1. ABSTRACT
Sexual selection is the competition seen within a species in order to reproduce successfully.
These pressures in a species have been classified by three different mechanisms: pre-copulation,
post-copulation/pre-fertilization, and post-fertilization pressures. We used these three
classifications to analyze the sexual selection of the blue-footed booby (Sula nebouxii). The
booby is a prime candidate for this study as they have been shown to have intrasexual selection
in both sexes, participate in extra-pair mating, and continually reassess their current pairings.
This study also compared these selection pressures to other seabirds, the Laysan albatross
(Diomedea immutabilis) and the emperor penguin (Aptenodytes forsteri), to show the differences
across species. This uniqueness of the boobies can be seen in their selection of mates by foot
color and courtship dance, but not in their sexual dimorphic calls. We conclude with a call to
action against climate change as boobies reproductive success is intensively linked to ocean
temperatures and food availability.
The importance of sexual selection on a species has been studied since Charles Darwin in his
book published in 1871 called “The descendant of man, and selection in relation to sex.” Since
then, many studies have gone on to support Darwin’s views on sexual selection and continue to
discover more intricacies involved. The basis of sexual selection is due to the competition for
mates in order to successfully reproduce and pass on their genes. Sexual selection can be seen at
three separate times: pre-copulation, post-copulation/pre-fertilization, and post-fertilization
(Cunningham and Burkhead 1998). The mechanisms controlling sexual selection vary greatly

2. from species to species and many species still remain unstudied. In this paper we will focus on
the topic of sexual selection in relation to the blue-footed booby (Sula nebouxii).
The blue-footed booby is a socially monogamous seabird that is found on arid islands off the
coast of Central and South America (Harris 2001). The booby is widely recognized because both
males and females have blue feet that they use in their courtship displays. After courtship and
copulation, females lay two eggs 5-7 days apart (Harris 2001). Once the chicks hatch these birds
practice bi-parental care of the young and take turns feeding and guarding the young (Nelson
1978). Boobies are monogamous for life and can liveto around 18 years of age (Kim et al.
2011). The sexual selection pressures in this species were, until recently, thought to be
completely based onintrasexual selection but a study by Torres and Velandon (2005) found it to
be mutual selection in both species. In this study we aim to focus on the mechanisms of sexual
selection at the 1) pre-copulation, 2) post-copulation/pre-fertilization, and 3) post-fertilization, in
boobies. The main question of this study is: what variables influence the sexual selection in the
blue-footed booby? We will look to explain this selection by three unique traits of the booby
which are their feet, mating ritual, and calls. FEET
Foot color-
Female blue-footed boobies use foot color to select their mates and to constantly reassess these
pairing. Foot color plays an important role in this reassessment because the preferred bright
green foot color is also an indicator of good health. While blue pigmentation was discovered to
be structural and based entirely on genetics, green has beenselected for because yellow
pigmentation in the feet is an indicator that carotenoids are present. Carotenoids are can only be
produced through a good diet and are essential in the immune system.Therefore, well-fed and

3. healthy boobies are the only ones that are capable of allocating this yellow pigmentation to their
feet, and are seen as more desirable to the opposite sex. The selection of females for mates with a
greener foot coloration serves as a type of pre-copulation sexual selection. This coloration is
highly susceptible to environmental conditions though, and can take as little as 48 hours to
change the color of their feet (Torres and Velando 2003). Velando, Beamonte-Barrientos, and
Torres (2006) tested the influence of carotenoids after pairs had been established and the first
egg was laid. The males’ feet were painted a duller blue color as a signal of “bad health” in
accordance with a previous study by the same authors in 2003. Upon return to the nest,the mates
of the experimental males laid eggs that were smaller in volume than the control males.
Thisindicates that the females had adjusted their level of parental investment based on the
males’ foot color. This example shows that foot color can also be an example of post-fertilization
sexual selection.
There are several reasons females might adjust investment based on the presumed state of their
male partner. First, male and female boobies are equal partners in the rearing of chicks. If the
male is presumed to be unfithe cannot do his share in caring for the chick.. Additionally, weak
males are not desirable donors of genetic material. Old or sickly males’ genetic material is worth
less, and if a female perceives her young are not going to have the best chances, she immediately
lowers her investment. Decreasing the second egg size therefore isvery important, because in
broods with two chicks of significantly different sizes the larger chick will often commit
siblicide.. In blue-footed boobies siblicide will not occur when the chicks are comparable sizes..
By laying a smaller egg upon re-assessment of her partner, the female is choosing to only have
one chick sired by that particular father, ensuring she does not waste too many of her own
resources caring for substandard chicks by herself(Velando et al. 2006).The parental investment

4. in relation to foot color is also supported by Velando et al. (2005) in their cross-fostering
experiment comparing the foot color of the foster father and the genetic father. The results were
such that the both fathers’ sexual ornamentation accounted for 32% of variance in chick
condition. This shows that those with more attractive coloration sire healthier chicks and prove
to be more attentive fathers (Velando et al. 2005).Though blue-footed boobies are considered
socially monogamous, foot color can also affect the incidence of extra-pair mates, especially in
females. In some cases, females have been observed engaging in up to seven extra-pair
copulations with neighboring males during a single breeding season (Osorio-Beristain and
Drummond 1998). The occurrence of extra-pair matings has also been linked to the sexual
attractiveness of females, as females with brighter feet have been shown to attract more extra-
pair mates. Males with brighter feet are also less likely to be cuckolded (Torres and Velando
2005).
Female ornamentation-
Ornamentation is the use of brightly colored visual displays commonly seen in the males of a
species in order to attract females (Darwin 1871). This form of sexual selection has been
extensively studied and can be observed across the animal kingdom (Amundsen 2000). Female
ornamentation is not as common as males, but it is far from rare. When females adopt the same
elaborate coloration of the males it was thought that the favorable traits were highly heritable and
were therefore “accidentally” inherited by the females. This idea was presented by Charles
Darwin (1871) and was called “the laws of inheritance”. Darwin’s law of inheritance is not
widely accepted anymore as scientists continue to learn more about the role of female
ornamentation. In a study by Torres and Velando (2005) they showed that females’ blue feet
were a factor in the males’ choice for a partner as female with darker feet participated

5. significantly less in courtship with their mate (figure 2)This study also showed that the color of
the females’ feet greatly affected the chance of courting with extra-pair mates. More attractive
females were 5.4 times more likely to be courted by a male besides their mate (Torres and
Velando 2005). Males’ choice of a specific female based on foot color is another example of pre-
copulation sexual selection. This mutual selection of males and females has not been well
studied in birds, but has been found to be correlated with bi-parental care and is this is another
likely cause of the boobies mutual ornamentation (Guerra and Drummond 1995, Amundsen
2000).
Bi-parental care and monogamy-
Bi-parental care plays a large role in the monogamous nature of the blue-footed booby. Bi-
parental care is often an indicator of whether a certain species is monogamous, and the extent of
obligate parental care influences the amount of polygyny within a breeding colony. The level of
breeding synchronicity also influences polygyny in the breeding system, as well as the sex ratios
within a certain colony. Therefore, birds with a longer period of bi-parental care, synchronous
breeding seasons, and even-sex ratios are unlikely to participate in polygyny, whereas in colonies
with uneven sex ratios, asynchronous seasons, or uneven parental investment will likely have
some degree of extra-pair mating (Tershy and Croll 2000).
A good example of strict monogamy is the emperor penguin (Aptenodytes forsteri). Since their
harsh environment mandates breeding synchronicity and they have an equal amount of
investment by both parents, there is no recorded incidence of extra pair mating in this species.
This is also evidentby the lack of sexual dimorphism within the species, indicating the equal
nature of a permanently mated male and female pairing. (Jenouvrier et al. 2010) Boobies,
however, do not have this extreme level of synchronicity, and it is suggested that the amount of

6. breeding synchronicity varies per colony. They also often have uneven sex ratios that make it
easier to engage in extra-pair mating even though they share in parental effort. This is evidenced
by a particularly unusual case study where several blue-footed booby nests were observed being
shared, each time with two females and one male. This unusual group nesting behavior was said
to be a result of more females than males in the breeding colony, and while it resulted in more
females having mates, it did not improve overall breeding success. One trio of parents were
reported to have successfully raised three chicks, while the others either broke up prior to
incubation or abandoned the nest, resulting in no reproductive success. (Castillo-Guerrero et al.
2005)
Though monogamy is favored in marine bird species, there are some trade-offs that suggest
promiscuity can also be beneficial. It has been hypothesized that monogamy evolved in boobies
and other seabirds because their dependence on the ocean mandates one parent toleave the nest
for long periods of time, making it impossible to rear the chick without aid from both parents.
Unlike many monogamous seabirds (including other species of boobies), blue-footed boobies are
capable of raising more than one nestling to adulthood. This is due to the fact that boobies
typically live in places where food is abundant, like the Galápagos.Boobies are sexually
dimorphic in their size and foraging behavior, with the larger females bringing home greater
catches and males foraging more frequently from closer distances. (Wittenberger and Tilson
1980) However, boobies are not strictly monogamous, and while there are theories about the
benefits of promiscuity and extra-pair mating, it is not certain why blue-footed boobies
developed this behavior. As discussed earlier, constant re-assessment of pairs and the ability to
move on to new partners does allow females to ensure the highest quality of her chicks. Extra-

7. pair mating can also serve to increase genetic diversity and ensure those individuals who were
unable to find mates before the abilityto pass on theirgenes.
Blue-footed boobies are also interesting because the females are typically up to 30% larger than
males and therefore are the determining sex when it comes to extra-pair copulations. While it is
questionable if females increase their fecundity by engaging in these behaviors, it is suggested
that females may participate in extra-pair mating during her fertile period to increase the genetic
quality of her offspring (Osorio-Beristain and Drummond 1998). It is also probable that male age
has an effect on extra-pair mating in females, as male attractiveness and overall fitness decrease
as males become senescent at around 10 years of age. As pairs have been known to spend an
average of 5-6 breeding seasons together, it is likely that males become senescent in this time
and increase the likelihood of extra-pair mating by females (Kim et al. 2011). Taking on extra-
pair mates due to the altered condition of the male is a further example of constant reassessment
of the pair as well as post-fertilization sexual selection.
MATING RITUAL
The Bonds of Dance
Courtship displays are another way that females of a species have been shown to choose their
mates. These dances involve complicated auditory and visual stimuli in the attempt of winning a
mate ( ). These types of courtship rituals can be found in mammals, reptiles, amphibians, fish,
and some insects, but is predominantly found in birds ( ). One such example of this can be
seen in the dance of the Laysan albatross (Diomedea immutabilis). The courtship displays
involves a variable pattern of movements with correlated honks and whistles that are performed
by both the male and females (Meseth 1975). The origin of these movements is thought to be
derived from other behaviors such as nest building, comforting behaviors, and locomotion that

8. were adapted to serve a reproductive purpose (Meseth 1975). The dance of the blue-footed
boobies shows many parallels to the Laysan Albatross, with some key differences.
Blue-footed boobies have four main actions in their courtship ritual: landing, presenting nesting
material, parading, and sky-pointing (Nelson 1978). The following description is from a personal
observation made on Isabela Island, Galapagos Islands. The first step is always initiated by a
male where he lands in front of the female with his feet splayed in front of him. He then presents
a gift in the form of a stick or other nesting material to the female. The male then will flaunt his
feet to the female in a high stepping display called parading. This exaggerated stepping is usually
done in conjunction with a high pitched whistled call. The final move of this sequence is a
motion called the “sky point” where the male and female boobies will lift their heads toward the
sky while opening their wings. When a female accepts a male she will follow his lead and
accompany his whistle with her trumpet (Nelson 1978). This dance does not always lead to
copulation, but that is the end goal (Osorio-Beristain and Drummond 1998). The order of these
motions does not vary, unlike the dance of the albatross (Meseth 1975). Similar to the
aforementioned albatross, the motions in the boobies dances have probably transformed from
other behaviors. The presenting of nesting material to the female is most likely derived from dual
parental investment in the raising of the young. The parading must be used to display their blue
foot as it is the main visual cue for sexual selection. The albatross described by Meseth (1975)
also did a motion similar to sky pointing that he attributed to a pre-flight motion. Like the
albatross, boobies are monogamous birds and will perform their courtship dances at the
beginning of a breeding season to reaffirm previous breeding pairs (Meseth 1975).
CALLS
Sexually Dimorphic Calls-

9. Calls in birds are most commonly used by males to attract a mate, but these signals have been
shown to also be used for mate recognition and pair bond maintenance in many species
(Dentressangle et al. 2012). The call of many penguins species (Spheniscidae) have been shown
to serve all of these purposes (Robinson et al. 1993). Robinson et al. (1993) suggests that the
evolution of these individualized calls was mainly influenced by the noisy environment of the
nesting colonies. Like penguins, Blue-footed boobies are monogamous colonial nesting seabirds
with bi-parental care (Nelson 1978). For these reasons it is also suggested that boobies developed
their individual sexual dimorphic calls. Unlike penguins though, the difference of the calls
between the species is greatly noticeable. Female boobies produce a loud trumpet sound while
males have a high pitched whistle (Nelson 1978). These calls are used during the courtship ritual
as well as a greeting back to the nest after a foraging trip (Nelson 1978). Dentressangle et al.
(2012) showed that calls can vary individually on 10 acoustic variables and that mates could
recognize these minute differences. This recognition is imperative as the main visual cues in the
boobies, their blue foot, are highly variable based on their health status (Torres and Velando
2003). In order to achieve these individualized calls females tend to vary the harmony and pitch,
while males change the frequency of their whistle (Dentressangle et al. 2012). While it is still
unknown why the calls vary between sex, it is thought to be because of the size dimorphismin
females that are usually 30% larger than males (Dentressangle et al. 2012). It has not been shown
if these calls influence the formation of new pairs, but it is essential to maintain a breeding pair
from year to year. Therefore, unlike other species the call of the blue-footed booby is not thought
to be a pre-copulation form of sexual selection.
Conclusion:

10. Our goals in this study were to highlight the unique aspects of sexual selection in the blue-footed
booby, as well as determine the devices driving this selection. We also strove to answer the ways
in which sexual selection is manifested in blue-footed boobies, and how these birds are unique in
their mating behaviors in comparison to other marine bird species.
Blue-footed boobies are incredibly unique in their mating behaviors as compared to other
seabirds. Sexual selection by these birds is a multi-faceted process that includes assessment of
foot color by both females and males in order to determine health, virility, and parental
investment in chicks, as well as an evaluation of the potential partner’s courtship dance. Blue-
footed boobies also frequently reassess their partners once mated and throughout their time
together in order to determine parental investment and the frequency of extra-pair mating. Calls,
though important in identification and reaffirming pair bonds, do not play a large role in
courtship like they do in other bird species.
Through our research on mating behaviors in blue-footed boobies, we became aware of how El
Niño and climate change events can seriously impact reproductive success in this incredibly
selective species. In learning about how malnutrition and lack of carotenoids in the system can
result in a failure to court, we realized that El Niño events, which cause serious food shortages,
can seriously reduce mating success. Wingfield et al. (1998) recorded that the 1992 El Niño
resulted in 100% breeding failure in blue-footed boobies for that year. That year, ocean surface
temperature raised an average of 1ºC, yet resulted in less pairs mating and an 89% abandonment
of nests if eggs were laid; of those not abandoned the rest of the chicks all died before they could
fledge (Wingfield et al. 1998). (Wingfield et al. 1998) This led us to look toward the future of
blue-footed boobies, as a likely increase in ocean temperature caused by climate change will
cause a collapse of their whole breeding system. We feel that research on these creatures is

11. invaluable as much is left unknown about them and especially about their sexual selection. We
believe that this is a critical point worldwide and if we are to take strides to reduce our global
impact, it must be soon, as even 1ºC can make the difference between life and death for the blue-
footed booby.