Trade-offs between age-related breeding improvement and survival senescence in highly polygynous elephant seals: Dominant males always do better


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Abstract

Life-history trade-off theory predicts that current reproduction can negatively affect survival and future reproduction. Few studies have assessed breeding costs for males of polygynous species compared to females, despite substantial variation in breeding success among individual males (e.g. subordinate cf. dominant breeders). Specifically, differentiating between the cost of attending breeding events, and the additional cost of successfully securing and mating females is lacking.We investigated whether trade-offs are present in the highly polygynous male southern elephant seal Mirounga leonina using 34 years of individual-level data. We compare age-specific survival, recruitment and future breeding success probabilities of pre-breeders (males yet to recruit) and breeders (subordinate and dominant social ranks) using multievent models.Pre-breeders and breeders of overlapping ages had similar survival probabilities, suggesting that there was no attendance cost for early recruits. In addition, the probability of recruiting as a dominant breeder never exceeded recruitment probability as a subordinate breeder of the same age. Therefore, older pre-breeders that delayed attendance costs generally did not improve their breeding success (probability of being dominant) at recruitment more than younger recruits. Rather, recruitment age may be a function of individual quality, with lower quality individuals requiring more time to socially mature.When comparing subordinate and dominant breeders, we found clear evidence for survival senescence, with subordinate breeders having a higher baseline mortality. In contrast, age-specific future breeding success (probability of being dominant at t + 1) increased with age, with dominant breeders maintaining higher subsequent breeding success than subordinate breeders.The opposite trends in survival and future breeding success for both subordinate and dominant breeders may indicate a lifetime, population-level trade-off. However, we found no evidence to suggest that being a dominant breeder consecutively (and having a higher accumulated breeding cost) accelerated the rate of senescence when compared to individuals that were previously subordinate.Thus, males experienced actuarial senescence regardless of social rank, with dominant (and possibly high quality) breeders showing a reduced trade-off between survival and future breeding success. We make several novel contributions to understanding polygynous male life histories and southern elephant seal demography.IsifinyezoUmbono ngokomlando wokuphuceka kwempilo (life-history trade-off theory) uqagela ukuthi ukuzalana kwanamuhla kungaba nomthelela ongemuhle ekuphileni kanye nasekuzaliseni okuzayo. Izifundo ezimbalwa sezike zabhekisisa izindleko zokuzalanisa kwezilwane zesilisa ezithatha isithembu uma ziqhathaniswa nezesifazane, noma kunokuhlukahlukana ekuzalisaneni okube yimpumelelo kuzo zesilisa (e.g. ezingaqavile cf. ezizalisa ngokubusa). Ikakhulukazi, ukuhlukanisa phakathi kwezindleko zokunakekela isikhathi sokuzalisana nezindleko ezengeziwe zokuthola ngokuyimpumelelo isifazane esizozalisa akukho.Sibhekisise ukuba ukuphuceka kwokuphila (trade-offs) kukhona yini esiliseni esiqavile esithatha isithembu kwizindlovu zezimvu zolwandle zaseningizimu (Mirounga leonina) sisebenzisa ubufakazi besilwane ngasinye obufikelela kwiminyaka engama-34. Siqhathanise ukuphila okuqondene nobudala, ukuheha kanye namathuba okuzalanisa okuyimpumelelo okuzayo nezizalanisa ngaphambi kwesikhathi (izilisa ezisazoheha) kanye nezizalanisi (eziqavile kanye nezingaqavile ngokwesikhundla senhlalo) kusetshenziswa izindlela ezilandelanayo (multievent models).Ezizalanisa ngaphambi kwesikhathi kanye nezizalanisa ngokujwayelekile ezineminyaka ecishe ifane zibe namathuba afanayo okuphila, lokhu kutshengisa ukuthi bekungekho ukunaka kwezindleko zabahehiwe ngaphambilini. Ukungeza kulokhu, ithuba lokuheha njengomzalisi oqavile alizange lidlule lelo lokuheha njengomzalisi ongaqavile oseminyakeni efanayo. Ngakho-ke, lezo ezindala ezizalise ngaphambi kwesikhathi futhi ezidonse izinyawo ngezindleko zokunakekela azibanga nokuthuthuka empumelelweni (yamathuba okuba ngoqavile) yokuzalanisa ngesikhathi sokuheha ukudlula lezo ezisencane. Kunalokho, iminyaka yokuheha ingahle ibe umongo wekhwalithi ngayinye, kuthi lezo ezisezingeni eliphansi ngokwekhwalithi zidinga isikhathi esiningi ukuze zikhule ngokomphakathi.Uma kuqhathaniswa lezo ezingaqavalile kulezo eziqavile ekuzalaniseni, sithola ubufakazi obusobala maqondana nokonakala okuhambisana neminyaka (senescence), kuthi lezo ezingaqavile ekuzalaniseni zikwizinga eliphezulu lokufa. Ngokuqhathanisa, impumelelo yokuzalisa okuzayo eqondene nobudala (ithuba lokuqava libe limi ku t + 1) linyuke kanye neminyaka, eziqavile ekuzaliseni ziqhubeka ngamanani aphakeme ekuzalaniseni kunalezo ezingaqavile.Ukuthambekela okuphambene (opposite trends) ekuphileni kanye nasekuzalaniseni okuzayo okuyimpumelelo kwazo zombili eziqavile nezingaqavile ekuzalaniseni kungase kukhombise isikhathi sokuphila kanye nokuphuceka kwempilo (trade-off) okusezingeni leqembu. Noma kunjalo, asitholanga ubufakazi obutshengisa ukuthi uma uqavile ekuzalaniseni kuzokwenzeka ukuba (futhi uma unezindleko eziphezulu zokuzalanisa) kusheshise ukonakala okuhambisana neminyaka (senescence) lapho kuqhathaniswa khona nalezo ebezingaqavile ngaphambilini.Kanjalo ke, ezesilisa zibe nokonakala okuhambisana neminyaka (senescence) yazo kungakhathalekile ukuthi isikhundla senhlalo siyini, nalezo eziqavile (futhi mhlawumbe ezingeni eliphakeme lekhwalithi) ekuzalaniseni ziveza ukwehla ekuphucekeni kwempilo (trade-off) kanye nasekuzalaniseni okulandelayo okuyimpumelelo. Siletha okuningi okusha ekuqondeni izindlovu-zolwandle zesilisa eziseningizimu ezithatha isithembu ngempilo-mlando yazo.

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