Mating behaviour of two polygamous shorebird species in the Arctic

Krietsch, Johannes (2023): Mating behaviour of two polygamous shorebird species in the Arctic. Dissertation, LMU München: Fakultät für Biologie

Abstract

The mating behaviour is a critical part of an individual’s life, given that it directly influences the reproductive success of sexually reproducing organisms. In nature, we can observe a fascinating diversity of mating strategies shaped by sexual and natural selection. Understanding the evolution of this diversity is a central part of evolutionary biology. Nevertheless, the underlying evolutionary and ecological factors that shape the evolution of this diversity are still not fully understood, and on top of that we even lack detailed knowledge of the mating behaviour for many species. In this dissertation, we enhance our understanding of the mating behaviours and strategies of two polygamous shorebird species. The focus of our studies are the polyandrous red phalarope Phalaropus fulicarius and the polygynous pectoral sandpiper Calidris melanotos, two sympatric breeders of the Arctic tundra with distinctly different mating systems. More specifically, we investigated the social and genetic mating system of the red phalarope, by quantifying the number of social and genetic mates and describing the copulation behaviour in this context. This allowed us to test the “sperm storage hypothesis”, which predicts that extra-pair paternity in sequentially polyandrous species is mainly the result of sperm stored by females from within-pair copulations with a previous mate. Next, we investigated mate guarding behavior in red phalaropes under consideration of the male and female perspective and in relation to breeding phenology, time relative to mean clutch initiation, and to other mutually exclusive behaviours, like incubation or mate searching. Furthermore, we investigated if the apparently nomadic movements of pectoral sandpipers between potential breeding sites are influenced by the prevailing wind conditions. Finally, I describe the mating system of both species in detail, discuss how sexual selection shapes the mating strategies in both species and sexes, and discuss how environmental conditions influence mating strategies. To accomplish this, we recorded in great detail the mating behaviour of red phalaropes during the time span of three breeding seasons in Utqiaġvik, Alaska. We caught and colour-banded nearly all individuals and documented all breeding attempts within our study site. Eggs were collected and artificially incubated to prevent data loss due to predation events, which allowed us to perform comprehensive paternity analysis. Furthermore, modern tracking devices allowed us to follow individual movements and pair-wise association patterns continuously, which allowed us to study variation in mate guarding behaviour. Additionally, we used a previously published data set of pectoral sandpiper movements from two breeding season, in combination with wind data from a global reanalysis model to investigate the influence of wind conditions on breeding site sampling behaviour. We found lower rates of social polyandry and genetic polyandry in red phalaropes than previous studies reported. Overall, 7% of females (11/162) had multiple social mates and extra-pair paternity occurred in 11% (37/334) of the nests. Our paternity analysis and behavioural observations provide limited evidence for the sperm storage hypothesis. Our findings indicate that stored sperm from a previous mate does not significantly contribute to extra-pair paternity in this sequentially polyandrous species. Instead, extra-pair paternity was generally due to two mechanisms: firstly to extra-pair copulations by both sexes during the period between pair establishment and early incubation; then to rapid mate switching by females in the context of attempts to acquire multiple care-giving males. We show that red phalarope pairs were almost continuously together in the days before clutch initiation and showed no sex-bias in separation movements, which suggests that both pair members guard their mate. Still, limited sexual conflict arises through biases in the operational sex-ratio and a trade-off with male nest attendance. We found no clear relationship between mate guarding intensity and the occurrence of extra-pair paternity. Our analysis on the breeding site sampling behaviour of pectoral sandpipers suggests that the wind conditions influence movements in two ways. First, stronger wind support led to increased ground speed and was associated with a longer flight range, and second, males had a higher chance of flying in the direction with more favourable wind conditions. In conclusion, we found that extra-pair paternity in red phalaropes can mainly be explained by female strategies to acquire multiple mates and that in this non-territorial socially polyandrous species, mutual benefits of mate guarding might be the process underlying the evolution of a brief but strong social pair bond, with the unique purpose of producing a clutch for a care-giving male. Polygamy in both species is likely influenced by the length of the breeding season and its spatiotemporal variation throughout the breeding range, as well as local operational sex ratios. Large scale breeding site sampling behaviour can be influenced by the prevailing wind conditions in pectoral sandpipers and consequently effect local breeding densities. Both species are characterised by strong intrasexual selection and direct fitness benefits seem to play a more important role than indirect fitness benefits in mate choice. Still, it is necessary to consider the interplay of intrasexual selection and mate choice to completely understand the factors shaping sexual selection, especially in the context of sex-specific biases in the operational sex ratio and individual strategies to maximise reproductive success from the male and female perspective.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: sexual selection, mating systems, shorebirds, Arctic, red phalarope, pectoral sandpiper, Alaska
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Fakultät für Biologie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 28. November 2023
• 1. Bericht­erstatter:in: Kempenaers, Bart