Offspring sex, hatching order, and brood reduction: different strategies lead to different sex ratios?

BARRIONUEVO, MELINA; MORGENTHALER, ANNICK; MARCHISIO, NAHUEL; MILLONES, ANA; FRERE, ESTEBAN

Conferencia; 11th International Penguin Conference; 2023

Rockhoppers (RP, Eudyptes chrysocome chrysocome) areobligate brood reducers with large egg-size dimorphism within a clutch (20%)and highly biased survival towards first hatchlings belonging to the largereggs. Magellanic penguins (MP, Spheniscus magellanicus) are facultativereducers with a more similar egg-size dimorphism . Taking into account thehatching order within a brood, we molecularly sexed MP hatchlings at IslaQuiroga (Sta.  Cruz, Argentina) from 2010to 2012 (n=117 broods) and RP hatchlings at Isla Pingüino (Sta. Cruz,Argentina) in 2015, 2018 and 2022 (n=68 broods). We found that 1.3% of the RPcouples raised both nestlings, being first hatchlings the ones that survivedmost. In MP 46% of the couples raised both nestlings, with first hatchlingssurviving 10% more than second hatchlings. We found, for both species, anunbiased overall hatchling sex ratio (MP = 0.53, RP = 0.49). But, while the sexratio of first hatchlings was 0.48 (X2=0.07, p=0.79) in RP and 0.56 (X2=1.75,p= 0.18) in MP, second hatchlings' sex ratio was 0.36 (X2=2.95, p=0.08) and0.67 (X2=12.9, p<0.001), respectively. We found an unbiased sex ratio offirst hatchlings in both species, while second hatchlings are biased towardsfemales in RP and towards males in MP. This result is probably related to thereduction strategy and survival probability of second hatchlings which isdifferent for each species; together with a possible lower cost to raisefemales (which are smaller) for RP, leading to a bias towards females in secondhatchlings in case conditions are optimal.Libro de resúmenes adjunto p.100