Tucked into almost every cell of an animal's body there are fierce little engines that turn food into energy. These little power plants, called mitochondria, fuel animal activities by generating ATP (adenosine triphosphate) that animals use to contract their muscles or send nerve impulses so they can get moving and do things. For any species, some individuals will be high performers – you know the type – they run the fastest or jump the highest, others are more ho-hum and average, and some drag their heels not doing well at all. Scientists already know that this range or variation in animal performance is partly caused by the number of mitochondrial engines animals have and how effective their mitochondria are at turning food into energy. We also know from lab studies that couch potatoes can actually increase their performance when faced with energetically demanding situations. Yet, it is unclear whether the number of mitochondria or how effective they are at pumping out energy can change over time within wild animals.

This is an important question because some stages in life require more energy than others; for example, reproduction is infamous for being energetically draining. Antoine Stier, Pierre Bize, Bin-Yan Hsu and Suvi Ruuskanen from universities in Finland and the UK set out to address this very question by studying pied flycatchers while they were caring for their offspring. The team collected female birds that were keeping their eggs warm in nest-boxes at a field site near Turku, Finland. They took a small blood sample containing cells with mitochondria for later analysis and returned the mother to continue incubating her eggs. The team then returned 10 days later to collect blood once again when the eggs had freshly hatched and the mothers were scrambling to care for their chicks, allowing them to test whether there was a change in mitochondria number or performance given the task at hand. The blood samples allowed the researchers to measure two things. First, they measured the number of mitochondria in each bird's blood cells by comparing the amount of genetic material that came from mitochondria to the amount of genetic material from the rest of the cells. Second, they measured how efficient or high performing a bird's mitochondria were by testing how well they produced ATP given the amount of fuel available.

Stier and colleagues found that the birds had fewer mitochondria while they were tending to their nestlings compared to when they were incubating the unhatched eggs. However, the mitochondria performed more efficiently (produced more ATP with less fuel available) while the birds were busy wrangling their nestlings. A female's ‘energy budget’ is likely much tighter when she has nestlings, because she has to feed both herself and a nest full of hungry, begging chicks.

The researchers also found the birds were consistent across contexts: those with the most and the highest performing mitochondria during incubation, also had the most and the highest performing mitochondria during the nestling phase. Overall, the team's findings suggest that wild animals may have some wiggle room to modify the amount and performance of their mitochondria across contexts. However, they are also stuck with the cards they were dealt and are consistently either ‘high’ or ‘low’ performers in all scenarios.


Plastic but repeatable: rapid adjustments of mitochondrial function and density during reproduction in a wild bird species
Biol. Lett.