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The accumulation of circulating CO2 across a series of dives is thought to prolong surface recovery, thereby reducing foraging efficiency. This can be empirically tested by experimentally altering circulating gas tensions in diving seals and quantifying the effect on net rate of energy gain. In the present study, six grey seals (Halichoerus grypus) voluntarily dove in a simulated foraging setup, swimming from a breathing chamber to and from an underwater feeder. During surface intervals, seals were exposed to ambient, hypercapnic (high CO2), hypoxic (low O2) or hyperoxic (high O2) respiratory gas conditions. The effect of gas condition on net rate of energy gain, dive behaviour, respirometry-derived energy expenditure, post-dive circulating lactate concentration, and digestion (indicated by circulating triglyceride concentrations) was quantified. Net rate of energy gain significantly decreased under hypercapnia, likely owing to extended surface recovery durations, rather than underlying changes in energy expenditure or other post-dive metabolic processes. Extended surface durations reflected the slower rate of CO2 elimination relative to O2 uptake. Our findings show that the accumulation of CO2 is a significant limiting factor to net rate of energy gain in grey seals. Furthermore, we provide evidence of both digestion and anaerobic metabolism during diving, which contrasts with previously hypothesised optimal foraging strategies. Phocid seals are therefore not limited by digestive activity or the accumulation of lactate during short foraging bouts.<\/jats:p>","DOI":"10.1242\/jeb.251718","type":"journal-article","created":{"date-parts":[[2026,4,2]],"date-time":"2026-04-02T08:04:07Z","timestamp":1775117047000},"source":"Crossref","is-referenced-by-count":0,"title":["Accumulation of CO2 limits energy gain in freely diving grey seals"],"prefix":"10.1242","volume":"229","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-6084-9378","authenticated-orcid":false,"given":"Eva-Maria S.","family":"B\u00f8nnelycke","sequence":"first","affiliation":[{"name":"Scottish Oceans Institute, University of St Andrews 1 Sea Mammal Research Unit , , St Andrews, Fife KY16 8LB ,","place":["UK"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Joanna L.","family":"Kershaw","sequence":"additional","affiliation":[{"name":"Scottish Oceans Institute, University of St Andrews 1 Sea Mammal Research Unit , , St Andrews, Fife KY16 8LB ,","place":["UK"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9773-2755","authenticated-orcid":false,"given":"Gordon D.","family":"Hastie","sequence":"additional","affiliation":[{"name":"Scottish Oceans Institute, University of St Andrews 1 Sea Mammal Research Unit , , St Andrews, Fife KY16 8LB ,","place":["UK"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Carol","family":"Sparling","sequence":"additional","affiliation":[{"name":"Scottish Oceans Institute, University of St Andrews 1 Sea Mammal Research Unit , , St Andrews, Fife KY16 8LB ,","place":["UK"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Steve","family":"Balfour","sequence":"additional","affiliation":[{"name":"Scottish Oceans Institute, University of St Andrews 2 Sea Mammal Research Unit Instrumentation Group , , St Andrews, Fife KY16 8LB ,","place":["UK"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2451-1820","authenticated-orcid":false,"given":"Ryan","family":"Milne","sequence":"additional","affiliation":[{"name":"Scottish Oceans Institute, University of St Andrews 1 Sea Mammal Research Unit , , St Andrews, Fife KY16 8LB ,","place":["UK"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5378-7773","authenticated-orcid":false,"given":"Simon E. 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Chris","family":"McKnight","sequence":"additional","affiliation":[{"name":"Scottish Oceans Institute, University of St Andrews 1 Sea Mammal Research Unit , , St Andrews, Fife KY16 8LB ,","place":["UK"]}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"237","published-online":{"date-parts":[[2026,5,8]]},"reference":[{"key":"2026050806042924500_JEB251718C1","doi-asserted-by":"publisher","first-page":"1199","DOI":"10.3389\/fphys.2019.01199","article-title":"Natural tolerance to ischemia and hypoxemia in diving mammals: a review","volume":"10","author":"Allen","year":"2019","journal-title":"Front. 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