DOI: 10.5061/dryad.8w9ghx3jn
title: Energetic limits: Defining the bounds and trade-offs of successful energy management in a capital breeder

description: 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. 
description: 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.