Tail-Feather Length in Birds Is Sometimes a Sexually Dimorphic Trait

Multiple Choice

Tail-feather length in birds is sometimes a sexually dimorphic trait. That is, the trait differs substantially for males and for females. Researchers measured tail-feather length (the R1 central tail feather, in mm) in male and female long-tailed finches either raised in an aviary or caught in the wild. This observational study does not have a balanced design, particularly because finches caught in the wild were more difficult to obtain. A total of 52 finches were studied.
A partial ANOVA table is provided below, along with an interaction plot displaying the group means.
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$$\begin{array}{l}\begin{array} { l r r r } \text { Factor } & \text { Type } & \text { Levels } & \text { Values } \\\text { Malefemale } & \text { fixed } & 2 & \text { f, m } \\\text { Origin } & \text { fixed } & 2 & \text { aviary, wild }\end{array}\\\\\text { Analysis of Variance for feather length }\\\\\begin{array} { l r r r r r } \text { Source } & \text { DF } & \text { SS } & \text { MS } & \text { F } & \text { P } \\\text { malefemale } & 1 & 2015.5 & 2015.5 & & \\\text { origin } & 1 & 251.7 & 251.7 & & \\\text { interaction } & 1 & 15.6 & 15.6 & & \\\text { Error } & 48 & 4076.9 & 84.9 & &\end{array}\end{array}$$ S = 9.21600 R-Sq = 47.32% R-Sq(adj) = 44.03%
The interaction effect is nonsignificant, so we can interpret the main effects. Using a significance level of 0.05, what do the results suggest about the main effects of sex and origin?

A) Sex affects mean tail-feather length significantly in finches, but origin does not.
B) Origin affects mean tail-feather length significantly in finches, but sex does not.
C) In finches, sex affects mean tail-feather length significantly, and origin affects mean tail-feather length significantly, but the two influences work independently.
D) Sex affects mean tail-feather length significantly, and origin affects mean tail-feather length significantly, but the influence of origin is different for male and female finches.