Life history tradeoffs in relation to the degree of polyandry and developmental pathway in Pieris napi (Lepidoptera, Pieridae)

Authors

  • Panu Välimäki,

  • Arja Kaitala


P Välimäki (panu.valimaki@oulu.fi) and A. Kaitala, Dept of Biology, Univ. of Oulu, PO Box 3000, FI-90014 University of Oulu, Finland.

Abstract

Pieris napi females have different heritable reproductive tactics. Polyandrous females have higher lifetime fecundity, whereas monandrous ones start to reproduce at a faster rate. Butterfly larvae are time-constrained in seasonal environments. Thus, polyandry is expected to be associated with fast juvenile development, which may result in biased mortality due to physiological costs. We compared how females with varying degrees of polyandry allocate between duration of larval period and achievable size in directly developing and over-wintering generations. Offspring survival and growth were monitored under a high density and low quality diet. Polyandrous females developed at a faster rate than monandrous ones, regardless of developmental pathway. The growth rate of female offspring correlated with their mothers’ degree of polyandry, which underpins polyandry and monandry as distinct strategies with life history differences reaching beyond mating frequency. The high growth rate of polyandrous females resulted in a short larval period among directly developing females, and in large size within an over-wintering cohort. A change in either the duration of the larval period or pupal mass had no significant effect on the other, emphasising that growth rate is not necessarily a simple outcome of the tradeoff between development time and size at maturity. The correlation between the degree of polyandry and juvenile growth rate diminished when larvae were exposed to environmental stress, which offers an explanation why juvenile mortality was decoupled from mating tactic. We conclude that polyandry is a strategy that allows larvae to utilise optimal conditions in a more effective way than monandry. As a consequence, polyandrous females either achieve larger size or they mature faster. This gives them a double advantage over monandrous ones within an over-wintering generation or diminishes the effects of asynchronous hatching of offspring within a directly developing generation. Possible costs of high growth rate are discussed.

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