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1 December 2005 Egg size-number trade-off and a decline in oviposition site choice quality: Female Pararge aegeria butterflies pay a cost of having males present at oviposition
M. Gibbs, L. A. Lace, M. J. Jones, A. J. Moore
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Abstract

Once mated, the optimal strategy for females of the monandrous butterfly, Pararge aegeria, is to avoid male contact and devote as much time as possible to ovipositing, as there is little advantage for females to engage in multiple matings. In other butterfly species the presence of males during egg laying has been shown to affect aspects of oviposition behavior and it has been suggested that repeated interference from males has the potential to reduce reproductive output. The aim of this study was to assess the effects of male presence during oviposition on reproductive output and behavior of a population of P. aegeria obtained from Madeira Island, Portugal, and maintained in the laboratory. Two experiments were performed where females were housed individually in small cages. Experiment 1 examined how social factors influenced the egg laying behavior of females. To do this the presence or absence of males was manipulated and egg size and number was measured over the first 14 days of oviposition. It was observed that when males were present during oviposition females made a trade-off between egg size and number. Experiment 2 examined how social factors affected oviposition site choice. Again, male presence/absence was manipulated, but in this experiment where the female laid her egg in relation to host quality was scored, and the size of the egg laid was measured. In the absence of males females selectively positioned their larger eggs on good quality host plants. However, selective oviposition was no longer observed when females were in the presence of males. We suggest that P. aegeria females from the Madeira Island population are adapted for a flexible oviposition strategy, governed by external cues, allowing a trade-off between egg size and number when the time available for egg laying is limiting.

Introduction

The choices that a female makes at oviposition can greatly affect the survival and development of her offspring (Awmack and Leather, 2002; Fox and Czesak, 2000; Réale and Roff, 2002; Thompson and Pellymyr, 1991). At oviposition a female may vary her egg laying in response to many factors including; her own state, e.g. the quantity of nutritional reserves, the environmental conditions she encounters, e.g. host quality (Heimpel and Rosenheim, 1995), in response to trade-offs between clutch size and adult survival (Roff, 1992), and current and future reproduction (Rosenheim, 1999).

One frequently studied reproductive trade-off is that of propagule size versus propagule number. The classic Smith & Fretwell (1974) model predicts a negative relationship between propagule size and number, and suggests that for each species a particular combination must exist to maximize fitness. Many studies have since shown that in unpredictable environments selection will favor a bet-hedging reproductive behavior where a range of propagule sizes and numbers are produced to increase the lifetime reproductive success (Clutton-Brock, 1991; Fox and Czesak, 2000; Roff, 1992; Stearns, 1992). Studies on various insect species demonstrate that larger offspring should be produced when conditions for offspring growth and survival are poor (Awmack and Leather, 2002; Carriére and Roff, 1995; Fischer et al., 2003a; Fischer et al., 2003b; Fox et al., 1997; Leather and Burnand, 1987; Rotem et al., 2003; Savalli and Fox, 2002; Takakura, 2004). However, the model of Mangel et al. (1994) predicts that mothers should choose larger clutch sizes, thereby producing small less fecund offspring, when their offspring inhabit a poor environment. Therefore, the time when mothers should choose larger offspring over smaller offspring (and vice versa) is still subject to debate.

Some insect species adjust the size-number trade-off of their offspring according to the quality of the hosts that they encounter. The time available for females to search for optimal oviposition sites has the potential to greatly affect the decision-making process of females. Behavioral plasticity during oviposition enables females to increase the rate at which hosts are found when time is limited. However, the strategies employed to increase host encounter rate may result in a decline in the quality of hosts on which eggs are laid (Papaj, 2000). Time limitation therefore also has the potential to affect this trade-off. At high male densities, repeated male harassment of females has the potential to reduce the time available for oviposition, and may result in a reduction in the reproductive success of females (Boisseau, 1996).

Preliminary data collected by Boisseau (1996) suggested that the presence of males during the first week of oviposition may cause Pararge aegeria (L) females to be engaged in a high level of activity, thus minimizing the time available for oviposition and oviposition resting. Although further studies are required to confirm this finding, this observation may provide an explanation for the cryptic post-copulatory behavior of P. aegeria females, with active avoidance of males (Wickman and Wiklund, 1983). Other butterfly species also show active avoidance of males once mated (Baguette et al., 1996; Odendaal et al., 1989; Shapiro, 1970; Wickman, 1986). This post-copulation behavior may have evolved to reduce the potential cost (to the offspring) of the differing mating strate