Hugh Drummond

Senescent birds redouble reproductive effort when ill: confirmation of the terminal investment hypothesis

This study reports an experimental confirmation of the terminal investment hypothesis, a longstanding theoretical idea that animals should increase their reproductive effort as they age and their prospects for survival and reproduction decline. Previous correlational and experimental attempts to test this hypothesis have yielded contradictory results. In the blue-footed booby, Sula nebouxii, a long-lived bird, after initial increase, male reproductive success declines progressively with age. Before laying, males of two age classes were challenged with lipopolysaccharide to elicit an immune response, which induced symptoms of declining survival prospects. Reproductive success of immune-challenged mature males fell, while that of immune-challenged old males showed a 98% increase. These results demonstrate that senescent males with poor reproductive prospects increase their effort when those prospects are threatened, whereas younger males with good reproductive prospects do not.

Parent-offspring cooperation in the blue-footed boody (Sula nebouxii): social roles in infanticial brood reduction

SummaryReproduction in the blue-footed boody was examined for evidence of parent-offspring conflict over infanticidal reduction of the brood. Parental investment was analysed by measuring clutch characteristics, and chick growth and mortality in four seasons. Direct observations were made of behavioral development to determine the social roles of family members. The modal clutch was two similar-sized eggs, which hatched 4.0 days apart due to a 5.1-day laying interval and immediate incubation of the first egg. On average, senior chicks grew faster than their sibs in years of good or poor growth (Fig. 2), maintaining the initial size disparity for at least 65 days (Fig. 1). Differential mortality of junior chicks was associated not with poor personal growth, but with a 20–25% weight deficiency of the senior sib, implying siblicidal brood reduction triggered at a weight threshold. Senior chicks established behavioral dominance through low-frequency pecking, but ordinarily did not eliminate their sibs nor substantially suppress their begging (Fig. 3), even when their own growth was 16% below potential. Parents fed dominant chicks more frequently than subordinates, but did not intervene in inter-sib aggression, even when it reached a siblicidal level. The weight and possibly the dominance relation between sibs was inverted in 12% of pairs. The theoretical prediction of conflict over elimination of the junior chick was not supported; rather, parents and senior chick cooperate, as if their fitness interests were congruent. Further, provisional tolerance of the junior chick by its underweight senior sib is consistent with “self-sacrifice” to increase the latters inclusive fitness.

Training siblings to be submissive losers: dominance between booby nestlings

Abstract Second-hatched chicks in two-chick broods of the blue-footed boody, Sula nebouxii , usually remain subordinate to their siblings throughout the nestling period, even those that outgrow the firsthatched chick. Initially, the superior size and maturity of the first-hatched chick enable it to dominate, but subsequently other factors could be involved. The idea that a chicks own social experience determines whether it behaves aggressively or submissively was tested by pairing unfamiliar chicks in the field. In brief trials, dominants (chicks with a history of dominance) tended to behave aggressively and subordinates (chicks with a history of subordination) tended to behave submissively, whatever the prior social experience of the partner. However, in chicks lacking prior social experience with a sibling, direction of dominance depended on relative size or age. When experience was pitted against size, eight of 12 experimental subordinates permanently paired with younger and smaller dominants initially behaved aggressively, although only one subordinate succeeded in becoming dominant. None of the nine control subordinates that were paired with older and larger dominants showed aggressive behaviour. In conclusion, the results of this study demonstrate that agonistic roles are determined by experience, although subordinates may become aggressive when they detect a personal advantage. When subordinates do become aggressive they are competitively inferior to smaller dominants.

Reversed sexual size dimorphism and parental care: minimal division of labour in the blue-footed booby

Reversed sexual size dimorphism in avian species (females larger than males) may be an adaptive consequence of different roles of males and females in parental care. We examined the alleged division of labour in two-chick broods of the blue-footed booby, using behavioural observation and frequent weighing of chicks. In the first week of parental care, males and females fed broods at similar frequencies and provided similar masses of food, but females brooded more than males when broods were 5-10 d old. Subsequently, females provided a greater mass of food and frequency of feeds than males until chicks were at least 35 d old (mass) and 60 d old (frequency), while attending the brood for just as much time as males until chicks were at least 35 d old. Males and females did not differ in the tendency to feed (frequency and mass) the first-hatched chick differentially. In nearly all components of parental care examined here, and in other studies, the females contribution is equal to or greater than the males. Only in clutch attendance and nest defence does the male contribute more than the female, but his small size seems unlikely to enhance performance in these activities. Overall, small size appears potentially to limit male provisioning of the brood, and is unlikely to be an adaptation for division oflabour in parental care. This result casts doubt on the relevance of the division-of-labour hypothesis for adult size dimorphism.

Sexual Size Dimorphism and Sibling Competition: Implications for Avian Sex Ratios

The blue-footed boobys (Sula nebouxii) first-hatched chick aggressively dominates and outgrows its sib and sometimes kills it when food is in short supply. Differential male mortality is expected since females grow to be 27% heavier than males and should have an advantage in sibling competition in mixed-sex broods. Surprisingly, when the male hatches first, even though his sister outgrows him, he usually sustains dominance over her throughout the nestling phase, and he grows and survives normally. When the sister hatches first, despite a large, persistent sibling size disparity the male grows and survives normally. These observations contradict the prevailing hypothesis that in sexually dimorphic birds that practice siblicidal brood reduction the smaller sex suffers differential mortality as the result of its disadvantage in sibling conflict in mixed-sex broods. Such species consequently are not expected to modify their primary sex ratio or associate sex with hatching order in response to biased mortality in brother-sister conflict.

Female-biased mortality in nestlings of a bird with size dimorphism

1. Explanations for sex-biased mortality include higher vulnerability of the heterogametic sex and greater susceptibility of one sex to food shortages. 2. We recorded daily mortality of male and female nestlings in 376 broods of the blue-footed booby, Sula nebouxii, a species where females are 32% heavier than males. We predicted: (i) higher mortality of last-hatched females than last-hatched males; and (ii) greater increase of mortality for last-hatched females than last-hatched males through the season. 3. Last-hatched females suffered greater mortality than last-hatched males, although the difference was only significant in three-chick broods. Male and female mortality was: 19% and 26%, and 24% and 44% in two-chick broods and three-chick broods, respectively. 4. Last-hatched females suffered a greater increase of mortality than last-hatched males as the season progressed, but the difference was only significant in broods of three chicks. 5. Pooled data from broods of two and three chicks showed that the risk of death increased substantially with hatching sequence and hatching date, although the increase was different for male and female chicks. Male and female mortality was similar in early broods, however, as the season progressed female mortality increased significantly with hatching sequence, while the increase in mortality of first-, second-and third-hatched males did not differ. 6. These results support the idea that blue-footed booby females are more vulnerable to food-related stress than males because of their larger size.

Buffered Development: Resilience after Aggressive Subordination in Infancy

Do aggressive dominance and subordination in vertebrate broods and litters affect development? We examined 1,167 fledglings from two‐chick broods of the blue‐footed booby (Sula nebouxii), a species in which the first‐hatched chick dominates with violent attacks throughout the nestling period and subordinates suffer lower fledging success, but if both broodmates survive, they grow to the same size. There was little evidence that dominant fledglings were more likely to recruit into the breeding population than were subordinate fledglings, and there was no evidence that dominant and subordinate recruits differed in their age, date, brood size, or nest success at first reproduction or in their summed brood sizes or total nest success over the first 5 yr or first 10 yr of life. Compared with dominants, subordinate fledglings were less prejudiced by late hatching and established clutches earlier over the first 10 yr, and subordinate recruits had 33% larger broods over the first 5 yr. However, in broods where both chicks fledged, accumulated reproductive success for chicks up to age 5 yr was similar for dominants and subordinates. Exercising dominance throughout infancy apparently does not fortify a chick for the future and may incur a long‐term cost, and suffering violent subordination throughout infancy has little or no prejudicial effect and may even steel a chick for adult life.

Variably male-biased sex ratio in a marine bird with females larger than males

When the costs of rearing males and females differ progeny sex ratios are expected to be biased toward the less expensive sex. Blue-footed booby (Sula nebouxii) females are larger and roughly 32% heavier than males, thus presumably more costly to rear. We recorded hatching and fledging sex ratios in 1989, and fledging sex ratios during the next 5 years. In 1989, the sample of 751 chicks showed male bias at hatching (56%) and at fledging (57% at ˜90 days). Fledging sex ratios during the five subsequent reproductive seasons were at unity (1 year) or male-biased, varying from 56% to 70%. Male bias was greater during years when mean sea surface temperature was warmer and food was presumably in short supply. During two warm-water years (only) fledging sex ratio varied with hatching date. Proportions of male fledglings increased with date from 0.48 to 0.73 in 1994, and from 0.33 to 0.79 in 1995. Similar results were obtained when the analysis was repeated using only broods with no nestling mortality, suggesting that the overall increase in the proportion of males over the season was the result of sex ratio adjustments at hatching. The male-biased sex ratio, and the increased male bias during poor breeding conditions supports the idea that daughters may be more costly than sons, and that their relative cost increases in poor conditions.

Senescence of Maternal Effects: Aging Influences Egg Quality and Rearing Capacities of a Long‐Lived Bird

Senescence could depress prenatal and postnatal capacities of mothers to invest in offspring. Longitudinal observations on the blue‐footed booby (Sula nebouxii) revealed a quadratic effect of female age on fledgling production and cohort differences in rate of reproductive decline. By swapping clutches between females of different ages, we tested whether reproductive senescence is due to decline in egg quality or capacity to care. As laying mothers aged, egg size, ulna length of 5‐day‐old chicks, and ulna growth of second chicks up to age 30 days declined, and as rearing mothers aged, ulna growth and cellular mediated immune response of second chicks diminished. Oddly, senescent females (>11 years) produced more fledglings when rearing offspring of middle‐aged females (8–11 years) than when rearing offspring of senescent or young females. Thus, senescence reduced egg quality and rearing capacities, and reproductive success of senescent mothers depended on prenatal effects associated with the age of the laying mother. Reproductive senescence of boobies may involve constraints on resources allocated to reproduction as well as adaptive adjustment of provision and care according to offspring value, implying that negative effects of senescence on offspring survival can be ameliorated by plasticity in postlaying or postnatal care.

Archiving primary data: solutions for long-term studies

The recent trend for journals to require open access to primary data included in publications has been embraced by many biologists, but has caused apprehension amongst researchers engaged in long-term ecological and evolutionary studies. A worldwide survey of 73 principal investigators (Pls) with long-term studies revealed positive attitudes towards sharing data with the agreement or involvement of the PI, and 93% of PIs have historically shared data. Only 8% were in favor of uncontrolled, open access to primary data while 63% expressed serious concern. We present here their viewpoint on an issue that can have non-trivial scientific consequences. We discuss potential costs of public data archiving and provide possible solutions to meet the needs of journals and researchers.