Understanding seasonal variation in reproductive investment is crucial to explore life-history strategies of a given species. We measured the timing of reproduction and the reproductive output of recently captured females to test whether the conclusions on seasonal shifts of reproductive investment made in the laboratory can apply to field populations in a lacertid lizard Takydromus septentrionalis. Female T. septentrionalis started producing eggs in April and ceased reproduction in August. During the midreproductive season (May to July), more than 60% of field-captured females were gravid. The females produced more eggs and thus had heavier total egg mass in May than in July, although egg mass and hatchling size did not vary seasonally. The seasonal variation in clutch size did not induce seasonal shifts in trade-offs between clutch size and egg size. In general, these results are largely consistent with data collected in the laboratory and support the conclusion of previous studies in this species that seasonal shifts in reproductive output are determined ultimately by natural selection. However, Relative Clutch Mass (RCM) of recently captured females was greater than that of females in captivity, which suggests that RCMs could be affected noticeably by proximate factors. Therefore, when explaining the evolutionary strategies of an organism based on intraspecific comparisons of RCMs, investigators should measure them in a common-garden environment so as to control the potential influence of environmental factors.
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