This suggests a threshold, or minimal investment level, but no great advantage to extra investment above this level. Such a pattern would make sense if the female were able to produce offspring just above this threshold level and then maximise fecundity. Strong and positive relationships between female size and fecundity is well known for woodlice and is also true for P. pruinosus , so the tactic appears to invest the threshold amount in each offspring and then maximise number. The significant range in offspring size within broods appears to have little adaptive value. Measures within a species or a population can be misleading because they cannot evaluate the contribution of a function or adaptation to fitness unless it can be compared with another.
Fitness is a relative concept and so can only be assessed through comparison.
In the offspring size example the comparison between individuals may have been significant for mean values but these statistical differences had no apparent fitness consequences for offspring. A search of the literature shows that numerous studies have measured reproduction in woodlice, especially for temperate species, in an attampt to explain population dynamics or life history tactics. Between 1990 and 1994 we were also able to collect data on female size, fecundity, reproductive allocation and offspring size for several tropical species. Comparing these data there are several significant patterns. As well as showing a linear relationship within species and populations, fecundity also shows an allometry with female size across species.