Phenotypic assortative mating and within-pair sexual dimorphism and its influence on breeding success and offspring quality in Magellanic penguins

Abstract

We examined within-pair sexual dimorphism and phenotypic assortative mating in Magellanic penguins (Spheniscus magellanicus) breeding in six colonies located on the Patagonian coast (Argentina). All measured phenotypic traits except the number of pectoral spots differed between the sexes; bill depth and flipper length were the most and least dimorphic traits, respectively. We found assortative mating by bill depth and body mass. The similarity in body condition within pairs was close to significant. When we performed separate correlations for birds that bred successfully, i.e., raised one or two offsprings, and birds that did not attempt to breed or bred unsuccessfully, only the successful breeders showed assortative mating by body mass. In addition, we attempted to relate the body size of each member of the pair and the degree of sexual dimorphism within pairs to the breeding performance of individuals, which was measured as brood size, and body condition and immunocompetence of offspring. We found that pairs that were less dimorphic in flipper length raised more offspring. This effect was due to female flipper length per se and not to the relative difference in flipper length between members of the pair. Females with larger flippers had a higher probability of raising two chicks. No effects of body measurements or degree of sexual dimorphism on body condition or T-cell-mediated immune response of offspring were found. We discuss these results in the context of potential factors responsible for the maintenance of sexual size dimorphism in this species.Résumé : Nous avons étudié le dimorphisme sexuel et le choix d'un partenaire en fonction du phénotype chez des couples de Manchots de Magellan (Spheniscus magellanicus) dans six colonies reproductrices de la côte de la Patagonie (Argentine). Tous les caractères phénotypiques mesurés différaient chez le mâle et la femelle d'un même couple, à l'exception du nombre de taches pectorales; la hauteur du bec était le caractère le plus dimorphe, la longueur des ailerons, le caractère le moins dimorphe. Le choix d'un partenaire se faisait en fonction de la profondeur du bec et de la masse corporelle. La similarité de la condition physique chez le mâle et la femelle d'un couple était presque significative. En faisant des corrélations séparées chez les oiseaux à reproduction réussie et chez les oiseaux non reproducteurs ou les oiseaux qui avaient raté leur reproduction, seuls les oiseaux à reproduction réussie (i.e., qui avaient élevé un ou deux petits) avaient choisi leur partenaire en fonction de sa masse corporelle. Nous avons également tenté de relier la taille de chaque partenaire du couple et l'importance du dimorphisme sexuel au sein du couple avec la performance reproductrice individuelle en mesurant la taille des couvées, la condition physique et la compétence immunitaire des rejetons. Ce sont les couples les moins dimorphes quant à la longueur des ailerons qui ont élevé le plus de petits. Cet effet est attribuable à la longueur per se des ailerons de la femelle et n...

Foraging behaviour of Magellanic Penguins during the early chick‐rearing period at Isla de los Estados, Argentina

“…, Agnew & Kerry , Forero et al . , Walker & Boersma ). Despite this, birds displayed only slight sexual differences in foraging behaviours, albeit with large interannual variation.…”

“…We calculated the index of sexual dimorphism using the morphometric measurements from individuals equipped with GPS loggers following Forero et al . (). We also used regression analysis between diving and tracking parameters (trip duration, percentage of time diving and time in the bottom phase per hour of dive) and the sum of chick mass and its squared term (in case the relationship was not linear) at the time of the recorded foraging trip, to assess whether the foraging behaviour of parents changed with the age of chicks.…”

“…, Agnew & Kerry , Forero et al . ). In seabirds, dietary differences between dimorphic sexes (Clarke et al .…”

Natural selection on morphology varies among years and by sex in Magellanic Penguins (Spheniscus magellanicus)

“…The lack of selection observed in females, and no evidence for selection on flipper length independent of selection on body size, may help maintain phenotypic variation. Forero et al (2001) speculated that female Magellanic Penguins either had high heritability for flipper length or that females with larger flippers had higher survival, leading to low sexual dimorphism for flipper length. Since heritability of flipper length is lower in females than in males and the estimate is very small (depending on the test used), the low sexual dimorphism of flipper length appears to result from higher survival of females with longer flippers.…”

Using stable isotopes of nitrogen and carbon to study seabird ecology: applications in the Mediterranean seabird community

“…Age and sex are potential factors that could influence such differences. However, assessing trophic differences between adults and chicks using conventional dietsampling methods is virtually impossible, and also the lack of molecular methods for sexing birds have prevented these kinds of studies until recently (Ellegren, 1996;Forero et al, 2001Forero et al, , 2002aTella et al, 2001;Bertellotti et al, 2002). Recent advances in stable isotope analyses and molecular sexing have permitted the exploration of these sources of variability in several seabird species.…”