Thursday, May 25, 2017

Coral snake mimic loses pattern in absences of coral snake model

Tobago's Erythrolamprus ocellatus above. Trinidad's coral snake 
mimic E.  
aesculapii below.
Losses of adaptations in response to changed selective pressures are evolutionarily important phenomena but relatively few empirical examples have been investigated in detail. To help fill this gap, Hodson and Lehtinen took advantage of a natural experiment in which coral snake mimics occur on two nearby tropical islands, one that has coral snake models (Trinidad) and one that lacks them (Tobago).

The Tobago snake's pattern represents a loss of an adaptation in response to changed selective pressures. Relatively few empirical examples of adaptation loss have been investigated in detail. Hodson and Lehtinen took advantage of a natural experiment in which coral snake mimics occur on two nearby tropical islands, one that has coral snake models (Trinidad) and one that lacks them (Tobago). On Tobago, an endemic coral snake mimic (Erythrolamprus ocellatus) exists but has a relatively poor resemblance to coral snakes. Quantitative image analysis of museum specimens confirmed that E. ocellatus is a poor mimic of coral snakes.

To address questions related to the functional importance of this phenotype, the authors conducted a field experiment on both islands with snake replicas made of clay. These results clearly indicated a strong inter-island difference in predator attack rates where snake replicas that resembled coral snakes received protection in Trinidad but not in Tobago. Color patterns from museum specimens confirmed that E. ocellatus is indeed a poor mimic of coral snakes in many respects, especially in regards to the relative proportions of colors and the lack of discrete band. This implies that the classic coral snake mimicry adaptation has been degraded in this species. Field experiment revealed that E. ocellatus replicas were not protected from predator attacks on Tobago (where no coral snakes occur) compared to controls. However, on Trinidad (where coral snakes do occur) we found the expected lower attack rate on coral snake and mimic replicas compared to controls. Thus, E. ocellatus does not just look like a poor mimic to human eyes, its predators show no evidence of avoiding it.

Further, a molecular phylogenetic analysis of the ancestry of E. ocellatus revealed that this poor coral snake mimic is deeply nested in a clade of good coral snake mimics. Therefore the lack of coral snakes on Tobago altered the selective environment such that the coral snake mimicry adaptation was no longer advantageous. The failure to maintain this ancestral feature in allopatry provides a compelling example of how losses of complex adaptations can occur.

Citation
Hodson EE, Lehtinen RM. 2017. Diverse Evidence for the Decline of an Adaptation in a Coral Snake Mimic. Evolutionary Biology. 2017:1-0.

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