Biology Department Research Group Terrestrial Ecology Unit (TEREC) _____________________________________________________________________________________ FISSION-FUSION DYNAMICS IN THE FACULTATIVE COOPERATIVE BREEDING PLACID GREENBUL (PHYLLASTREPHUS PLACIDUS) Rori Sys Studentnumber: 01302591 Supervisor(s): Prof. Dr. Luc Lens Scientific tutor: MSc Laurence Cousseau Master’s dissertation submitted to obtain the degree of Master of Science in Biology Academic year: 2018 - 2019 Introduction Fission-fusion dynamics within social species are a relatively well observed phenomenon (Silk et al.; 2014 (1)). Fission includes behaviours where groups either split up in subgroups or just completely dissipate. Fusion behaviour on the other hand is characterized by the forming of new groups or the growth of a group. Cooperative breeding is a type of breeding-behaviour where more than two individuals provide care at a single nest. In a recent study it was observed that at least 9% of all passerines show this behaviour (Leedale et al.; 2018 (2)). Numerous hypotheses try to explain the origin of this behaviour, including the ‘inclusive fitness hypothesis’ (cf. infra). The role of the environment is also still under debate. We will be looking at this type of behaviour through the lens of fission-fusion dynamics. The breeding pair is the centre of any cooperative breeding group. Changes to it will affect the entire group significantly. Fission-fusion behaviour surrounding the breeding pair encompass the forming of the pair and their reasons for staying together or breaking up. Pairs can stay together for multiple breeding seasons, sometimes even for entire lifetimes, like in the case of the bearded reedling (Griggio and Hoi ;2011 (3)). However, it is not always clear what the main benefits are of choosing to stay together. A general hypothesis for pair fidelity is that with bonding time the familiarity increases between the pair. This familiarity would increase the cooperation between the pair and positively impact their breeding success, which would encourage not switching to a new partner (Gabriel et al.; 2012 (4)). Results of this hypothesis are mixed. Some studies seem to confirm this. In a study by Griggio et al (2011 (3)) an increase in breeding success with pair bond duration of bearded reedlings was found and in another study by Gabriel et al. (2013) (5) similar results in Steller’s jays were observed. Other studies showed increases in breeding success only after the first year, indicating that after the pair is more familiar with each other no further big increases in breeding success happen, as observed in kittiwakes (Naves et al., 2007 (6)) and barnacle geese (Larsson et al., 1991 (7)), which can live together for many years. Breeding success has also been related to site familiarity (Naves et al., 2007 (6)) and the overall experience of the breeding pair (Orell et al., 1993 (8)). It is clear that the reasons for pair stability can be species-specific and very complex. Divorce, a clear example of fission, seems to be a general strategy to obtain a better breeding position. This principle is titled the “better option hypothesis” (Ens et al.; 1993 (9)). In great tits, the chance of divorce was higher if the clutch size was smaller, indicating either a lack of experience or compatibility of the couple. (Perrins and McCleery; 1983 (10)). The initiation of the divorce is often dependent on the sex of the breeder as it is often the female breeder that actively chooses to divorce. In domestic pigeons, female breeders were observed to choose new mates after failed breeding attempts while males stayed at their nesting site (Wosegien; 1996 (11)), and in Willow Tits, females were observed to choose more experienced males after divorce (Orell et al 1993 (8)). In several temperate bird species, including the mallard (Cezilly and Nager;1995 (12)), divorce was related to extra-pair paternity, possibly as a mean of mate sampling by the female breeders suggesting evidence for the better option hypothesis. However, this female choice-based fission is not always the cause for divorce as it seemed to be more accidental, as in the death of either breeder, like in the case of the barnacle goose (Larsson et al.; 1991 (7)). Alternatively, the divorce of a pair can be forced by other females in species like kiwis, because a strong female bias of the species (Taborsky et al. 1999 (13)). 2 Another reason for divorce could also be to avoid inbreeding and to increase the heterozygosity of the offspring. (Kempenaers 2007 (14)) Females have been observed to leave their nest after raising young and go to other nesting locations. Several hypotheses try to explain this female promiscuity. Firstly, female breeders might have to choose between direct and indirect advantages for their young. The care that their partner gives to their young would be the direct advantage. While the genetic benefits that can be acquired through extra-pair copulations are indirect genetic benefits to the young. Secondly, extra-pair copulations might also be an attempt to increase the heterozygosity of the young. Thirdly, it can be an effort to reduce inbreeding and thus to increase hatching success. Lastly, in some birds mating with multiple males can cause sperm competition with the best sperm winning the race. Of course, this is all limited by the ability of individuals to make choices as demonstrated in a study by Wheelwright et al.; 2006 (15). Young females could recognize their father but not their brothers since they had only seen them with juvenile characteristics. They were less likely to mate with their fathers than with their brothers as a consequence. Although inbreeding is generally avoided, breeding females were sometimes observed to breed with their sons, generally only for one year. (Wheelwright et al.; 2006 (15)) Furthermore, as the breeding pair is the central unit of any cooperatively breeding group its stability will be a key component of fission-fusion dynamics that the group undergoes. In a study on marmosets (C. Lazaro-Perea et al.; 2000 (16)) the loss of a breeding female had two possible outcomes for the group. If no adult females were present in the group, a replacement female from outside the group was readily accepted. If multiple other females were in the group, they fought for dominance and kept out new immigrants. The number of extra-group copulations rose in this case and ultimately the group could fission along sexual lines. In the pied babbler, longer pair bonds decreased the amount of immigration the group received, showing a clear connection between pair stability and group dynamics, as was observed in a recent study by Wiley and Ridley (2018) (17). They suggest that this happens because groups with pairs that have been together longer are more related on average and consequently they see a decrease in intra-group reproductive competition as well as in intra-group conflict. In cooperatively breeding species fission-fusion dynamics can be especially complex. Besides the reasons for the breeders to stay together or not, the non-breeding individuals also have their own reasons for staying in a group or dispersing to join or form new groups. The decision to delay dispersal by certain new-borns is especially important for cooperatively living birds. Birds stay in their natal nest until they are physically ready to risk dispersal but often choose to stay regardless. This is linked to several benefits (Lewis et al.; 1990 (18)). Firstly, better chances at survival and a better place in the queue for a breeding position. In siberian jays (Ekman et al.; 2002) (19) it was observed that fathers provided a safe haven for their young. This allowed them to gain an advantage for possible breeding positions in their natal zone. When replaced by a non-related father the subordinates of the old father tended to leave. In superb fairy wren (Lewis et al.; 1990) (20) dispersal by new subordinates and old ones was mostly limited by mate choice and available habitat of good quality. Leaving the natal territory seems to be beneficial when subordinates cannot inherit a breeding position and when habitats are available with the possibility of better breeding positions. In pied babblers (Nelson-Flower et al.; 2018) (21) subordinates that stayed had greater survival rates and could inherit breeding positions if they had a high social ranking. If subordinates did choose to disperse to other groups the motivations were different dependent on their sex. Females joined groups with high productivity while males joined groups with breeding vacancies and to avoid conflict within their old group. The benefit for unrelated subordinates to enter groups will mostly rely on which social status they can acquire in a new group (Nelson-Flower et al.; 2018) (21). 3 The benefits of staying or leaving aside, a proportion of subordinates also choose to help with raising the young of the breeding pair. Both related and unrelated subordinates have been observed helping the young. The main benefit for related helpers could be 'inclusive fitness'. According to the hypothesis of inclusive fitness the helpers raise their own fitness by increasing the breeding success of the related breeding pair. In fairy wrens different types of helping behaviour have been observed (L. Brouwer et al.; 2014) (22). Female helpers feed the young fixed amounts of food, while male helpers compensate when a decrease in the breeding pairs’ feeding rate occurs. In the first case, the females increase the breeding success of the current year while, in the second case, the males increase the survival chance of the breeding pair and thus their lifetime breeding success, because the breeders can put more resources into their own survival and into their future breeding attempts. In both cases related helpers can gain an indirect fitness benefit.