If morphs differ in their MMR profiles, they may also differ in their net allocatable energy available for growth and reproduction, and hence in their fitness[21, 25, 31]. This in turn could constitute a mechanism for the maintenance of a colour polymorphism. We found that colour phenotype significantly contributes to variation in MMR profile (Table 1). This pattern of colour contributing to variation in metabolic rate and, in particular, the pattern of the black morph from VPA having a significantly lower metabolic rate than the striped morph at 10°C (Figure 1b) agrees with a study by Moreno , who found that the black morph had a lower SMR than the striped morph at 15°C. For thermal niche specialization to directly maintain the polymorphism however, both morphs must have a niche where they are favoured, and our study did not find evidence of this. Within the range of temperatures that we studied we did not find a consistent pattern across populations in either metabolic response or in thermal niche choice.
The data collected in this study provided the ability to differentiate morphs with even subtle differences in metabolic rate and thermal profile. While differences between morph could be detected we did not find evidence that these differences support a hypothesis that morphs are adapted for different thermal optima. We only found significant differences in MMR between the morphs at 10°C for animals from VPA and in that population there was no evidence of separate thermal niches. For separate niches to exist each morph would have to have a thermal niche in which it is favoured. This pattern of response was evident in neither the O'Neil nor VPA population. Furthermore, given that the influence of phenotype on MMR profile varied significantly between locations, thermal niche specialization is unlikely to be a uniform force directly maintaining the polymorphism in these populations. The existence of significant variation between morphs in MMR profile across temperature (Table 1), does however indicate a linkage of some sort between morph type and thermal optima. The existence of significant interactions between morph phenotype and location in the determination of thermal profile also indicated that while potentially important, any relationship between phenotype and thermal specialization is not consistent across populations.
This study demonstrates that black and striped morphs of P. cinereus are sometimes found in different thermal microhabitats. The two morphs seem to segregate into different thermal habitat in some locations (marginally significant for VPA in Figure 3). However, the difference is once again dependent upon the census location and season (Table 4). Differences between morphs in their thermal microhabitat distribution are consistent with data suggesting that morph frequencies can vary with air temperature  and with temperature proxies such as climate  and season . Furthermore, since we find that patterns of thermal segregation are not consistent, we confirm the contradictory patterns of niche preference in these previous studies. Lotter and Scott  found striped morphs in lower frequencies in regions defined as "warm", and Moreno  found that striped morph frequency decreased with increasing temperatures. In contrast, Test  found that striped morph frequency increased with increasing temperatures through the summer. We conclude from these previous studies that our results accurately reflect a relationship between thermal habitat segregation and colour morph that is inconsistent from population to population.
This study, as well as Moreno's study  found the black morph to have a lower metabolic rate under certain conditions. Either chromosomal or pleiotropic linkage between colour phenotype and metabolism could be responsible for this relationship. If pleiotropy is responsible for the relationship between these traits then there should be a consistent relationship across all populations. However, if chromosomal linkage is responsible, linkage could be inconsistent from population to population since linkage pairing could vary from population to population. Because we find differences in the relationship between MMR and morph type between locations, it appears that chromosomal linkage is more likely to be responsible for the association of colour and metabolism than pleiotropy. Correlated response to selection on colour morphs should, in this case, vary from location to location depending on the particular linkage within a given population or region.
Alternative explanations for the maintenance of the polymorphism given evidence (here and ) that the black morph has at times a lower maintenance expenditure remain unexplored. Possible explanations for maintenance of such colour polymorphisms are extensive and have been thoroughly reviewed by Roulin  and Hoffman and Blouin. Roulin has pointed out that proximate adaptive explanations for the existence of a colour polymorphism may fall into three classes: historical, direct, or indirect effects . A historical association between colour and other fitness related traits, due to divergent selection during allopatry, might be carried into the present. The apparent mosaic of various associations between colour morph and metabolic rate indicate that no single historical event is responsible for the polymorphism in P. cinereus.
There may be some direct selection on the colour polymorphism in P. cinereus. Differences in solar heating are unlikely to be of any importance in this species as is common in many species  since the species is not active during daylight. It is possible however that colour is important in defense or mating. Such direct selection on colour may support the polymorphism or it may balance indirect selective effects due to correlated response to selection on thermal niche. The potential for selection on the all red 'eurythristic' morph as a Batesian mimic of red spotted newts has been demonstrated [33, 34]. However the eurythristic form is too geographically restricted to account for the more common striped vs. black polymorphism. Striped morphs could be more cryptically coloured than the black morphs or, since predators often prey on the most common prey form, frequency-dependent selection can be maintaining the polymorphism even without mimicry . No studies, however, have yet documented direct selection supporting the common striped/black polymorphism.
This and previous studies [7, 10, 11] have found evidence for thermal niche differentiation of the two different colour morphs. This provides the opportunity for a correlated response of to selection on temperature specific metabolic rate. While this study fails to find evidence for unique thermal optima for the two morphs over typical summer field temperatures, seasonal dynamics may provide a more complex selective regime than examined here. The differences between populations in the way in which the colour morphs respond to temperature indicate that while correlated response to selection may be an important part of the dynamics of the polymorphism it is not a consistent one. We cannot eliminate the possiblity of plastic response of metabolic rate to environmental factors upon which morphs might be segregating. This type of influence has been documented in Tawny Owls  although in this case we have no evidence supporting such a plastic response. We speculate insead that some sort of indirect response to selection on thermal niche is probably important in maintenance of the polymorphism, but that historical linkage associations between the morphs and the genes under selection may be locally variable.