Mate-Guarding in Siberian Jay (Perisoreus Infaustus)
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Mate-guarding in Siberian jay (Perisoreus infaustus) Mate-guarding hos Lavskrikor (Perisoreus infaustus) Leo Ruth Faculty of Health, Science and Technology Biology Independent Research Project 15 hp Supervisor: Björn Arvidsson and Michael Griesser Examinor: Larry Greenberg 26/08-16 Serial number: 16:102 Abstract Mate-guarding is performed by many monogamous species, a method used by individuals to physically prevent competitors of the same sex from mating with their partners. This behaviour is most often displayed during the fertile period (i.e. when females can be fertilized). In this study I focused on the genetically and socially monogamous species, the Siberian jay (Perisoreus infaustus), in which I observed mate-guarding behaviour. The Siberian jays did change their behaviour and increased their aggression in the fertile period, a sign of mate-guarding. This result also suggests that even socially and genetically monogamous species do increase their aggression during the fertile period. This indicates that fidelity still requires an investment in mate-guarding to limit extra-pair mating opportunities. Mate- guarding should then be possible to find in species where there is at least a theoretical opportunity for extra-pair matings. Sammanfattning Mate-guarding är en metod använd utav många monogama arter, metoden används för att fysiskt hålla konkurrenter utav samma kön borta ifrån sin partner för att försäkra sin egen parning. Denna metod beskådas oftast under tiden honan är fertil. I denna studie fokuserade jag på den genetiska och sociala monogama arten Lavskrika(Perisoreus infaustus) där jag observerade mate-guarding beteende. Lavskrikans beteende förändrades mellan perioden då honan icke var receptiv och hon var fertil, aggressionen ökade för båda könen under den fertila perioden vilket är ett tecken utav mate-guarding beteende. Detta resultat föreslår att socialt och genetiskt monogama arter ökar sin aggression under tiden honan är fertil, det föreslår även att trofasthet fortfarande kräver hög investering utav mate-guarding för att minska riskerna utav otrohet. Mate-guarding beteende borde då finnas hos arter med en teoretisk möjlighet för otrohet. Introduction In theory, monogamy is quite straight forward and only requires having one mate, in real life, however, it is not that simple. Infidelity occurs in many species across all taxa, including socially monogamous ones, a phenomena called extra pair copulation (EPC) (Westner et al 1990). Both of the sexes in a monogamous pair can benefit from mating with individuals outside the pair. There are several benefits associated with EPC for females, for example; an extra-pair male may have good genes, or it could be an insurance in case of infertility from their partner (Griffith et al 2002). Males have the possibility to mate with an large number of females and usually takes the opportunity to mate if this option is available. In species with bi-parental care, females have a higher initial investment than males, but males may contribute with helping to raise offspring (Trivers 1972). In pairs where female successfully mate with an extra-pair male, males pay a fitness costs by raising offspring that are not their own. To reduce the occurrence of female EPC, males use mate-guarding. Mate guarding occurs in species with bi-parental care, predominantly during the fertile period where males keep in close vicinity of the female and aggressively chase other males away from this territory (Beecher & Beecher 1979 ; Birkhead. 1979). Similarly, female increase their aggression towards other females, to reduce the opportunities for her mate to engage in EPC (Birkhead. 1979). Most monogamous species use some form of biparental care where both sexes invest in the rearing of offspring (Brotherton & Komers 2003). Here, I investigated mate guarding in a socially and genetically monogamous bird species, the Siberian Jay (Perisoreus infaustus). This species is year-round territorial and live in small flocks with only one mating pair and some retained offspring but usually also includes immigrants in the flock (Blomgren 1971; Ekman et al 1994). Retained offspring may stay up to five years within the flock, dominant siblings displace subordinate brood mates out of the flock. Retained offspring disperse if the opportunity for breeding arrive, but they do not act as helpers. Incubating female are fed by males (Ekman et al 1994). Even if the retained young does not act as helpers they are more tolerated by the breeders then immigrants (Ekman et al 1994). The breeders also have a difference tolerance level in immigrants, aggression is for instance higher against same sex immigrants (Ekman & Sklepkochy 1994). Siberian jays are single brooded, monogamous and has high male assistance in offspring rearing, three criteria suggesting a higher probability of mate-guarding (Barsh 1976). The Siberian jays are hence a prime candidate to study mate-guarding. Since guarding increases in the fertile period we would expect a change in the behaviour when this period begins. Materials & methods Study site The data was collected in a natural population of Siberian Jays in an area located near Arvidsjaur in Northern Sweden (65°40 N, 19°0 E). This population has been studied thoroughly from 1989 onwards (Griesser et al 2013; Griesser et al 2015). The study site is split into two areas; the first area is located in the nature reserve Reivo where the habitat is unmanaged boreal forest (Fig. 1), the second area is on the borders of Reivo, here the habitat is mixed with managed and unmanaged boreal forests (Fig. 1). All individuals within the area have been ringed continuously since 1989 with four rings for individual recognition (Griesser et al. 2012). Figure 1. Map over the study area. The smaller map in the upper left corner displays where in Sweden the area is located. (1) Is the unmanaged area in the nature reserve Reivo. (2) The area with mixed habitat outside Reivo. Data collection Data used in this study was collected before and during the fertile period of female Jays, in March 2016, by observing social interactions in flocks on standardized feeders during 15 - minutes. The feeders are made up of one main lateral stick with two horizontal sticks attached to allow up to five individuals to forage at the same time and are baited two pieces of pig fat. The feeders were placed close to the forest edge or a tree and birds were attracted to the feeders by whistling. I assessed the aggressive interactions between flock members by using four criteria (Ekman et al 1994): (1) feeding together (individuals landing on the feeder and feeding without any sign of aggression), (2) displacement (an individual is forced away from the feeder by another member of the flock), (3) patiently waiting (individuals sitting close by waiting for a feeding opportunity) or (4) chasing (an individual is chased away from the feeder and the area followed by the aggressor). Observations were recorded on a scoring sheet, noting the social interactions between all individuals,. The observer was placed 10 – 20m away from the feeder to not disturb the Jays and was always equipped with a binocular. Statistical analysis Data were collected in 35 groups. I used linear mixed models in the package lme4 (REF) in R version 3.2.5 (R development team 2016) and used model selection to find the models with the highest explanatory degree. Model selection was run using the package MuMIn (Barton 2016), parameters where obtained after averaging over the set of best fitting models (ΔAICc < 2). Given the rarity of chasing events, I only assess the factors that influenced the number of displacements, using a Poisson distribution. The models always included two random effects: identification (ID) and territory to avoid repeated samples of the same individual and on the same territory. The independent fixed terms used in the statistical analyses are as follows: rank (breeder, non-breeder), sex of recipient (female, male, unknown), rank of recipient (breeder, non-breeder), time in group (in months), habitat (see below), group size (see below), season (before and during the fertile period) kinship between aggressor and recipient (kin, non-kin, unknown) and an interaction between group size and kinship (Caraco 1979). The habitat was defined by the proportion of unmanaged forest in a territory. Flocks living in territories with a higher quality habitat has higher survival, this could change social relationships in the group (Griesser et al 2006). Group size could inflict aggressive behaviour because in larger groups each individual has to fend for themselves while foraging and when group sizes increase the food resources might decrease (Caraco 1979). Results Model selection revealed that several factors influenced the aggression between individuals. The number of displacements was best explained by the rank of recipient, period (non-fertile vs fertile period of the female), rank, the kin relationship between actor and the recipient, sex of the recipient and time the recipient had stayed in the flock (Tab. 1). Male breeder aggression changed distinctly between the non-fertile and the fertile-period. During the non-fertile period aggression was focused on females displacing on average 0,5 females per 15-minute and non against males (Fig. 2). In the fertile period however males displaced only other males averaging 0,67 displacements per 15 minutes (Fig. 2). Females also increased their aggression towards the same sex in the fertile period (Fig. 2). Male breeder aggression towards immigrant were higher than kin in both periods, in the non-fertile period the general aggression decreased (Fig. 3). Female breeders displayed higher aggression towards kin in the non-fertile period and against non-kin the fertile period (Fig. 3). Time spent in the flock did affect aggression , most aggression was against members that had less than a year in the flock. FB on female FB on male Female breeder kin Male breeder kin Female breeder non-kin Male breeder non-kin 0,8 0,7 0,7 0,6 0,6 0,5 0,5 0,4 0,4 0,3 0,3 0,2 0,2 AVERAGE DISPLACEMENT 0,1 AVERAGE DISPLACEMENT 0,1 0 0 Non-fertile period Fertile period Non-fertile period Fertile period Figure 4.