Phylogenetic and dietary patterning in the morphology of avian colic ceca

In birds, the colic ceca appear as a pair of blind sacs that arise from the junction of the ileum and colon. The length of the ceca is variable among species, ranging from absent to as long as 50% the length of the small and large intestine. We tested the hypotheses that cecal length is related to diet composition, and that phylogenetic relationships are not reflected in cecal morphology. We collated published data on relative cecal length, dietary category, and body mass from 150 avian taxa. To test whether these data patterned phylogenetically, we mapped the character states for each of the 150 avian taxa onto a recently published supertree and recorded the number of evolutionary steps. We then generated 1000 and 10,000 random simulated trees, and mapped the data onto them. None of the simulations required as few evolutionary steps as the real supertree (p<0.001), indicating a significant phylogenetic signal in avian colic cecal morphology. The correlation between diet composition and relative cecal length was not significant (pairwise correlation and Pagel's analyses). However, this could be due to the absence of clades with a large variability in both cecal length and diet. We also found that birds within a given clade have a similar relative cecal size, which further suggests that phylogenetic relationship dominates dietary function. Future work within Galloanserae will examine the relationship between dietary function, cecal size, and phylogenetic relationships. We will specifically test whether plasticity in cecal length is a response to dietary alterations.