Differential Haemoparasite Intensity between Black Sparrowhawk (Accipiter melanoleucus) Morphs Suggests an Adaptive Function for Polymorphism

Abstract

Recent research suggests that genes coding for melanin based colouration may have pleiotropic properties, in particular conveying raised immune function. Thus adaptive function of polymorphism may be associated with parasite resistance. The black sparrowhawk Accipiter melanoleucus is a polymorphic raptor with two morphs. Over most of its range the light morph is commonest, however within the recently colonised Western Cape of South Africa the dark morph predominates. The species breeds in winter throughout South Africa, however unlike in the rest of the species' South African range, the Western Cape experiences a winter rainfall regime, where arthropod vectors which transmit haematozoan parasites may be more abundant. We hypothesise that the higher frequency of dark morph birds in this region may be due to their improved parasite resistance, which enables them to cope with higher parasite pressure. If so, we predict that dark morph black sparrowhawks would have lower parasite burdens than light morph birds. Within our population the prevalence of the two most common haematozoan parasites was high, with 72% of adults infected with Haemoproteus nisi and 59% of adults infected with Leucocytozoon toddi. We found no difference in prevalence for either parasite between adult morphs, or between chicks of different parental morphs. However, within adults infected with H. nisi, infection intensity was significantly higher in light morphs than dark morphs. This suggests that dark morphs have lower parasite loads than light morphs due to resistance rather than morph-specific habitat exploitation. Greater resistance to Haemoproteus parasites may therefore be one of the mechanisms through which dark morph black sparrowhawks have a selective advantage in this region and may explain why they are most common in our study area. In other regions, the cost to benefit ratio may be in favour of the light morph, where parasites are less abundant or virulent.

Family morph matters: factors determining survival and recruitment in a long‐lived polymorphic raptor

“…) or parasite exposure (Lei et al . ). It is therefore logical that parental morphs could strongly influence both survival and recruitment.…”

“…Parental morph was the other key variable. Numerous studies have shown differential life-history traits in melanin-based polymorphic species, which has led to the hypothesis that different morphs may be adapted to different environments, that may vary, for example in food abundance, social interactions (Roulin 2004b), weather conditions (Karell et al 2011) or parasite exposure (Lei et al 2013). It is therefore logical that parental morphs could strongly influence both survival and recruitment.…”

Immune responses link parasite genetic diversity, prevalence and plumage morphs in common buzzards

“…Several studies have indeed found correlations between melanin pigmentation and immune responses in wild vertebrates as a possible consequence of the POMC pleiotropy. However, the precise molecular link to the humoral response is so far unknown and the documented patterns are, at least partially, contradictory (Gangoso et al 2011(Gangoso et al , 2015Jacquin et al 2011Jacquin et al , 2013Lei et al 2013;Roulin et al 2000).…”

Fecundity selection does not vary along a large geographical cline of trait means in a passerine bird

“…The most conspicuous coloration trait in male pied flycatchers is the melanin‐based dorsal coloration that varies from completely brown to black (Drost, ; Lundberg & Alatalo, ). Melanin coloration is often linked with reproduction (Meunier et al ., ), stress sensitivity (Almasi et al ., ), parasite resistance (Lei et al ., ), and the costs of parasite infections (Karell et al ., ). Several studies have found context‐dependent selection on melanin‐based coloration (Roulin et al ., ; Dreiss et al ., ; Karell et al ., ; Dreiss et al ., ).…”