Growth Prior to Thermogenesis for a Quick Fledging of Adélie Penguin Chicks (Pygoscelis adeliae)

Abstract

The evolutionary trade-off between tissue growth and mature function restricts the post natal development of polar birds. The present study uses an original integrative approach as it includes gene expression, plus biochemical and physiological analysis to investigate how Adélie penguin chicks achieve a rapid growth despite the energetic constraints linked to the cold and the very short breeding season in Antarctica. In pectoralis muscle, the main thermogenic tissue in birds, our data show that the transition from ectothermy to endothermy on Day 15 post- hatching is associated with substantial and coordinated changes in the transcription of key genes. While the early activation of genes controlling cell growth and differentiation (avGHR, avIGF-1R, T3Rβ) is rapidly down-regulated after hatching, the global increase in the relative expression of genes involved in thermoregulation (avUCP, avANT, avLPL) and transcriptional regulation (avPGC1α, avT3Rβ) underlie the muscular acquisition of oxidative metabolism. Adélie chicks only become real endotherms at 15 days of age with the development of an oxidative muscle phenotype and the ability to shiver efficiently. The persistent muscular expression of IGF-1 throughout growth probably acts as a local mediator to adjust muscle size and its oxidative capacity to anticipate the new physiological demands of future Dives in cold water. The up-regulation of T3Rβ mRNA levels suggests that circulating T3 may play an important role in the late maturation of skeletal muscle by reinforcing, at least in part, the paracrine action of IGF-1. From day 30, the metabolic shift from mixed substrate to lipid metabolism, with the markedly increased mRNA levels of muscle avLPL, avANT and avUCP, suggests the late development of a fatty acid-enhanced muscle non-shivering thermogenesis mechanism. This molecular control is the key to this finely-tuned strategy by which the Adélie penguin chick successfully heads for the sea on schedule.

Sex-Based Differences in Adélie Penguin (Pygoscelis adeliae) Chick Growth Rates and Diet

“…A rapid increase in both number and size of both fat and muscle cells is largely responsible for mass growth in Adélie Penguin chicks, with fat storage becoming more important and rapid in the second half of chick rearing [ 52 ]. The ability to both quickly deposit and later mobilize these fat reserves could provide selective advantage to individuals coping with such a short breeding season [ 52 , 53 ]. Considering our results in this context, it also may be that males need to have a greater capacity for increasing the mass of adipose tissue, which could benefit them later in life during several stages of reproductive effort when long-term fasting is required (more so in males than females [ 24 ]).…”

Feet first: Adaptive growth in magellanic penguin chicks

“…At Punta Tombo, young chicks die from hypothermia following precipitation events (9-23 days of age) and from hyperthermia later in the breeding season, when temperatures at the colony can reach over 37°C (Boersma & Rebstock, 2014, Holt and Boersma, Personal communication). Thermoregulation is costly (e.g., up to 31% of metabolic rate in Adelie penguin chicks: Chappell et al, 1990;up to 35% of metabolic rate in Eurasian blackcap Sylvia atricapilla: Węgrzyn, 2013) and rapid growth may reduce the costs of thermoregulation by decreasing surface-to-volume ratio and by creating muscles that can produce heat through shivering (Cheng & Martin, 2012;Dégletagne et al, 2013;Pereyra & Morton, 2001;Stahel et al, 1987;Starck & Ricklefs, 1998). In older chicks, having large flippers, feet, and bills aids in heat loss (Starck & Ricklefs, 1998;Symonds & Tattersall, 2010;Tattersall, 2016;Whittow & Rahn, 1984).…”

The 12-day thermoregulatory metamorphosis of Red-winged Blackbirds (Agelaius phoeniceus)

“…Although Red-winged Blackbird expression of avUCP was also significantly elevated after hatching, the expression levels peaked at 3 dph and remained constant through 7 dph, rather than progressively changing as in the Adélie penguin where relative expression remained very low and stable until 15 dph then increased progressively to reach peak levels by 60 dph (Dégletagne et al 2013). Walter and Seebacher (2009) observed significant increase in avUCP mRNA levels after hatching in chicken skeletal muscle and postulated that rather than a thermogenic role, avUCP is involved in locomotory activity.…”