Biology 342 Fall 06
Cannibalism is prevalent in situations of limited resources. These resources can be food, mates, shelter, etc. In order to obtain resources for themselves, some animals resort to cannibalism to reduce competition.
One such example of this competitive cannibalism exists in larval tiger salamanders (Ambystoma tigrinum). There are two morphs of tiger salamanders, one that is a typical morph that feeds on invertebrates and a cannibal morph that preys on conspecifics on a regular basis. Studies show that the cannibalistic morph is induced by stimuation from other larval tiger salamanders or from starvation. This morph can be reversed if conditions change within a certain critical period. The stimulation from other larvae must be tactile, chemical, and visual in order for the morph to be cannibalistic. In a crowded situation, in which all of these cues from other larvae are present, the larvae will become cannibalistic and eat their conspecifics. Identical results can be obtained with heterospecifics as well, which were A. texanum in this study. Once one cannibal morph is present, the likelihood that other cannibals will form is significantly increased.
This study, however, does not provide evidence about the specific fitness advantages. The larvae could be reducing competition by eating competitors, or they could be responding to crowding and thus limited resources by eating larvae around it because food resources are strained.
(Hoffman and Pfennig 1999)
Another example of competitive cannibalism more directly linked to lack of food resources comes from a study of Mormon crickets. Millions of Mormon crickets can be found in migratory bands that can extend up to 10 km in length. Cannibalism is prevalent in these migratory bands and a study by Simpson et al. points to the lack of resources as the trigger (2006). When so many crickets are present in one situation, resources become greatly limited. Many of these crickets resort to eating conspecifics. The most limiting resources are protein and salt, and within these bands the crickets themselves are the major source of nutrients in the field. They eat wounded, molting, and freshly dead insects, but also participate in active predation. During active predation the prey cricket tries to defend itself by kicking with the hind legs, thus wounded or immobile crickets are at greater risk. Protein consumption also inhibits locomotion, so crickets that have just consumed protein are moderately immobile and also at a greater risk.
Providing these crickets with constant access to protein and salt for five hours resulted in a significantly reduced amount of cannibalism. Thus the cannibalism is a means of gaining access to resources that the cricket is lacking. This activity greatly increases the cannibalistic cricket's fitness. Being within these migratory bands, even in the presence of cannibals, is better than leaving the group and exposing a sole cricket to heterospecific predation. These crickets have adapted to limited resources by finding the next best source: each other (Simpson et al. 2006).
Another such example of using conspecifics as a resource/removing competition occurs in the larvae of the pond-breeding salamander Hynobius nebulosus. These larvae have a high frequency of cannibalism based on a high population density at the time of hatching. This results in a potential food shortage if all of these larvae are to grow and develop in one small pond. Older larvae can eat younger larvae because they have enlarged or modified mouth structures for cannibalism. The older larvae attack the younger and smaller larvae head first and swallow them whole (Kusano et al. 1985). Their fitness is increased by reducing the number of competitors and also possibly from the nutrition gained by the consumption of other larvae, although the paper by Kusano et al does not discuss the fitness advantages explicitly.