Development time (egg deposition to adult ecolosion)
A total of 3481 (1733 females) and 4460 (2245 females) adult mosquitoes emerged in the photoperiod-alone experiment (Exp. 1) and the photoperiod and temperature experiment (Exp. 2), respectively. In both experiments, males developed faster than females (Exp. 1: F
(1,3474) = 36.6, Exp. 2: F
(1,4450) = 30.8, P < 0.01) in analyses within species across treatments. In both experiments, differences between species, treatments, and species-by-treatment interaction were all significant (Exp. 1: F > 31, Exp. 2: F > 4.5, P < 0.037).
Photoperiod only: Experiment 1
Development time of female An. arabiensis increased by 1.5 days, from 13.0 d to 14.5 d, with decreasing day-length from the wet to the dry season (F
(1,775) = 91.4, P < 0.001; Fig. 1a). In contrast, female An. coluzzii developing under wet-season conditions had the longest development time (14.2 d) while those developing under the induction (wet-to-dry transition) photoperiod had the shortest (12.1 d, F
(
1,952) = 66.9, P < 0.001; Fig. 1a), amounting to 15% reduction in developmental time.
Photoperiod and temperature: Experiment 2
Development time of female An. arabiensis differed minimally across all 4 photoperiod-temperature combinations (F
(
3,415) = 3.1, P = 0.026; Fig. 1b) whereas, for female An. coluzzii, the effect of treatment was highly significant (F
(
3,1822) = 16.4, P < 0.0001). However, females developing in the photoperiod-temperature “matched” treatments [i.e. wet-season photoperiod with wet-season temperature (10.9 d), and dry-season temperature with dry-season photoperiod (10.8 d)] were not significantly different from each other (F
(
1,1822) = 0.5, P = 0.82). Moreover, the development time of the two “matched” treatments were significantly shorter (F
(
1,1822) = 47.3, P < 0.0001) than the two “mismatched” treatments (wet-season photoperiod with dry-season temperature and dry-season photoperiod with wet-season temperature), which were not different from each other (F
(
1,1822) = 0.4, P > 0.55; Fig. 1b).
Body size and lipid reserves
At adult emergence, positive correlations were found between female body size (measured as wing length), dry mass, and lipid mass across species and treatments (An. arabiensis Exp. 1: 0.34 < r < 0.69, 0.002 < P < 0.15, n = 20–23; Exp. 2: 0.81 < r < 0.85, P < 0.0001, n = 20; and An. coluzzii Exp. 1: 0.54 < r < 0.95, P < 0.019, n = 18–20; and Exp. 2: 0.56 < r < 0.64, P < 0.0001, n = 38). However, the relative lipid content (proportion of lipids from total dry mass) was negatively correlated with body size in An. arabiensis (Exp. 1: -0.58 < r < -0.55, P < 0.11, n = 20–23; Exp. 2: -0.69 < r < -0.53, P < 0.0001, n = 20) whilst the correlation was not significantly different from zero in An. coluzzii (Exp. 1: -0.37 < r < -0.36, P < 0.12, n = 18; Exp. 2: -0.16 < r < 0.29, P < 0.085, n = 38). These results suggest that upon emergence, in An. arabiensis females there is a trade-off between body size and lipid content, whereas in An. coluzzii lipid content is relatively constant across body size.
Experiment 1
Photoperiod significantly affected female body size (wing length) for both An. arabiensis (F
(
2,22) = 14.3, P = 0.0001) and An. coluzzii (F
(
2,17) = 14.3, P = 0.0002). However, the direction of the change differed between the species; under the dry-season photoperiod, female An. arabiensis were smallest while An. coluzzii were largest (Fig. 2a). Female An. arabiensis were not significantly different in dry mass at emergence between the three photoperiod treatments (F
(
2,22) = 2.8, P = 0.085), although the trend followed that exhibited by their body size (Fig. 2c). In contrast, female An. coluzzii under the dry-season photoperiod had a significantly higher dry mass at emergence than those in either the wet-season or the wet-to-dry transition treatments (F
(
2,20) = 13.5, P = 0.0002; Fig. 2c). The absolute lipid mass at emergence was not significantly different among treatments for both species (F < 2.38, P > 0.12; Fig. 2e), although the trends are qualitatively similar to those of dry mass. Moreover, relative lipid content was not significantly different between treatments of both species (F
(
2,20) = 3.45, P > 0.059 and F
(
2,19) = 0.64, P > 0.53 for An. arabiensis and An. coluzzii, respectively; Additional file 1: Figure S2).
Experiment 2
Temperature (F
(1,19) = 13.2, P = 0.0023) but not photoperiod alone (F
(1,19) = 2.3, P = 0.15) or the interaction between photoperiod and temperature (F
(1,19) = 1.2, P = 0.30) significantly affected body size (wing length of female An. arabiensis. Female An. arabiensis raised under the wet-wet treatment were the largest while those raised under the dry-dry treatment were the smallest (Fig. 2a). In contrast, photoperiod alone (F
(1,34) = 6.8, P < 0.014) and the interaction between photoperiod and temperature (F
(1,34) = 4.4, P < 0.04) significantly affected body size (wing length) for female An. coluzzii, while temperature alone was non-significant (F
(1,34) = 0.0, P = 0.96). Similar to the results of Exp. 1, female An. coluzzii raised under the dry-dry treatment were larger than those raised under the wet-wet treatment (Fig. 2b). Temperature (F
(1,19) = 20.9, P = 0.0003) but not photoperiod (F
(1,19) = 2.1, P = 0.17) or their interaction (F
(1,19) = 0.2, P = 0.66) significantly affected dry mass at emergence for An. arabiensis females. Female An. arabiensis raised under the wet-season temperature treatments were significantly heavier than those females raised under the dry-season temperature treatments (Fig. 2d). For female An. coluzzii, the effect of photoperiod on dry mass at emergence was marginally significant (F
(1,34) = 4.1, P = 0.052), while temperature and the interaction term were both non-significant (F
(1,34) < 1.4, P > 0.24). However, the qualitative differences in dry mass at emergence for An. coluzzii were similar to those of Exp. 1 (Fig. 2d). Unlike Exp. 1, body lipid mass at emergence was significantly affected by the treatment for both species (Fig. 2f). For female An. arabiensis, lipid mass at emergence was significantly affected by temperature (F
(1,19) = 25.3, P = 0.0001) but not by photoperiod or the interaction term (F
(1,19) < 0.08, P > 0.8); female An. arabiensis had higher amounts of lipids under both wet-season temperature treatments as compared with the two dry-season temperature treatments (Fig. 2f). By contrast, for female An. coluzzii, photoperiod (F
(1,34) = 47.4, P < 0.0001) and the interaction term (F
(1,34) = 22.9, P < 0.0001), but not temperature alone (F
(1,34) = 2.7, P = 0.11), significantly affected lipid mass at emergence. Female An. coluzzii raised under the dry-dry treatment had the highest amount of lipid at emergence (Fig. 2f), similar to the trend seen in Exp. 1 (Fig. 2e). Moreover, relative lipid content was also not significantly different between treatments in An. arabiensis (F
(1,19) < 1.7, P > 0.19) but significantly differed between treatments in An. coluzzii (F
(1,34) = 10.1, P < 0.0001). Notably, under shorter photoperiod and lower nightly temperature relative lipid content was highest in An. coluzzii (Additional file 1: Figure S2).
Adult longevity
Longevity (from adult ecolosion to death) of a total of 1473 females consisting of 676 and 797 of An. arabiensis and An. coluzzii, respectively (including 74 and 81 censored mosquitoes, respectively) was measured in Exp. 1. Longevity of a total of 2041 females consisting of 392 and 1657 of An. arabiensis and An. coluzzii, respectively (with additional 84 and 229 censored mosquitoes, respectively) was measured in Exp. 2. Overall mean longevity of An. arabiensis and An. coluzzii in the first and second experiments were 19.4 d (SE = 0.39, max = 55 d) and 20.5 d (SE = 0.38, max = 63 d), 26.5 d (SE = 0.52, max = 59 d), and 25.8 d (SE = 0.26, max = 64 d), respectively.
Experiment 1
For An. arabiensis, no significant difference in female longevity was found between the three photoperiod treatments tested (Wilcoxon χ
2 = 0.94, df = 2, P = 0.59; Fig. 3b), with mean longevity under the wet-season photoperiod at 20.0 d (SE = 0.84), under the transition photoperiod at 19.2 d (SE = 0.62), and under the dry-season photoperiod at 19.3 d (SE = 0.62). In contrast, photoperiod significantly affected longevity of female An. coluzzii (Wilcoxon χ
2 = 30.04, df = 2, P < 0.0001; Fig. 3a), with dry-season mosquitoes having the highest mean longevity (23.1 d, SE = 0.64), wet-season mosquitoes having the shortest mean longevity (18.0 d, SE = 0.60), and transition photoperiod mosquitoes having intermediate longevity (20.6 d, SE = 0.74).
Experiment 2
Unlike Exp. 1, mean longevity of female An. arabiensis was significantly affected by treatment (Wilcoxon χ
2 = 30.2, df = 3, P < 0.0001), with higher longevity under dry-season photoperiod/wet-season temperature (32.0 d, SE = 1.3), as compared with the wet-season photoperiod/dry season temperature (23.5 d, SE = 1.1), and wet-wet (24.8 d, SE = 0.96) treatments (Fig. 3d). Notably, the dry-dry treatment (27.0 d, SE = 0.77) was intermediate and statistically similar to all other treatments (after Tukey-Kramer multiple test adjustment: P > 0.087). For An. coluzzii, treatment also significantly affected female longevity (Wilcoxon χ
2 = 85.7, df = 3, P < 0.0001; Fig. 3c). In contrast to An. arabiensis, for An. coluzzii the highest mean longevity was exhibited under the dry/dry treatment (30.1 d, SE = 0.68), which was significantly greater than all other treatments (after Tukey-Kramer multiple test adjustment: P < 0.039). The next highest mean longevity was achieved under dry photoperiod/wet temperature (26.9 d, SE = 0.51), which was higher than the two wet-season photoperiod treatments, which were also not significantly different from each other (wet-dry = 24.2 d, SE = 0.46 and wet-wet = 23.8 d, SE = 0.47). Initial adult density did not affect longevity of An. arabiensis (P > 0.90) or An. coluzzii (Wilcoxon χ
2 = 0.5, df = 1, P > 0.48).