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The adaptive significance of aggressive behavior in the Nazca Booby
Photo credits: Antariksh Mahajan
Galapagos Sophomore College 2015
By Meghana Rao
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Introduction
The Nazca booby, Sula granti, is one of the three species of boobies that inhabits the
Galapagos Islands. Galapagos is home to the Nazca booby, the blue-footed booby, and the redfooted booby. While none are endemic, the Nazca booby differentiates itself from the other
species by its size, with a wingspan exceeding five feet. Due to its large body size, the Nazca
booby is often found on cliffs, where height assists with propelling them into flight. They also
have distinct brown facial features and a yellow bill and are pelagic feeders. Between the months
of August and November and November through February, they build ground nests and breed
monogamously on the island of Genovesa and Espanola, respectively. Nazca boobies lay two
eggs per nest and have a long period of hatching asynchrony, the five-day period between the
hatching of the first and second egg (Galapagos Conservancy 2015).
Nazca booby spotted on a cliff
The blue-footed booby, Sula nebouxxi
The Nazca booby partakes in obligate siblicide. Obligate siblicide is the routine killing of
the younger sibling by the older sibling. In practice, the older chick generally pushes the younger
chick out of the nest moments after its birth, where the chick starves to death, in the presence of
the parents. The process is known as “obligate” because it occurs regardless of external factors
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such as abundant food supply or negligible size dimorphism. In contrast, the blue-footed booby
partakes in facultative siblicide, where siblicide only occurs if the food supply cannot support
both offspring. The aggressive behaviors of the Nazca booby observed during obligate siblicide
extend into adulthood, when non-parental adult visitor behavior (NAV behavior) becomes
ubiquitous (Anderson 1990).
Non-parental adult visitor behavior (NAV) is when non-breeding adults seek out
unattended chicks and initiate various interactions with them. Non-breeding adults display keen
interest in non-familial chicks. Research done by Anderson depicted that 51.5% of NAV
behavior is aggressive, 46.3% is affiliative (grooming), and 12.2% is sexual (Anderson 2004).
This behavior is widespread in large breeding groups, and most adults will initiate NAV at least
once in it during their lifetime. Chicks become victims of NAV between the first 30 and 80 days
of their lives, because this is when parents begin foraging for food and leaving them unattended.
NAV behavior strays away from the traditional benefits of interaction between adult and nonrelated young (Anderson 2004). Generally, interactions between non-related adults and young
are motivated by nutritional benefits, reduction of future competition by killing the young, or
kin-selected actions. NAV behavior serves as an anomaly, posing the question of how and why it
occurs.
An adult Nazca booby amidst the opuntia
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Currently, researchers seek to better understand the relationship between the Nazca
booby’s obligate siblicide practice and NAV behavior practice. The physiological implications of
both aggressive behaviors catalyze questions regarding causation, evolutionary significance, and
potential permanent neuroendocrine alterations. Recent research on Nazca booby aggression
addresses factors related to the physiological understanding of aggression in vertebrate
mammals. In vertebrate animals, androgen levels and corticosterone levels regulate aggression.
Androgens are sex steroid hormones, including testosterone and progesterone, and corticosterone
is a corticoid hormone secreted by the adrenal gland and comprises the primary avian stress
response (Nelson 2006). Understanding the role of hormones and the adaptive significance of the
aggressive behavior of the Nazca booby is a growing field of research
Hypotheses
In this research, two hypotheses focusing on siblicide and NAV behavior will be pursued. They
are:
1. Lethal aggression levels in Nazca boobies are beneficial during the neonatal stage because
they enable brood reduction.
2. Aggression becomes maladaptive in adulthood because it catalyzes a cycle of harmful nonparental adult visitor interactions (NAV).
Each hypothesis will be evaluated through reading and analysis of existing literature.
Findings
Hypothesis 1:
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To address the first hypothesis, the
significance of brood reduction through obligate
siblicide must be established. Research
demonstrates that the optimal brood size for the
Nazca booby is one. A single chick is more likely to
pass on its genes to future generations in
comparison to a chick raised with a sibling. In 1962,
Reduced
Dorward proposed the egg-insurance hypothesis to
Enlarged
Figure 1: Proportion of
clutches that produced at
least one hatchling for
is ideal (Dorward 1962). Nazca boobies hatch
reduced and enlarged clutch
sizes (Anderson and
approximately 60% of the eggs they lay due to infertility, and therefore, theClifford
second 2001,
egg provides
p. 343)
explain the persistence of two-egg clutches if one chick
an insurance policy if the first fails to hatch. In 2001, Clifford and Anderson experimentally
tested the egg-insurance hypothesis. In their experiment, they manipulated clutch sizes, creating
reduced clutches of one egg by removing eggs and then creating enlarged clutches by placing
removed eggs into natural one-egg clutches. Figure 1 depicts reduced and enlarged clutches
versus the probability of each hatching at least a single egg. Enlarged clutches had higher rates of
hatching at least one egg, demonstrating that it is
beneficial for the Nazca booby to lay two eggs (Clifford
and Anderson 2001).
As outlined in the introduction, androgen levels
and CORT levels regulate aggression in vertebrate
animals. In 2001, Tarlow studied CORT activity during
Nazca booby brood reduction. In the research, a colony of
2-egg
clutch
1-egg
clutch
2-egg
clutch
Figure 2: CORT levels in A
and B chicks and one-egg
clutches (Tarlow 2001)
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10,000 boobies were studied on Espanola. Researchers observed the nests daily, collecting blood
samples. Their findings, depicted in Figure 2, demonstrate that CORT levels are higher in Achicks (1st born) in comparison to B-chicks (2nd born). Single chicks from one-egg clutches had
even lower levels of CORT than B-chicks. The research suggests that the differing CORT levels
may illuminate inherent differences in aggression levels in the chicks based on their relationship
to the siblicide process (victim vs. perpetrator vs. not involved).
Research by Grace and Anderson in 2014 explains the significance of higher baseline
CORT levels in the Nazca booby. Grace and Anderson discovered that high baseline levels
correlate to increased likelihood of partaking in “gardening” and “aggression,” where gardening
is anxiety and agitation-related moving of nest material and aggression consists of general biting
and jabbing motions. The research done by Grace and Anderson and Tarlow indicate that Nazca
boobies are born with inherent varying levels of aggression that enable their implementation of
the evolutionarily beneficial obligate siblicide.
In addition, studies have compared factors that influence aggression in both Nazca and
blue-footed boobies. The comparison of the Nazca to the blue-footed booby provides effective
evidence towards explaining the Nazca booby’s inherent aggression since the blue-footed booby
does not partake in obligate siblicide. Instead, they partake in facultative siblicide, where
siblicide only takes place in the case of limited food supply. In 2008, Mueller compared levels of
androgens in Nazca boobies and blue-footed boobies moments after birth through blood
sampling. The results, observable in Figure 3, demonstrate that the Nazca booby has
approximately three times the androgen level of the blue-footed booby. In addition, first-born
chicks (2ECA), the ones that initiate siblicide, had higher levels of androgens than second born
(ECB). The differences correspond to distinct variations in exposure during the embryonic
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period, not upregulation, since the measurements were taken at birth. Through the comparison to
the less aggressive relative, this study supports the hypothesis that the Nazca booby possesses a
greater inherent aggression that enables
obligate siblicide.
For hypothesis one, the evidence
demonstrates an evolutionary need for
obligate siblicide and physiological
explanations for inherent differences in the
Figure 3: Comparison of Androgen Levels at
Birth for Blue Footed vs. Nazca Booby
(Muller 2008, p.8)
aggression that enables it. Heightened levels
of CORT and androgens in A-chicks and 2-
egg clutches in comparison to B-chicks and one-egg clutches suggest varying aggressiveness
based on the bird’s role in siblicidal act. The research demonstrated lower levels of androgens in
the blue-footed boobies coupled with heightened levels of CORT and androgens at a young age
for the Nazca booby, with levels higher in the older sibling. These increased levels are present
during times of siblicide and not present in birds that partake in facultative siblicide, indicating a
relationship between heightened aggression during the neonatal stage and the ability of the
booby to commit obligate siblicide, supporting the hypothesis.
Hypothesis Two:
The second hypothesis seeks to understand the adaptive significance
of the continuation of aggressive behavior into adulthood, as it manifests
itself in NAV behavior. Researchers have identified a relationship between
siblicide and NAV. Mueller’s research in 2008 elucidates the relationship; in
Figure 4: Relationship between number of
NAV events committed by siblicidal and nonsiblicidal chicks (Muller 2008, p. 3)
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a two-egg clutch where both eggs hatch (2ECA+) and siblicide occurs, the older chick is likely to
initiate more NAV events than in a two-egg clutch where only one egg hatches (2EC-) and no
siblicide occurs. Chicks that commit siblicide are more likely to commit NAV.
In 2011, Mueller further explicated the nature of NAV behavior and defined it as a “cycle
of violence.” As seen in Figure 5, the total number of NAV events experienced as a nestling
directly correlates to an increase in the number of NAV events initiated by the same bird as an
adult, creating a positive feedback loop, illustrated below as well.
Interpretation of the Nazca
booby cycle of violence from
chick to adulthood
Figure 5: The Cycle of Violence, relationship
between NAV events performed as an adult and
experienced as a nestling (Mueller 2011, p. 5)
As observed by Mueller in 2008, siblicide may play an activational role for NAV behavior.
However, even non-siblicidal chicks partake in NAV, indicating further causes.
In 2003, Tarlow studied the hormonal activity during NAV behavior on the island of
Espanola. The researchers observed NAV behavior opportunistically and initiated human
interruption to take blood samples when observed. Blood sampling findings shown in Figure 6
demonstrate that CORT levels were significantly higher in adults initiating NAV behavior than
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in non-NAV adults. The increased CORT levels during NAV mirror the increased CORT levels
observed in siblicidal chicks and indicate the involvement of inherent aggression levels.
In 2011, Anderson et al researched the CORT levels in chicks during NAV interactions.
He observed that NAV victims experience a five-fold increase in CORT during NAV
interactions in comparison to baseline levels, as seen in Figure 6. The CORT levels remain at
2.8-fold the baseline level the following morning and take weeks to return to regular levels,
revealing the magnitude of the impact of NAV behavior on the Nazca booby chicks.
Figure 6: CORT levels immediately after NAV for victim (left) and the morning after (right)
(Anderson et al 2014, p. 83)
The stress response activated in the chicks during NAV is known as the HPA stress axis
response and is regulated by the hypothalamus, pituitary, and adrenal glands. During the
response, the adrenal gland excretes corticoid hormones as the final product of the negative
feedback cycle (Mitrovic 2015). Repeated activation of the HPA stress axis has produced longterm neuroendocrine changes in rats, permanently organizing future adult maltreatment in their
lives (Boccia and Pederson 2001). The HPA axis stress response is also observed in the human
cycle of violence, where children who are abused are more likely to become abusers. Abused
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women and children experience the same hypercortisol release followed by a slow decline that
NAV victim chicks experience. Nazca boobies serve as a non-human model for the cycle of
violence, an unusual behavior pattern that may span across multiple vertebrate classes.
Currently, there is no direct explanation for NAV behavior. While the hypothesis proposes
that NAV behavior is maladaptive, the frequency of its occurrence and its widespread integration
within the life cycle of the Nazca booby makes it difficult to interpret as such. When a Nazca
booby is not mating, it engages in NAV behavior. It is possible that the NAV trait emerged due
to a strong positive selection for siblicide, which may permanently organize the chick to initiate
NAV behavior when older. In contrast, NAV behavior may be weakly selected against due to the
nonfatal nature of the harm it induces (Anderson 2004). While the evidence presented
illuminates a strong correlation between siblicide and NAV behavior and a correlation between
NAV behavior and permanent hormonal damage, it does not support the labeling of NAV
behavior as maladaptive.
Conclusion and Recommendations
This research studied literature regarding the adaptive significance of obligate siblicide
and NAV behavior. The first hypothesis that was investigated proposed that the lethal aggression
levels in Nazca boobies are beneficial because they enable brood reduction. Research by
Anderson and Clifford established the necessity for brood reduction and the benefits of a twoegg clutch. Tarlow demonstrated how first born chicks have higher CORT levels than second
born chicks and chicks born in one-egg clutches. Higher baseline CORT levels correlate to an
increased likelihood of partaking in aggressive behavior. In addition, Nazca boobies have three
times higher androgen levels than blue-footed boobies at birth, which are facultatively siblicidal.
The increased baseline androgen and CORT levels in first-born Nazca boobies suggest that they
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possess an inherent aggression that enables obligate siblicide, supporting the hypothesis. The
second hypothesis proposed a correlation between youthful and adult aggressive behaviors and
hypothesized that NAV behavior was maladaptive. Muller’s work demonstrates that siblicidal
birds are more likely to perform NAV behavior when older and that a cycle of violence emerges.
There is a direct correlation between the number of NAV events a chick experiences and the
number of events they initiate when they are older. CORT levels are also significantly higher
than the baseline in adults performing NAV and are increased by five-fold in chicks who are
victimized. The cyclical and ubiquitous nature of NAV behavior makes it difficult to label as
maladaptive. Rather, it is potentially a trait that is weakly selected against de to strong selective
pressures for obligate siblicide, which may catalyze the cycle of violence through hormonal
alterations. Therefore, the evidence does not support the second hypothesis.
Future research regarding Nazca booby aggression can lead to a better understanding of
human behavior. Since the Nazca booby serves as one of the few non-human models of the cycle
of violence, it could be beneficial to observe and understand physiological and physical aspects
of it. Through gaining a deeper understanding of a parallel cycle of violence, researchers may
discover the mechanisms that break the cycle. Also, it would be valuable to understand how
NAV behavior spreads and whether it can spread between species through observing the
interactions between the Nazca booby and other booby species.
Acknowledgements
Thank you so much Bill, Mari, and Annette for unconditionally supporting our group and
for selflessly and patiently teaching us about the Galapagos. Thank you so much for instilling in
me a newfound love for the natural world, for opening my eyes to hundreds of new species and
their evolutionary adaptations, and for giving me the opportunity to visit a surreal place. In
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addition, I have found a family of passionate peers through this process, and I could not have
possibly dreamed of a better experience.
“Nazca booby, living just to fly the ocean” – Don’t Stop Competin’ by Isabella Jibillian
Please note the Nazca booby in the left corner of the cliff
Photo credits: Isabelle Crary
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