Animal Behavior/Coevolution
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[edit] Evolutionary Arms Races
As in any complex scenario, explanatory attempts will tempt us to consider ad-hoc explanations that are intuitively appealing but in many instances spectacularly wrong. There is no alternative to using such explanations as functional hypotheses and to subject them to rigorous experimental examination. Moreover, we need to look beyond the currrent status and consider the likely paths such a system of co-evolution may have undergone in the past. Our final consideration needs to go to the end points of this arms races. Has it ended in a stalemate rather then with the demise of one of the protagonists?
[edit] The Rough-skinned Newt and the Common Garter Snake
Newts are generally slow-moving animals with few defenses. Forming a major component in the diet of Garter Snakes a system of co-evolutionary adaptations appears to have given rise to a spectacular case of an evolutionary arms race. Rough-skinned Newts, common on the western coast of North America, produce extremely high amounts of Tetrodotoxin, a potent blocker of voltage-gated sodium channels in nerve cell membranes. With levels of toxin far exceeding what is needed to kill any other conceivable predator, it is only ingested by garter snakes with a high degree of immunity to the toxin throughout much of the newt's range. In the early stages of this interaction, some newts with an ability to excrete tedrodotoxin may have gained a relative advantage over their conspecifics. With predation pressure selecting for higher levels of poison in newts, snakes with some level of resistance to the poison were deriving a relative advantage. Such resistance emerged through a mutation in sodium channel structure, which retain much of its functionality yet made it less susceptible to a block by tedrodotoxin. Continuing to select for higher levels of toxin on one hand and increased tolerance to it in snakes, the present system may have emerged. The system's progression has, however, entered a stable end point when further increases in immunity to the toxin could only be achieved with a concurrent decrease in neuronal functioning. At present, individual snakes with the highest levels of resistance to the toxin are also characterized by a reduction in motor abilities. Although able to handle even the most toxic newts, this ability is hard-won as it renders them at much greater risks of predation themselves.
[edit] Brood Parasitism
Common cuckoos (Cuculus canorus) in Europe, and brown-headed cowbirds (Molothrus ater) in North America lay their eggs in the nests of other species and leave them to the care of their respective host. A Cuckoo female searches for an unguarded nest of another species and then replaces an egg in it with one of her own. The mother then abandons her egg leaving it to be raised by the nest's natural owners. Clearly, lacking a distinct ability to identify and remove foreign eggs from among its clutch appears maladaptive.
As an example of an evolutionary arms race, cukoos have adapted their kleptoparasitic strategies to the host behavior. Upon return, hosts may reject the foreign egg if it is a poor match, added at the wrong time of day, or the cukoo's presence has been witnessed. Different cuckoo strains exhibit distinct preferences for a specific host species which provide a close visual match to their own eggs. Likewise, hosts have adapted to selection from cuckoos where species suffering high amounts of parasitism are much better at identifying and removing cuckoo eggs than those who suffer little. When parents remove an egg from their nest they do risk removing one of their own or breaking some of the others in the process. Whether taking such risks will be worth it depends on the overall rate of parasitism and the ability to discriminate foreign from own eggs.
In contrast to cuckoos, brownheaded cowbirds exhibit little preference for nests of a specific song bird species. Moreover, their eggs fail to show close mimicry to those of their hosts. Removal of parasitic eggs is much more variable, high in some species and rare in others. Although cowbird fledglings do not evict their nest mates, up to 50% of the nests may be parasitized in some areas.
The lack of co-evolution in brood parasitism of cowbirds compared to that of cuckoos is puzzling. Do these two cases represent different stages of the arms race? Are cowbirds in the early parts of the game while cuckoos have had a chance to adapt for a much longer time? Has the recent change in North America landscape from forest to agricultural lands brought cowbirds into contact with a much greater number of new hosts? Alternatively, both species may be at an evolutionary equilibrium with stabilizing selection. Differing degrees of rejection may well depend on their respective costs of rejecting vs. accepting any given egg.
A basic problem for all brood parasites resides in the fact that young birds raised by parents of another species will be unsuitable as tutors for its own songs. Cowbirds, which may parasitize the nests of over 200 different species, are nevertheless able to learn the right species-specific song.
