Table 2.

Multiple regressions for copulation duration for each of the four species

Copulation duration
	Intercept			Relative frequency of heterospecifics			Total number of flies			Interaction: Relative frequency of heterospecifics × Total number of flies
	Estimate	t value	P-value	Coefficient	t value	P-value	Coefficient	t value	P-value	Coefficient	t value	P-value
Drosophila yakuba	45.41	30.847	<1 × 10−10	0.13	−5.469	5.30 × 10−8	−2.05 × 10−2	−8.625	1 × 10−10	1.33 × 10−4	3.458	5.58 × 10−4
Drosophila santomea	46.62	47.808	<1 × 10−10	2.07 × 10−2	1.312	0.190	2.87 × 10−4	0.182	0.855	−2.60 × 10−5	−1.022	0.307
Drosophila simulans	41.18	27.189	<1 × 10−10	−0.12	−4.694	2.92 × 10−6	−9.78 × 10−3	−4.005	6.50 × 10−5	8.01 × 10−5	2.030	0.043
Drosophila sechellia	15.36	19.299	<1 × 10−10	8.56 × 10−4	0.067	0.947	−5.27 × 10−4	−0.411	0.681	−6.26 × 10−6	−0.302	0.763

A multiple linear regression was fitted to study the effect of number of heterospecifics (F_i), the population size (S_j), and the interaction between these two variables (F_i × S_j) in the copulation duration of each of the four species in experimental cages (). The two factors and the interaction were significant for D. yakuba and D. simulans but not for D. santomea or D. sechellia. Significant values (P < 0.05 df = 1496) are marked in bold.