Contributions of marginal offspring to reproductive success of Nazca booby (Sula granti) parents: tests of multiple hypotheses

Abstract

While obligate siblicide is a phylogenetically widespread behavior, known from plants, insects, birds, and other taxa, with important implications for life history evolution, comprehensive evaluations of its costs and benefits to parents are rare. We used 12 years of breeding and band resight data to evaluate the importance of several potential benefits that marginal offspring (the usual victims of obligate siblicide) could provide to parent Nazca boobies (Sula granti), a seabird. We found no evidence for the resource-tracking hypothesis: 99.95% of two-chick broods were reduced to one chick before fledging, and the single exceptional brood probably lost one chick between fledging and independence. Behavioral observations indicated that siblicidal aggression caused most mortality of marginal chicks, and at least contributed to the remainder. We also found no evidence that marginal offspring provide a food resource for other family members. Marginal chicks benefit parents via adoption into other families, and possibly also in the context of progeny choice, but these benefits are minor compared to the insurance that marginal chicks provide against early failure of core (first-hatched) offspring. Further evaluation of the Insurance Egg Hypothesis showed that marginal and core offspring are functionally equivalent in the absence of sibling interactions, and that core offspring incur no detectable costs from behaving siblicidally. Nazca boobies are truly obligate brood reducers, with parents receiving principally insurance benefits from marginal offspring, but many birds and other taxa exhibiting persistent, unconditional sibling aggression do not exhibit universal brood reduction. Insurance is only one of several potential benefits that marginal offspring can confer on parents, and a multi-hypothesis approach to decompose the different types of benefits is required to understand the evolution of clutch size in other obligately siblicidal species.

Offspring growth and parental care in sexually dimorphic Nazca boobies (Sula granti)

“…If similar SSD exists among offspring, then food requirements should differ for sons and daughters. Obligate siblicide in this species reduces brood size to one shortly after hatching (Anderson 1989;Humphries et al 2006), so brood size is effectively one for the purposes of studying parental effort, and sex-dependent outcomes of sibling competition (e.g., Arroyo 2002;Muller et al 2005) are not a complicating factor. Nazca boobies are among the longest-lived birds (Anderson and Apanius 2003).…”

“…Offspring were considered to have fledged successfully (reached independence at approximately age 160 days) if they survived long enough to attain juvenile plumage (at approximately 100 days), since mortality after attaining juvenile plumage is rare (Humphries et al 2006). The jth size measurement for the ith nestling Ageij…”

Mirror-Induced Behavior in the Magpie (Pica pica): Evidence of Self-Recognition

“…Within the Nazca booby population, more than half of the clutches contain two eggs in most years [29,30], and 68% of twoegg clutches produce two hatchlings [11], so more than 1/3 of all Nazca boobies experience the additional early androgen exposure associated with upregulation of T during obligate siblicide [3]. Given the evidence that neonatal birds with altricial development experience organizational effects from post-hatching exposure (the first 30% of the nestling period) to steroid hormones [31], the early exposure of highly altricial Nazca boobies to a high androgen level is expected to coincide with a sensitive period in development.…”

REPRODUCTION AND IMMUNE HOMEOSTASIS IN A LONG-LIVED SEABIRD, THE NAZCA BOOBY (Sula granti)

“…No more than one chick is raised per year, though two-hatchling broods are frequently produced and rapidly reduced to one by obligate siblicide (Anderson 1989a, Humphries et al 2006. Both males and females exhibit survival costs of reproduction after raising their single-chick broods (Townsend and Anderson 2007a).…”

“…Our empirical model, the Nazca Booby (Sula granti), is a tropical seabird with a low annual reproductive rate (Humphries et al 2006), a monogamous mating system , and biparental care (Anderson and Ricklefs 1992) that breeds on remote islands (Anderson 1993) like other pelagic seabirds, such as albatrosses (Diomedeidae) and penguins (Spheniscidae; Weimerskirch 2002). With high annual survival and slow actuarial senescence, the Nazca Booby provides a suitable model to investigate the reproductive-self-maintenance tradeoff in a long-lived species (Anderson and Apanius 2003).…”