ANÁLIS Y DISCUSIÓN

Populations founded by females from the downstream locality were just as successful as those founded by females from the upstream locality. Size of first brood was greater for females from the upstream, low-predation, ‘K-selected’ wild population, as would be expected from previous studies of wild fish (Reznick & Endler, 1982; Reznick & Bryga, 1987; Reznick, 1990, Reznicket al., 2001). However, this initial difference in life history strategy does not appear to have a bearing on the probability of establishment or population size after one or two years.

The evolutionary history of the founders did not affect the mean condition factor of fish in mesocosm populations at either 12 or 24 months post-introduction.

Mesocosm population age-structure differed according to evolutionary history, with those populations founded by downstream females containing a significantly greater proportion of juveniles. This may be a consequence of the ‘fast’ life history traits that are characteristic of downstream populations, such as larger brood sizes (Reznick & Endler, 1982). In a resource-limited environment, this may lead to a more juvenile-dominated population even in the absence of predation pressure. Identifiable males from the downstream-founded mesocosms were significantly smaller than those from upstream- founded mesocosms, suggesting that descendents of downstream fish mature faster and/or at a smaller size (Reznick & Endler, 1982). However, differences in male size disappeared by 24 months post-introduction.

Population differences in the size of newborns are well documented; they are associated with various factors including differences in predation regime (Reznick & Endler, 1982; Reznick, 1982). The wild populations in this study support the general findings of previous work, with upstream females giving birth to significantly longer young than downstream females. However, this difference does not persist in the mesocosm populations, where newborns of both evolutionary histories tend to be the same size as downstream wild newborns.

It seems that the effect of evolutionary history on the population and life history characters in mesocosm populations is relatively small, and any differences that do exist 12 months post-establishment, soon disappear. With the exception of population age- structure, by 24 months post-establishment there were no differences in any of the parameters measured between mesocosm populations founded by wild downstream or upstream fish.

3.4.3. Newborn performance

I found no significant differences in performance between the offspring of wild and mesocosm fish for most of the behavioural measures, which suggests that behavioural viability is maintained in the mesocosm populations. The persistence of antipredator behaviours in the absence of predation pressure has been found previously in highly

inbred ornamental guppy strains (Bleakley et al., 2006) as well as in wild-derived guppies reared in isolation (Paxton, 1996).

Original location had a significant effect on both evasion ability and reaction distance, with newborns born to fish originally descended from upstream locations being better at evading a net and reacting at a shorter distance from an approaching object. This is also reflected in the PC2 analysis, where there was a significant effect of evolutionary history. While behavioural differences between populations of guppies in Trinidad, especially between those from upstream and downstream localities, are unsurprising and well documented in the literature (Seghers, 1974; Templeton & Shriner, 2004; Magurran et al., 1995; Magurran et al., 1992; Magurran & Seghers, 1991), here the direction of the pattern is the opposite of what might be expected if I had tested adult guppies from the same localities. In all instances where there was a difference, it was those newborns born to, or descending from, fish from upstream locations - typically free from large predatory fish - that were in fact better at evading a net and reacting to an approaching stimulus.

Past studies have also emphasised that predation pressure on adult and juvenile guppies can differ markedly in the same location (Magurran & Seghers, 1990b). In this case, while the upstream locality in this current study supports fewer predators of adult guppies, the abundance of smaller species such as Rivulus hartii, and possibly higher levels of cannibalism due to higher densities and larger adults (Reznick & Endler, 1982), may lead to a greater selective pressure on newborn anti-predator behaviour here than in the ‘high predation’ location population further downstream.

Predator inspection appears to show the opposite pattern to the other behavioural measures. Here there was a significant effect of origin of mother, with closer and more frequent inspections conducted by those born to wild fish, while there was no evidence suggesting differences between upstream and downstream populations.

3.4.4. Courtship vigour

Direct comparisons between wild and mesocosm male courtship behaviours from this part of the study should be made with caution, as the experimental design does not provide a true control as it did with the newborn performance experiment. Here, it is not possible to separate the effects of genetics and experience, although in fact there were no significant differences between the two localities after the correction for multiple tests.

Wild fish performed almost twice the number of gonopodial thrusts during a 20 minute trial than the mesocosm males. This does not necessarily imply an effect of inbreeding; studies suggest that population differences tend to take the form of differences in risk sensitivity rather than in absolute behavioural differences (Magurran & Seghers, 1990a). As the mesocosm fish have never been exposed to a predator of adult fish within their lifetimes, whilst the wild fish probably have been, it makes sense that any differences in courtship behaviour would be more apparent in the wild fish, regardless of any potential inbreeding effects. Frequency of sigmoid displays did not differ with evolutionary history or immediate origin.

The relative effects of immediate origin or evolutionary origin appear to be different for different behaviours. Even within antipredator behaviours, it seems that differences in evasion and reaction behaviours are determined by evolutionary history (upstream or downstream), whilst for predator inspection behaviours whether they were born to wild or mesocosm parents proved to be more important and indicates some reduction in predator inspection behaviour in mesocosm newborns.