THE INFLUENCE OF FLUCTUATING RESOURCES ON LIFE-HISTORY - PATTERNS OF ALLOCATION AND PLASTICITY IN FEMALE GUPPIES

We investigated how resources are allocated to reproduction and how va riations in resource availability influence reproductive allocation, o ffspring number, and offspring size in guppies (Poecilia reticulata). Our goal was to evaluate how plastic these variables are in response t o environmental variation and to characterize the nature of this plast icity. Female guppies which had just given birth (litter 1) were assig ned to either high or low levels of food availability until they gave birth to their next litter (litter 2, interval 1). They were then rand omly reassigned to either high or low food with the constraint that th ere be equal numbers of individuals in each of four treatments: high-h igh, high-low, low-high, and low-low. They were maintained on this lev el of food availability until they produced their next litter (litter 3, interval 2). We analyzed variables that characterized the female af ter the birth of the third litter and the offspring in the third litte r. These were two-way analyses, with intervals 1 and 2 as the main eff ects and high vs. low food as the levels of each effect. The qualities of the third litter were influenced by both interbrood intervals, ind icating that the resources used for producing the litter were derived from both intervals. Specifically, higher food availability during eit her interval resulted in a significant increase in the number of offsp ring in litter 3, independent of the size of the mother. This result i ndicates that the number of offspring produced in a litter will be a f unction of both the immediate and the past environment. Lower food dur ing either interval resulted in an increase in the number of days betw een the second and third litters, indicating that, if resource availab ility is low, the female may delay the initiation of the next litter, allowing her to acquire more resources. Resource availability during b oth intervals also influenced how resources were allocated to individu al offspring. Females responded to low food during the first interbroo d interval by producing heavier offspring in litter 3. This increase i n mass was almost entirely attributable to an increase in fat reserves . Such a result could represent adaptive plasticity, if it can be demo nstrated that maternal fitness increases through the production of hea vier offspring in a low-food environment.