that attract females, but at the same time attract predators. Even those structures that don't obviously handicap their bearer take time and energy to develop. In other words, they have a cost. This tradeoff between self-preservation and the desire to mate has intrigued biologists for over a century.
Cichlids offer abundant opportunities to study many different examples of sexual selection and mate choice. Early works on this topic include Ken McKaye's (University of Maryland) study of several species of New World cichlids in Lake Jiloa. Nicaragua in the mid-1970s. He was able to show that in natural pairs, males are always larger than females (McKaye, 1986), likely as a product of mate choice. There is a long series of careful studies by George Barlow (University of California, Berkeley) on the complex relationships among mate choice, color, and size in the Midas cichlid (Barlow, 1986). Midas cichlids, Amphilophus citrinellus, are polychromatic, meaning that there is more than one color form within the species. The "normal" coloration is a combination of black and grayish-white vertical bars. "Gold" individuals lack the bars and range in color from solid yellow to orange or reddish-orange. Barlow's work shows that size is important in mate choice, but color is also important: golds prefer to mate with golds, but not always.
Frank Triefenbach and Murray Itzkowitz (Lehigh University) recently did an interesting experiment to examine the effect of size on mate choice, this time in the convict cichlid, Cryptoheros nigrofasciatus. They put pairs of convicts on either side of a divided aquarium. Later they removed the barrier dividing the pairs and observed what happened. In some cases, one pair took over a flowerpot placed in the center of the tank. In other cases, the original pairs re-sorted themselves, and one of the newly-formed pairs claimed the spawning site (= the flowerpot). Why? In some cases, the experimenters had used pairs composed of atypical larger male/smaller female combination, but in others they used mis-assorted pairs, i.e., pairs composed of a larger female and a smaller male. The re-sorting occurred most often in the latter case: the large male and the large female left their previous mates and paired together to take control of the flowerpot. Large size is a desirable trait in both sexes: larger males are better able to defend their territories, and larger females carry more eggs. The experiment demonstrates that these cichlids are constantly reevaluating their options, and if the conditions are right, they will abandon their existing mate to pair with a new and better partner. The fact that re-assortment didn't always occur is also interesting. It appears that a small female mated to a large male is sometimes able to dominate a larger female - normally not likely - and the small female may be able to restrict her mate from switch ing to the larger female.
Sometimes mate choice is more subtle, the preference being for a particular aspect of the potential partner's morphology. In New World cichlids, the nuchal hump on the head of male cichlids seems an obvious possibility to explore. In aquaria, large male New World cichlids - and occasionally females - develop such a hump. In the wild, only males do so, and the humps are usually smaller than those seen in captive specimens. Nonetheless, the hump is a conspicuous character that demands explanation.
In a number of neotropical species (such as Tomocichla tuba, a nuchal hump may function in sex recognition and mate selection. Photo by Ron Coleman.
George Barlow and Paul Siri considered many possible functions for the hump. It could be an organ of fat storage, offer mechanical advantage in fights, improve hydrodynamics, serve as an anti-predation device, or be the object of mate choice. First the researchers marshalled the facts. The hump changes size over time. It gets larger during courtship and actually shrinks during the parental phase. Males and females fight during the parental phase so if the hump was a fighting weapon, females should develop humps equal to those in males. Similarly, if the hump serves in fat storage or as protection from predators, why don't females develop them? Furthermore, the hump doesn't actually contain large volumes of fat, it is for the most part filled with water. The hydrodynamic explanation may apply to other kinds of fishes with humps (e.g., salmon), but the bulbous shape of the cichlid's hump is likely a hydrodynamic deterrent, if anything.
By offering female Midas cichlids a choice of rubber dummies of male Midas cichlids molded with differently-sized humps, Barlow and Siri found that females prefer males with humps over those without, but only up to a certain point. Once the hump becomes enormous, female preference declines. Several possible explanations exist. Hump size may serve as a measure of a male's condition or quality. Or, the hump may merely serve as a way to distinguish males from females. Many of the species with humps are highly monomorphic - males and females look alike - and the fact that the hump peaks in size at the time of pair formation strongly suggests a role in sex recognition. Nonetheless, the story is not finished because, as the researchers point out, well-fed fish develop larger humps: if sex recognition were the sole factor, the hump would develop to a specific size, but no further. The enormous humps seen on some captive males - and shown to be unappealing to females in the experiments - may not occur in the wild because it may not be possible for large males to afford such elaborate displays. Future experiments and observations will be needed to sort out the explanations for this intriguing character.
Mate choice has also been examined in a few African cichlids. Some male cichlids in Lake Tanganyika build a "bower," essentially a large volcano-shaped structure composed of fine sand, over which the male courts potential mates. Kenji Karino of the University of the Ryukyus (Japan) studied Cyathopharynx furcifer males at Mbemba in Zaire. Each C. furcifer male constructs his bower adjacent to other bowers, thus forming a large group of bowers called a lek. Females visit the lek and examine potential mates. There are males that hold territories without bowers, but females disregard them. Studies of bowers, in cichlids and other animals, particularly birds, have focussed on which aspects of the bower attract females. In some cases it is the size of the bower, in others, the presence of desirable objects. For example, female bowerbirds of one species have a particular liking for blue objects, while females of another species prefer green.
Karino measured many aspects of the bower as well as aspects of the males themselves. Males of C. furcifer have conspicuously long pelvic fin extensions, sometimes reaching beyond the tip of the anal fin. Karino observed 674 female visits to the lek, of which 161 resulted in circling behavior; of these, 43 resulted in spawning. Males were not chosen equally. Females did not mate preferentially with males holding bowers of a particular size or shape, but possession of a bower - any bower - was a requirement for mating. The females were much more picky about the males themselves.
Females were attracted to males that courted vigorously and to males with the longest pelvic fins. Even more important than length, however, was the symmetry of the pelvic fins. Females preferred males with long pelvic fins which were both exactly the same length! It appears that females use the courting phase to assess the symmetry of the fins in candidate males.
Why is symmetry so important? Because it is easy to judge. Comparing two fish that are not side-by-side can be difficult for a fish (and for humans). In essence, an animal which is symmetrical is demonstrating that it is of such high quality that it has withstood the onslaughts of nature, whether they be nutritional stress, fighting, or parasites, and still looks the same on both sides. Even the most discerning of females should recognize the value of such a male as a good choice of mate. Indeed in the last decade, research on many groups of animals has found symmetry to be important in mate choice. But symmetry is only one aspect, and there remain many interesting twists and turns on the road to understanding what makes a good mate.
- Barlow, G. W.; 1986; "Mate choice in the monogamous and polychromatic Midas cichlid, Cichlasoma citrinellum"; J. Fish Biol. 29A: 123-33.
- Barlow, G. W. and P. Siri; 1997; "Does sexual selection account for the conspicuous head dimorphism in the Midas cichlid?"; Animal Behavior; 53: 573-584.
- Karino, K.; 1997; "Female mate preference for males having long and symmetric fins in the bower-holding cichlid Cyatopharynx furcifer"; Ethology 103:183-192.
- McKaye, K. R.; 1986; "Mate choice and size assortative pairing by the cichlid fishes of Lake Jiloa, Nicaragua"; J. Fish Biol. 29A; 135-150.
- Triefenbach, F. and M. Itzkowitz; 1998; "Mate switching as a function of mate quality in convict cichlids, Cichlasoma nigrofasciatum" Animal Behavior; 55:1263-1270.