- Top of page
- Literature Cited
Studies of avian species have shown that maternal effects mediated by the transfer of egg hormones can profoundly affect offspring phenotype and fitness. We previously demonstrated that the injection of a physiological amount of testosterone (T) in the eggs of ring-necked pheasants (Phasianus colchicus) disrupted the covariation among male morphological traits at sexual maturity and positively affected male mating success. Here, we investigate whether egg T exposure affected adult male circulating T levels at the onset of the breeding season (reflecting gonadal maturation), and the relationship between circulating T and male traits. Egg T exposure did not affect pre-mating plasma T. T levels were not associated with the expression of secondary sexual and non-sexual traits or socio-sexual behaviour (social rank, overall fighting ability and mating success). However, wattle brightness decreased with increasing circulating T in males hatched from T-eggs (T-males) but not among control males. In dyadic encounters during the peak mating period, control males with higher pre-mating T levels had higher chances of being dominant over other control males. However, higher pre-mating T levels did not predict success in male-male competition in encounters involving T-males. We suggest that the long-term effects of egg T on male phenotype do not originate from differential gonadal maturation according to egg T treatment. Rather, prenatal androgens may have priming effects on functioning of target tissues, translating into differential phenotypic effects according to androgen exposure during embryonic development.
- Top of page
- Literature Cited
Maternal effects, which occur whenever mothers influence the development and phenotype of the progeny via maternal care and/or pre- and post-zygotic transmission of resources, constitute a fundamental source of phenotypic variation (Mousseau & Fox 1998; Wolf et al. 1998; Badyaev & Uller 2009; Wolf & Wade 2009). Moreover, they represent an important source of transgenerational phenotypic plasticity, allowing information about the environment experienced by the mother to be translated into adaptive phenotypic variation of the offspring (Badyaev & Uller 2009; Wolf & Wade 2009; Ho & Burggren 2010). Avian species constitute a highly suitable model to investigate the fitness consequences of maternal effects, because their cleidoic eggs contain a cocktail of substances of maternal origin, including hormones, vitamins and immune factors (Royle et al. 2001; Grindstaff et al. 2006; Groothuis et al. 2006; Rubolini et al. 2011), which have been shown to have both short- and long-term consequences for progeny development, phenotypic traits and fitness (Gil 2003; Groothuis et al. 2005; Groothuis & Schwabl 2008).
Maternal hormones, mostly steroids, vary markedly among clutches and between individual eggs in a clutch, often in a predictable fashion (Schwabl 1993; Royle et al. 2001; Groothuis et al. 2005; Groothuis & Schwabl 2008; Love et al. 2008; Rubolini et al. 2011). Such variation has paved the way for the experimental investigation of hormone-mediated maternal effects on offspring phenotype and fitness (Groothuis et al. 2005). Experimental manipulations are generally performed by increasing the hormonal egg content within the physiological limits via egg hormone injections, thus mimicking increased maternal transfer, and subsequently measuring the effects on progeny morphological and behavioural traits (Groothuis et al. 2005; Groothuis & Schwabl 2008). During ontogeny, exposure to steroids exerts ‘pleiotropic’ effects, by affecting different developmental pathways involving, for example, sexual, muscular and skeletal development, as well as metabolism and the immune system (see comprehensive reviews in Groothuis et al. 2005; Groothuis & Schwabl 2008). Early effects due to hormonal priming of target tissues may be long-lasting and contribute to shaping the adult phenotype (Groothuis et al. 2005; Groothuis & Schwabl 2008). Indeed, long-term studies showed that egg androgens affected male morphological and secondary sexual traits (Strasser & Schwabl 2004; Eising et al. 2006; Rubolini et al. 2006; Riedstra et al. 2013), female reproduction (Rubolini et al. 2007), as well as socio-sexual behaviour (Strasser & Schwabl 2004; Eising et al. 2006; Partecke & Schwabl 2008; Bonisoli-Alquati et al. 2011a; Schweitzer et al. 2013) and personality traits (Tobler & Sandell 2007; Ruuskanen & Laaksonen 2010). These effects, however, were often inconsistent across studies and species (Müller et al. 2008; Müller & Eens 2009; Bonisoli-Alquati et al. 2011b; Ruuskanen et al. 2012).
The specific mechanisms of action underlying long-lasting phenotypic effects of prenatal androgens are not well understood and are likely to be the result of both ‘organizational’ and ‘activational’ effects (Carere & Balthazart 2007; Groothuis & Schwabl 2008; Navara & Mendonca 2008). Indeed, steroid exposure during critical periods of embryonic development can cause permanent, irreversible modifications of the phenotype that last through adulthood, a so-called ‘organizational’ effect (Arnold & Breedlove 1985; Adkins-Regan 2007). Later on, at puberty or after sexual maturity, endogenously released steroids can (reversibly) stimulate behavioural traits, including those whose neural substrates were permanently affected by early exposure to maternal hormones, a so-called ‘activational’ effect (Arnold & Breedlove 1985; Adkins-Regan 2007).
For example, egg androgens may affect the development of the hypothalamo/pituitary/gonadal (HPG) axis, and thus, patterns of gonadal maturation and hormone secretion during adult life. Variation in circulating androgens in relation to early androgen exposure could alter adult socio-sexual behaviour and the expression of androgen-dependent secondary sexual traits. However, evidence that early androgen exposure affects functioning of the HPG axis is limited to the post-fledging stage, that is, well before sexual maturation. Elevated egg androgens caused higher plasma androgen levels in starling (Sturnus unicolor) nestlings close to fledging (Müller et al. 2007), but reduced plasma T in feral fowl (Gallus g. domesticus) chicks (Pfannkuche et al. 2011). Moreover, quail (Coturnix japonica) chicks hatched from T-injected eggs showed a tendency to excrete more T metabolites than chicks hatched from control eggs (Daisley et al. 2005). Studies analysing circulating hormones in sexually mature adult birds did not disclose any effect of elevated egg androgens (Partecke & Schwabl 2008; Riedstra et al. 2013; Schweitzer et al. 2013). However, early androgen exposure may affect adult behaviour and phenotype without affecting circulating androgen levels. This could be the case because early effects of in ovo androgens may affect the sensitivity of target tissues later in life (Carere & Balthazart 2007; Groothuis & Schwabl 2008), implying that similar adult circulating androgen levels can have differential persistent effects on androgen-dependent morphological traits and/or transient effects on behaviour according to early androgen exposure.
An alternative to the traditional ‘organizational/activational’ view of the long-term effects of in ovo hormonal exposure is that long-lasting effects of prenatal androgens represent indirect, cascading effects of early activational effects (Carere & Balthazart 2007). For example, early social experiences (involving, e.g., sibling interactions) affected by prenatal androgen exposure can ‘prime’ adult behaviour, independently of direct hormonal actions during adulthood (reviewed in Carere & Balthazart 2007).
In this study, we focus on pre-mating circulating T levels of adult male ring-necked pheasants (Phasianus colchicus) that were prenatally exposed to high T. Natural egg T levels were increased within the physiological limits by injecting in the egg albumen a T solution to mimic increased transfer of maternal egg T, and the morphology and behaviour of males hatching from T-injected eggs (T-males hereafter) were compared with those of males originating from control eggs (control males hereafter) (Bonisoli-Alquati et al. 2011a,b; Baratti et al. 2012).
We have previously shown on the same set of males that egg T affected the expression of wattle red colouration (T-males having less red, more orange wattles than controls), disrupted the covariation among several secondary sexual traits and between wattle colour and cell-mediated immune response (Bonisoli-Alquati et al. 2011b). It also affected male mating success, with T-males obtaining more copulations than control males, specifically with control females (Bonisoli-Alquati et al. 2011a). However, egg T exposure did not affect male social rank or overall success in male-male competition (Bonisoli-Alquati et al. 2011a).
The first aim of this study was thus to investigate whether circulating T levels at the onset of the breeding season (during a period of social stability; see 'Methods') differed between T- and control males: higher T levels in T-males at this time may suggest a permanent, organizational effect of maternal T on the HPG axis and/or gonadal maturation, with T-males maturing earlier than controls. Secondly, we investigated whether pre-mating T levels covaried with the concomitant expression of both sexual and nonsexual traits according to egg T treatment. A covariation between T levels and expression of male morphological traits might be expected, because in pheasants, the expression of important male ornaments, such as the size of the wattle (but not spur length), is dependent on circulating T (Briganti et al. 1999). Moreover, T may be immunosuppressive (Duffy et al. 2000; Roberts et al. 2004), and we therefore expected a negative covariation between circulating T and intensity of the cell-mediated immune response, as assessed by the standard phytohaemagglutinin skin testing technique (Lochmiller et al. 1993; Saino et al. 1997). Thirdly, we investigated whether pre-mating T levels predicted the odds of a male being dominant in subsequent dyadic aggressive encounters occurring during peak mating activity, and whether this relationship was dependent on egg T treatment. Our general prediction was that males with higher pre-mating T levels were more likely dominant in subsequent male-male combats. Indeed, circulating androgens predict aggressiveness in intrasexual conflicts in Vertebrates (Adkins-Regan 2005), and T implants increased the frequency of aggressive interactions in pheasants, with T-implanted males achieving higher rank (Briganti et al. 1999). Although we previously failed to detect a relationship between success in intrasexual conflicts and egg T treatment in the same set of males (Bonisoli-Alquati et al. 2011a), we argue that this could be due to the indirect effects of interindividual variation in T levels in modulating aggressive behaviour. The investigation of the effects of plasma T levels on aggressiveness in relation to egg T treatment may help elucidate the mechanisms behind the previously documented effects of egg T on male socio-sexual behaviour.