Which characteristics can influence the ability to obtain dominance rank

rank in social animals?

Particular traits associated with dominance in males, such as body mass, can also provide females with competitive benefits. Body mass is generally correlated with strength and ability to win contests in a range of species among arachnids (Wells, 1988), fishes (Enquist et al., 1990) and reptiles (Zucker & Murray, 1996). A relationship between body mass and competitive ability has also been demonstrated in mammalian species such as African elephants (Loxodonta africana) (Archie et al., 2006), dwarf mongooses (Helogale parvula) (Creel & Waser, 1994) and meerkats (Suricata suricatta) (Hodge et al., 2008). In some species such as naked mole rats (Heterocephalus glaber) and meerkats, females rapidly gain weight following a successful take-over (O'Riain et al., 2000; Russell et al., 2004),

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which is likely to be the result of gaining access to high-quality food resources and changes in hormone levels (Clutton-Brock et al., 2006; Russell et al., 2004). Increased body mass and size results in reproductive benefits in terms of the quantity and quality of offspring, with larger females producing larger and heavier litters (Russell et al., 2004); therefore the ability to obtain dominance rank is important for lifetime fitness in some species. Given that body mass appears to be important in obtaining dominance rank and that an increase in body mass can occur as a consequence of maintaining the dominant position, it is important to untangle these effects when measuring important characteristics that may predict the ability to obtain and maintain dominance rank. For example, higher ranking subordinate female meerkats tend to be larger and heavier than other subordinate females and are therefore perhaps better equipped to outcompete rivals when an opportunity for breeding position arises. Once the dominant position is obtained, the female is then likely to further increase in body weight (Hodge et al., 2008; Russell et al., 2004). Therefore female body mass should be measured prior to, during and following competitive take-over situations.

In chimpanzees (Pan troglodytes) (Pusey et al., 1997) and mountain goats (Oreamnos americanus) (Cote, 2000), dominance rank is thought to be related to age. Female bighorn sheep (Ovis canadensis) develop age related social ranks, but this is only evident up to the age of six years, the point in which females reach asymptotic mass (Favre et al., 2008). After this period, ewes are more likely to be dominant if they are larger than their rivals (Favre et al., 2008). As age has a strong positive association with body mass (Creel, 2001), it may therefore be difficult to extrapolate the most important factor in predicting competitive ability, particularly in relatively short lived species with variable adult body mass.

Female competitive behaviour can be influenced by in-utero exposure to androgens (such as testosterone), the levels of which increase with the number of male siblings in polytocous species such as house mice (Mus musculus domesticus) (Palanza et al., 2005). Pre natal exposure to testosterone is thought to result in masculinised genitalia and positively influence adult body weight in spotted hyenas (Crocuta crocuta) (Frank, 1986; Neaves et al., 1980; Tilson & Hamilton, 1984), which may provide competitive benefits for offspring (Dloniak et al., 2006). However, high levels of androgens may also negatively affect ovarian activity and subsequent fertility (Glickman et al., 1998; Packer et al., 1995), so a trade off in androgen exposure and production may be necessary. High

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levels of testosterone may not initiate competitive behaviour per se, but sustains the behavioural response to competition, and can be released in anticipation of competition, resulting in an increased frequency of aggressive behaviours and alertness (Bergmuller et al., 2010; Gleason et al., 2009; Wingfield et al., 1987). Testosterone levels may also increase following competitive interaction for successful winners, which can positively influence future competitive events (Oyegbile & Marler, 2005). In an experiment with bank voles (Clethrionomys glareolus), females were more likely to attack intruders in their home cage if they were injected with testosterone prior to the interaction to stimulate the winner effect (Kapusta, 1998). Circulating androgens may therefore serve to protect vulnerable offspring, particularly in species were intruders can be infanticidal (Hrdy, 1979; Maestripieri & Alleva, 1991; Palanza et al., 1996).

Morphological and physiological traits are not the only potential influence on female competitive behaviour. Personality traits may also correlate with competitive behaviour and survival chances (Bergmüller, 2010; Biro & Stamps, 2008); for example ‘boldness’ increases with winning experience in rainbow trout (Onchorhyncus mykiss) (Frost et al., 2007). In a study using field observations of wild chacma baboons (Papio hamadryas ursinus), females were defined as ‘nice’, ‘aloof’ or ‘loners’, based on the performance of seven behaviours, which correlated with stress levels and sociality with other group members (Seyfarth et al., 2012). Personality may therefore affect the amount of aggression an individual receives, as ‘nice’ females in Seyfarth et al’s (2012) study, were more likely to signal benign intent by grunting to lower-ranking females, whereas ‘loner’ females were more likely to avoid other group members, grunting primarily to higher-ranking females. Grooming behaviour can be an signal of submission in some species and therefore negatively correlate with competitive behaviour; for example to gain access to dominant female’s offspring in meerkats (Kutsukake & Clutton-Brock, 2006), to reduce aggression in bonnet macaques (Macaca radiata) (Silk, 1982), or as a signal of subordinate status in laboratory strains of mice (Gioiosa et al., 2009). Non-aggressive behavioural traits such as boldness or appeasement behaviours may therefore be important signals of competitive potential, however it is important to consider if the personality trait has been the cause or effect of female competitiveness. Without measuring personality extensively before and after competitive situations it is difficult to untangle the importance of traits on obtaining and maintaining dominance rank.

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