Nestling immunocompetence and testosterone covary with brood size in a songbird

Abstract

The social and ecological conditions that individuals experience during early development have marked effects on their developmental trajectory. In songbirds, brood size is a key environmental factor affecting development, and experimental increases in brood size have been shown to have negative effects on growth, condition and fitness. Possible causes of decreased growth in chicks from enlarged broods are nutritional stress, crowding and increased social competition, i.e. environmental factors known to affect adult steroid levels (especially of testosterone and corticosteroids) in mammals and birds. Little, however, is known about environmental effects on steroid synthesis in nestlings. We addressed this question by following the development of zebra finch (Taeniopygia guttata) chicks that were cross-fostered and raised in different brood sizes. In line with previous findings, nestling growth and cell-mediated immunocompetence were negatively affected by brood size. Moreover, nestling testosterone levels covaried with treatment: plasma testosterone increased with experimental brood size. This result provides experimental evidence that levels of circulating testosterone in nestlings can be influenced by their physiological response to environmental conditions.

Effects of early developmental conditions on innate immunity are only evident under favourable adult conditions in zebra finches

“…Inverse effects of brood size on lysis titres of low-cost foragers are in agreement with the negative relationships of brood size with PHA response (Alonso-Alvarez et al 2006;Naguib et al 2004) earlier observed in zebra finch nestlings (but not in adults, see Tschirren et al 2009). Several hypotheses have been invoked to explain these patterns.…”

“…Experimental brood sizes fell within the range normally observed in wild (Zann 1996) and laboratory (Naguib et al 2004) populations. We manipulated the brood size towards the extremes of the normal range in order to trigger sufficient variation in nestling environment between the two experimental groups.…”

“…Fargallo et al 2002;Hõrak et al 1999;Saino et al 1997). Second, lower immune responses in large broods might be caused by increased sibling competition resulting in an increase in testosterone (Naguib et al 2004) and a suppressed immune function (Boonekamp et al 2008;Fargallo et al 2007;Folstad and Karter 1992; but see Roberts et al 2004). Evidence for immunosuppressive effects of testosterone, however, is still lacking for zebra finches.…”

“…Klasing 2004;Lochmiller and Deerenberg 2000;Norris and Evans 2000;Sheldon and Verhulst 1996), immune function can be expected to be suppressed in large broods because of higher nutritional stress and increased sibling competition (e.g. Naguib et al 2004;Saino et al 1997). Yet, poor immunity can come at a price as the immune system is the main physiological mechanism to resist disease, and hence, to safeguard future survival and reproduction (reviewed in Møller and Saino 2004).…”

“…At a short term, being raised in a large brood reduces body sizes, growth rates, phytohaemagglutinin (PHA) inflammatory responses and survival of nestlings (e.g. Alonso- Alvarez et al 2006;de Kogel 1997b;Naguib et al 2004;Tschirren et al 2009), while at a longer term, it affects adult body size, metabolic rates, inflammatory responses and reproductive success (e.g. Alonso-Alvarez et al 2006;Tschirren et al 2009;Verhulst et al 2006).…”