1. Judicious management of energy can be invaluable for animal survival and reproductive success. Capital breeding mammals typically transfer energy to their young at extremely high rates while undergoing prolonged fasting, making lactation a tremendously energy demanding period. Effective management of the competing demands of the mother’s energy needs and those of her offspring is presumably fundamental to maximising lifetime reproductive success.

2. How does the mother maximise her chances of successfully rearing her pup, by ensuring that both her pup and herself have sufficient energy during this ‘energetic fast’? While energy management models were first discussed in the 1990s, application of this analytical technique is still very much in its infancy. Recent work suggests that a broad range of species exhibit ‘energy compensation’; during periods when they expend more energy on activity, their bodies partially compensate by reducing background (basal) metabolic rate as an adaptation to limit overall energy expenditure. However, the value of energy management models in understanding animal ecology is presently unclear.

3. We investigate whether energy management models provide insights into the breeding strategy of phocid seals. Not only do we expect lactating seals to display energy compensation because of their breeding strategy of high energy transfer while fasting, but we anticipate that mothers exhibiting a lack of energy compensation are less likely to rear offspring successfully.

4. On the Isle of May in Scotland, we collected heart rate data as a proxy for energy expenditure in 52 known individual grey seal (Halichoerus grypus) mothers, repeatedly across three years of breeding. We provide evidence that grey seal mothers typically exhibit energy compensation during lactation by down-regulating their background metabolic rate to limit daily energy expenditure during periods when other energy costs are relatively high. However, individuals that fail to energy compensate during the lactation period are more likely to end lactation earlier than expected.

5. Our study is the first to demonstrate the importance of energy compensation to an animal’s reproductive expenditure. Moreover, our multi-seasonal data indicate that environmental stressors may reduce the capacity of some individuals to follow the energy compensation strategy. 

The data included here represents a summary of 5,219 segments of collected heart rate data for 51 known female grey seals ('ID') during their lactation period on the Isle of May, Scotland using animal-borne heart rate monitors.  In short, these data summarize daily measures of minimum heart rate ('min_fH'; minimum value of mean heart rate, a proxy for background energy expenditures), mean heart rate ('mean_fH'; a proxy for daily energy expenditure), and auxiliary heart rate ('d.aux_fH', mean heart rate minus minimum heart rate; a proxy for auxiliary energy expenditure) in order to quantify energy management strategies during lactation across three separate breeding seasons ('Year') both within- and across-individuals. These heart rate metrics for each seal were determined by analysing multiple 15-min heart rate segments across a day ('n_seg') after processing and filtering of raw interbeat-interval data to remove any measurement artifacts.  All heart rate data were collected in real time across the breeding colony and logged on a portable base station computer. More information heart rate data collection and filtering can be found in the main text and supplement of JAE-2019-00935.