Singing capacity & biting force in Darwinian Finches.
Trade-offs are an essential survival tool for a species existing in a natural environment that has a limited amount of resources. The ecological theory giving a clear picture that species are able to coexist in the same ecological community if there are differences in their responses to limitations of resources (Egeblad and Werb, 2002). These differences are determined by the evolutionary traits of a species, ensuring their survival by adaptations to the environments due to natural selection or by cost effective measures that allow them to invest in certain aspects of their physiology or morphology that gives them a higher competitive factor in their ecosystem.
The Darwinian finches (finches of the Galapagos), arose from the South American single species through allopatric speciation. There are 14 known species that arose due to natural selection, allowing occupation of different environments by the birds as well as different ecological niches. This had an imperative impact on the evolutionary traits that they exhibited to ensure survival in the ecological system they had occupied (Purves et al., 2003). Beak morphology also tended to differ due to the changes in diet, with half of the species feeding on seeds and the other half feeding on buds. Seed eaters possess bills that are adapted for harvesting and crushing seeds whilst bud eaters have got bills adapted for grasping and wrenching buds off the trees (Purves et al., 2003), and such an evolutionary trait has an effect on the singing capacity of the finches.
A divergence from the usual singing capacity noted in the original mainland finches may also have induced isolated mating in the different species due to bill function with some studies indicating the preference of males based on song parameters by the females of the Geospiza finches (Grant & Grant, 1998). The morphology of the bill would determine the singing capacity of the finch thus suggesting a trade-off of some of the finch species reproduction for growth and development of the finch by feeding. Studies have shown the difference in jaw movement patterns which are a large contributor to the biting force as well as the determinant of the singing capacity. The large ground finch, Geospiza magnirostris, moving their large jaw at low repetition rates with small gap distances in comparison with the smaller beaked species, the warbler finch (Certhidea olivacea), that has high repetition rates and larger gap distances (Herrel et al., 2009). These rates of jaw movements indicating the difference in biting force and thus showing a competitive advantage of the G magnirostris over the C olivacea as it has a larger biting force allowing effective feeding. Larger jaws have been often associated with high force in jaw musculature, giving the finch a larger biting force (Herrel et al., 2005, 2009) and a narrower frequency bandwidth (Podos, 2001; Podos & Nowicki, 2004b). Trill diversity is restricted by mechanical performance of the bill hence a smaller bill would release mechanical limitations that would lead to a variety of singing performances (Podos et al., 2004).
When a bird sings, it is imperative to track the fundamental frequencies produced by the syrinx as the bird opens and closes its mouth (Nowicki, 1987; Nowicki & Marler, 1988) and this is regulated by the gap distances when opening the jaw. A wider gap resulting in larger frequencies and a smaller gap resulting in lower frequencies as well (Westneat et al., 1993; Hoese et al., 2000). The C olivacea has a high repetition rate and larger gap distances when opening their jaws, and previous studies show that this correlates with a high singing capacity as it gives higher vocal performance (Podos, 2001) due to lower vocal deviations. This gives the warbler finch a competitive advantage of population growth over a short generation time whilst also encompassing speciation by means of behavioural isolation since there is a need for mating call recognition.
Thus singing capacity is affected by the habitat of the finches which would also determine the morphology of the bird in time due to changes in the environment that would now allow the need to conform and adapt to the environment to ensure survival. This rapid change in environment would foster the variety in the trade-offs the Darwinian finches would take to ensure survival in their different habitats. The finches would trade-off singing capacity for biting force to survive in their different habitats.
REFERENCES
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