Introduction
An animal of a particular species may defend a territory from animals of other species or its own species. Mostly the territories are held by either individuals, a group or mated pair. Food is the overall key determinant for a species to hold the territory. The key factor for a particular species to dominate and defend its territory is best described by the benefits gained or lost in terms of energy. For example, territorial behavior of Broad-tailed (Selasphorous platycercus) and Rufous (Selasphorous rufus) hummingbirds research study has been used by different researchers to show the economics of territoriality.1
The species apply expensive territorial behavior to defend its territory. The defending mechanism may change of the quality of resource changes. That is, if by bad luck the cost of defending is high compared to that of resources gained then the territorial behavior diminishes and leaves.
The territorial defense intensifies with the quality of resources. Before carrying out the research on the economics of territoriality of Broad-tailed and Rufous hummingbirds Camfield (2006) predicted that the intruder pressure and the intensity of defense increases with increase in concentration levels of sucrose. The experiment were carried out using artificial feeders of sucrose of 10%, 20% and 30% with an aim of shedding light on how the defense mechanism of this two species occurs.
The different defense mechanisms of the species include; hovering, chasing the intruder and chip calls. The Broad-tailed are mostly displaced from their territories during mating period by the migrating Rufous hummingbirds.
The Rufous hummingbirds greater high-speed ability and high wing loading ratios is described as their territorial behavior to outweigh and chase the resident. On the other hand, the mating Broad-tailed do not want to loose a lot of energy in defending territories.2 This is clearly indicated by the fact that the female Rufous hummingbirds also chase the male broad-tailed. The migrating Rufous hummingbirds take two weeks and therefore it’s difficult to carry out the analysis based on a single individual. In addition, the sites with 30% feeders were not occasionally visited as a result of bad weather. The dependant variables used were not uniformly distributed and may have resulted in wrong conclusions.
The fresh incoming and aggressive Rufous hummingbird is in a good position to chase the specific incumbent and occupy the territory with economically less energy. The presence or absence of a certain feeders depended on quality of feeders. For example as expected, feeders of 10% sucrose were inhabited by the Broad-tailed and those with 30% were defended by Rufous hummingbirds. The number of chasers increased with increase in sucrose concentration and chip calls and hovering were applied in less concentrated sucrose levels. This is because at high quality feeders every species was persistent to defend its territory and also, the territory owners were very aggressive and robust to retain them. Rufous Hummingbirds are the dominant species compared to Broad-tailed Hummingbirds.
It is therefore economical for the Rufous Hummingbirds to first employ less energetic mechanism such as chip calls in response to intruders rather than apply the chasing mechanism which is not economically viable. On the contrary, it was observed that the Rufous hummingbirds first mechanism is to chase an intruder and chip calls are used later to intensify the chase. The use of artificial feeders or natural feeders may affect the results by changing the territorial behavior of the two or a particular species.
Promiscuity drives sexual selection in a socially monogamous bird
In accordance with Darwin3 explanation of the paradox of socially monogamous in birds, the results are of two consequences. One, some specific males may take charge of an area and dominate in obtaining the mating partners as others fail. This leads to the second consequence whereby the males may attract fecund females and produce more offspring. A number of researchers have suggested a solution to the above two paradox. The solution is for mating to occur to the pairs that are not socially paired with each other. The terminology used to describe this relationship is referred to as extra-pair paternity (EEP).
However EPP have several weaknesses and has become less important compared to the original Darwin proposal. Recent studies on EPP have shown that there is little effect on the opportunity for sexual selection. This often results if there is a negative covariance between extra-pair. Ecological factors may influence the mechanism of EPP by either constraining individual ability to engage in it or influence the ability of potential mates.
Other factors that have contributed to EPP becoming unpopular include; breading density, breeding synchrony and weather condition such as rainfall and snow. Despite the fact that the role of EPP in sexual selection is diminishing since few studies have allocated most extra-pair young to their sires, and because few studies have contrasted EPP with other potential sources of sexual selection, research is being carried out in this area to examine the effectiveness of EPP. The male mating of splendid fairy-wrens (Malurus splendens, Maluridae)4 showed a reproductive success on the effects of EPP. Earlier results have indicated that, splendid fairy-wrens breed cooperatively and are socially monogamous.
In splendid fairy-wrens, however, sexual selection has resulted to the reproductive success. First, EPP increased the reproductive success of some males, including several “nonbreeding” auxiliary males who sired young. For example, one male fathered 14 offspring in per annum, 10 of which were extra-pair. The results further support the importance of EPP to sexual selection.
The male (social monogamous) may mate with extra-pair females which results to a high reproductive achievement. However, the effects of EPP on sexual selection might be increased or decreased by variation across years. EPP seems to be important in this study of splendid fairy-wrens as they contribute strongly to the opportunity for sexual selection. In addition, partitioning the opportunity for selection into various parts has revealed that much of the variation in male mating success was due to variation in the ability to sire EPY and to avoid being cuckolded. In addition, the splendid fairy-wrens males with strong trait which related with EPP were most ideal by selection in this population.
EPP seems to be important in this particular study of the population of social monogamous species as opposed to Darwin. Moreover, a number of males with no social mates were advantaged as they were able to sire young ones through EPP, which contrast Darwin. In EPP the female quality variation is irrelevant as the result indicated, instead this was a sign of the stochastic effects of nest predation. Males who sired EPY through were higher when the effects of nest predation were included, but lower when they were excluded. Nest predation affects the strength of sexual selection acting through EPP.
In summary, the genetic promiscuity, acting through EPP or Darwin depends on a particular species. However in a socially monogamous species EPP has demonstrated to be effective. Other potential mechanisms for sexual selection such as biased breeding sex ratios and fecundity, might also contribute, but their effects are weak relative to that of reproductive promiscuity. Even though EPP may be unimportant in some social monogamous classification, few other studies have divided the sources of variance in male reproductive success.
Instead simple comparisons of variance in reproductive success to variance in actual reproductive success and this comparison do not quantify the effects of EPP. In addition, EPP cannot be disregarded and further work is recommended to know whether EPP can resolve the paradox of sexual selection in socially monogamous birds.
Work cited
Andersson, M. 1994. Sexual selection. Princeton Univ. Press, Princeton, NJ.
Tarvin, Tuttle, and Pruett-Jones S. 2007. Promiscuity drives sexual selection in a socially monogamous birds. Evolution 61(9):2205-2211.
Sandlin, A. (2000). Cue use affects resource subdivision among three coexisting hummingbird species. Behavioral Ecology 11(5): 550–559.
Camfield, F (2006). Resource value affects territorial defense by broad-tailed and rufous hummingbirds. Journal Field Ornithol 77(2):120-125.
Darwin, C. (1871). The descent of man, and selection in relation to sex. NJ: Princeton University Press.
Footnotes
- Camfield, F (2006). Resource value affects territorial defense by broad-tailed and rufous hummingbirds. Journal Field Ornithol 77(2):120-125
- Sandlin, A. (2000). Cue use affects resource subdivision among three coexisting hummingbird species. Behavioral Ecology 11(5): 550–559.
- Darwin, C. (1871). The descent of man, and selection in relation to sex. NJ: Princeton University Press.
- Camfield, F (2006). Resource value affects territorial defense by broad-tailed and rufous hummingbirds. Journal Field Ornithol 77(2):120-125