Abstract
Evolutionary theory predicts trade-offs between fecundity and mobility, but there is substantial lack of empirical evidence if and how basic mobility relates to fitness costs. In a field experiment, we investigated fecundity costs of mobility in a non-migratory, wing-monomorphic grasshopper, Stenobothrus lineatus, and at the same time tested for possible effects of reproductive state (egg-load) on the mobility. For 10 days, body weight and activity radius of 60 females were recorded daily and oviposition events were inferred from abrupt weight losses. We found a strong and significant relationship between the individual mobility and the time between egg pods laid (interpod period). Individual egg-laying was reduced by a rate of 0.36 eggs per day with each meter increase in mean daily activity radius. The trade-off was not biased by the size of the females, that is, constitution did not positively influence both offspring number and mobility. Egg-load had no significant influence on the individual distances travelled. We could demonstrate that mobility – as induced and selected for by foraging, thermoregulation, predator escape, shelter seeking, and reproduction – can be directly paid off by fecundity. This direct consequence of mobility on individual fitness was detected for the first time in a walking insect.
Highlights
Evolutionary theory predicts that among the life-history parameters of an organism, fecundity should be optimized versus parameters allowing future reproductions by tradeoffs to achieve optimum fitness (Stearns 1989; Kirkwood 2002)
The initial laboratory experiments revealed that lifetime fecundity of the single females of S. lineatus, as the totals number of eggs laid, was best related to the reciprocal interpod period, that is, the oviposition rate per day (Table 1; Pearson’s product moment correlation, R = 0.87, P < 0.0001), followed by reproductive period (R = 0.75, P < 0.0001)
Oviposition events in the field experiment were identified in 50 S. lineatus females, of which 47 could be included in the mobility analysis owing to their sequence of recaptures
Summary
Evolutionary theory predicts that among the life-history parameters of an organism, fecundity should be optimized versus parameters allowing future reproductions by tradeoffs to achieve optimum fitness (Stearns 1989; Kirkwood 2002). Current attempts to create an unifying paradigm of movement ecology (e.g., Dingle 1996; Clobert et al 2001; Holyoak et al 2008; Nathan et al 2008) show that there is substantial lack of empirical evidence, especially from field populations in invertebrates, how regarding investments relate to each other It is hardly known if and how basic mobility transfers to fitness costs (Holyoak et al 2008; Nathan et al 2008). Few attempts exist to separate two effects, that is, (1) the effect of mobility on the reproduction output as the basic trade-off, which could select for philopatry or non-dispersal behavior (e.g., Hanski et al 2006; Gu et al 2006; Saastamoinen 2007); and (2) the effect of reproductive state (egg-load) itself on the mobility (e.g., Olsson et al 2000; Veasey et al 2001; Plaut 2002)
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