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Why Do Aptenodytes Penguins Have High Divorce Rates?

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Why Do Aptenodytes Penguins Have High Divorce Rates? The Auk 116(2):504-512, 1999 WHY DO APTENODYTES PENGUINS HAVE HIGH DIVORCE RATES? JOi•LBRIED, 1 FRI2DI2RICJIGUET, AND PIERREJOUVENTIN CentreNational de la RechercheScientifique, Centre d'Etudes Biologiques de Chizd, 79360 Beauvoir sur Niort, France ABSTRACT.--Inlong-lived birds, monogamy is thoughtto enablebreeders to retainthe samemate from year to year,but exceptionsoccur. For example, King Penguins(Aptenodytes Downloaded from https://academic.oup.com/auk/article/116/2/504/5168774 by guest on 29 September 2021 patagonicus)and Emperor Penguins (A. forsteri) have much lower mate fidelity than do small- er speciesof penguins,despite their high rates of survival.Recently, Olsson (1998) suggested that divorcein King Penguinscould be adaptive.Although Olsson was the first to propose an adaptivefunction for divorcein this species,he wasunable to assessthe relationships amongindividual quality, date of arrival,mate choice, and breeding success. Accordingly, we studiedKing Penguins and Emperor Penguins to furtherexamine the determinantsand consequencesof divorce. Mate retentionwas not affectedby breedingperformance in the previousyear or by experience,and neithermate retention nor divorcehad significantcon- sequenceson chick production the following year. King Penguins were more likely to divorce asarrival asynchrony of previouspartners increased. In EmperorPenguins, this tendency to divorceoccurred only when females returned earlier than their previousmates. Most Em- perorPenguin pairs formed within 24 hoursafter the arrivalof males,which were outnum- beredby females.King Penguins that had nestedsuccessfully in theirnext to lastattempt werefavored as mates for onesthat had been unsuccessful, and individuals of bothspecies probablychose the best mates available. Most of ourresults for KingPenguins and Emperor Penguinssupported Olsson's (1998) conclusions in thatdivorce appears to beadaptive. Mate retentionin the absenceof a true nestingsite (neither species builds a nest)would be mal- adaptivefor thesespecies, which face strong time constraintsfor breeding.Therefore, di- vorceoccurs because costs of materetention are high.Aptenodytes penguins appear to have adoptedan optimaldivorce strategy in orderto adaptto their longbreeding cycle in a de- mandingenvironment. Received 27 June1998, accepted 30 October1998. SOCIALMONOGAMY is the predominantmat- theirchick on top of theirfeet. Only twospecies ing systemamong birds (Lack1968), and many occurin thisgenus, the King Penguin(Apteno- long-livedspecies have a longreproductive life dytespatagonicus) and the EmperorPenguin (A. span(Stearns 1992). High survivalmay enable forsteri).Being the largestspecies of penguins, monogamousspecies to retain the samemate theyhave a longbreeding cycle, part of which between successivebreeding attempts more takesplace during the australwinter (Stone- frequentlythan do specieswith highermortal- house1953, 1960). Consequently, they must op- ity (Rowley 1983). In addition, breeding suc- timize their reproductiveoutput during severe cessoften is enhancedby mate retention(e.g. conditions that involve a decline in food avail- Rowley 1983, Ens et al. 1996). ability and a four-monthwinter fastfor chicks High survival rates and long reproductive of the subantarcticKing Penguin(Stonehouse life spans are common in seabirds (Ricklefs 1960, Weimerskirchet al. 1992), and low tem- 1990),but mate fidelity varies amonggroups. peraturesand ice for the antarcticEmperor For example,albatrosses are very faithful to Penguin(Stonehouse 1953). In addition,both their partners (Rowley 1983, Warham 1990), speciesface strongtime constraintsfor breed- whereas frigatebirdsregularly divorce (see ing. Thus,laying after the endof Januaryat Iles Black 1996) between successivebreeding at- Crozetor after 1 Januaryon SouthGeorgia Is- tempts(Nelson 1976). This variabilityalso oc- land invariably results in breeding failure in curswithin the samefamily, as is the casefor King Penguins(Weimerskirch et al. 1992,Jou- penguins(Williams 1996). The genusApteno- ventinand Lagarde1995, Olsson 1996) because dytesis unique in that adults do not build a late-hatched chicks have not been able to store nest,incubating their singleegg andbrooding sufficient fat reserves before the austral winter (Stonehouse1960, Van Heezik et al. 1994, Jou- E-mail: [email protected] ventinand Lagarde1995). In themore synchro- 5O4 April 1999] Divorcein Penguins 505 nouslybreeding Emperor Penguin (Stonehouse posedthe "expensivefat-storing" hypothesis, 1953, 1960; Isenmann 1971; Weimerskirch et al. suggestingthat individualswould facea trade- 1992),only the earliest-hatchedchicks (i.e. be- off between the costs of divorce and the costs fore 10 August;Isenmann 1971) are able to de- of building up reserveswhile waiting for their part to seawhen the ice breaksin December, previouspartners so that materetention would havingcompleted their moltbut attainingonly occur.Although Olsson believed that divorce 50% of adult body mass(Isenmann 1971). was adaptivein King Penguins,his studystill Aptenodytespenguins are long lived, with a neededto be supplementedin somerespects. mean annual survival rate of 0.91 to 0.95 (Jou- In particular,he did not evaluateaccurately the Downloaded from https://academic.oup.com/auk/article/116/2/504/5168774 by guest on 29 September 2021 ventin and Weimerskirch 1991, Weimerskirchet eventual benefits (in terms of offspring pro- al. 1992).Yet, they show low mate fidelity be- duction) of extensivedivorce, and he did not tween years (15% in Emperor Penguin [Isen- analyze the relationshipsamong individual mann 1971,Jouventin 1971]; 19 to 29% in King quality,date of arrival,mate choice, and breed- Penguin[Barrat 1976, Olsson 1998]) compared ing success.Despite a lack of data on Emperor with otherpenguins (œ = 84%;Williams 1996). Penguins,Olsson (1998) also suspected that his For divorceto be adaptive,individuals that di- hypotheseswere relevantfor this species;like vorce should obtain some benefit; i.e. it would the King Penguin,Emperor Penguins have be "optimal" for an individual to divorce if its part-time pair bonds, and fat reservesplay a reproductivesuccess from breeding with a pre- major role during courtshipand incubation vious partner in the next year is lower than its (Pr•vost 1961, Isenmann 1971). Here we pro- averagefuture success(i.e. the averagefor all vide resultsfrom our long-termstudy of King future breedingattempts until the individual Penguins,supplemented with data from Em- dies; McNamara and Forslund 1996). peror Penguins,that enabledus to further as- Severalhypotheses have been put forward to sessthe adaptivevalue of divorcein thesespe- explain divorce in birds (e.g. Ens et al. 1993, cies. Choudhury 1995), including penguins (Wil- liams 1996).The "incompatibility"hypothesis STUDY AREA AND METHODS predictsthat pairs having low breedingsuccess are morelikely to divorce,and that eachof the From September1990 to March 1994,we conduct- ed a demographicstudy of King Penguinsin a small previouspartners should be more successful colony (750 pairs) on PossessionIsland, Crozet Ar- with its new mate (Coulson 1966, Rowley chipelago(46ø25'S, 51ø45'E). We attachedflipper 1983).In specieswith part-timepair bonds(i.e. bandsto 989 birds and checkedfor their presenceev- during part of the year only), the main reason ery otherday. To assessthe influenceof previousre- for pairs to split might be asynchronousre- productive successon divorce,we consideredpairs turns by previousmates (Coulson1972, Boe- to be successfulif at least one mate was seenfeeding kelheide and Ainley 1989, Davis and Speirs a chickafter September,or wasmolting after Novem- 1990). In his study of King Penguins,Barrat ber If the egg did not hatch,or if oneof the parents (1976)also hypothesized that divorceis caused was molting before Decemberor displaying in early December,the pair wasassumed to havefailed (Jou- by the asynchronousreturn of previouspart- ventin and Lagarde 1995). At best, King Penguins ners to the colony.However, his assumption can breed successfullyevery other year (Weimer- wasbased on a smallsample (three pairs), and skirchet al. 1992, Olsson1996). Therefore, individual his study was conducted during only one quality at the onsetof breedingcycle n was defined breeding season. Recently, Olsson (1998) from the reproductiveperformance at breedingcycle showed that divorce rates in King Penguins n - 2 (Olsson 1996). from South Georgia Island increasedas the Data on Emperor Penguinswere from studiescon- previousmates returned more asynchronously ducted in 1968 by P. Isenmann(pers. comm.)and in to the colony,confirming Barrat's (1976) hy- 1969by P. Jouventin.Field work was carried out at a pothesis.He alsoshowed that King Penguins colony(12,500 pairs) on PointeG•ologie, Terre Ad•- lie, Antarctica (66ø40'S,140øE), where flipper tags cameashore at the onsetof the breedingcycle were attached to 855 individuals between 1965 and with only half of their body reserves.Because 1967. From arrival until laying, daily observations they fast when on land, their fat storeswere wereperformed on bandedbirds that wereknown to closeto exhaustionat the end of their first stay have bred in 1967 and 1968. Thus, dates of arrival at in the colony.Accordingly, Olsson (1998) pro- the colony(in 1968 and 1969) and pair formation (in 506 BRIED,JIGUET, AND JOUVENTIN [Auk, Vol. 116 lO 8 8 28 1969) were known accuratelyfor each individual. lOO Pairsin which one matewas seenfeeding a chickaf- ter 15 November were considered successful. 80 Forboth species,observations
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