Evidence for within-individual energy reallocation in cold-challenged, egg-producing birds

Abstract

SUMMARYRecent studies have shown that the metabolic cost of avian egg production involves a 16-27% increase in metabolic rate (MR) above non-reproductive basal or resting values (BMR and RMR, respectively). To determine how the metabolic cost of egg production interacted with the costs of other essential processes (such as cold acclimation and active heat production), we measured the MR of non-breeding and egg-producing zebra finches (Taeniopygia guttata) while (a) warm-acclimated (to 19-21°C) and measured within their thermoneutral zone (at 35°C), (b) cold-acclimated (to 7°C) and measured at thermoneutrality (at 35°C, i.e. not actively producing heat), and (c) cold-acclimated and measured below thermoneutrality (at 7°C) (i.e. during active heat production). The metabolic cost of egg production was small (24% above BMR) compared with the additive costs of cold acclimation and active heat production (224% above BMR). Exposure to low ambient temperatures was accompanied by an increase in seed consumption (by 72%) and a decrease in locomotor activity (by 72%) compared with warm-acclimated, nonbreeding values. By contrast, egg production in heat-producing females was associated with an 11% decrease in MR and a 22% decrease in seed consumption compared with non-breeding thermoregulating values. Our data suggest that while the increase in MR associated with egg production is small in relation to the birds' capacity to increase MR in response to other energetically demanding processes, the addition of egg production to these metabolically costly activities may be enough to necessitate the use of energy-saving strategies, such as internal energy reallocation, to cope with the additional energetic demands.

The Cost of Sex: Quantifying Energetic Investment in Gamete Production by Males and Females

“…For males, our estimate of the cost of ejaculate production (0.4% of BMR) is similar to that previously reported for Japanese macaques (0.8–6%; [23]). For females, our estimate of the energetic cost of egg production (∼300% of BMR) is in close agreement with the range of estimates (∼20% to ∼200%) that have been presented for various species of birds [24], [25], [26]. If we assume that eggs are produced throughout the year, rather than only during the breeding season, our estimate of the energetic cost of egg biomass production decreases to about 50% of BMR.…”

Phenotypic compromises in a long‐distance migrant during the transition from migration to reproduction in the High Arctic

“…Therefore, not only do females invest more energy than males in the days following arrival, but they also incur a higher cost for body reconstruction and preparation for breeding during cold springs. In passerines, egg production is associated with a 22–27% increase in resting metabolic rate (RMR) (Chappell, Bech & Buttemer 1999, Nilsson & Råberg 2001, Vézina & Williams 2002, 2005; Salvante, Vézina & Williams 2010; Vézina & Salvante 2010) and the size and activity of reproductive organs would underlie this cost of reproduction (Vézina & Williams 2005; Vézina & Salvante 2010). As male red knots arrive on the breeding ground with relatively well‐developed gonads while females must build their reproductive machinery (Morrison, Davidson & Piersma 2005; Vézina & Salvante 2010), it is reasonable to assume that growth of reproductive organs and follicle development in females create the sex effect reported here on BMR.…”

The Case of the Missing Mechanism: How Does Temperature Influence Seasonal Timing in Endotherms?

“…In birds, the first evidence for a direct relationship between ambient temperature and timing of reproduction has only recently been demonstrated (Box 1; [23],[24]). In mammals it has, to our knowledge, not yet been experimentally shown that temperature influences timing of breeding per se, but there is evidence that temperature modulates photoperiodic effects on the reproductive system [25]–[27].…”