Bio Senior Sem

Running Head: ABNORMAL OR EVOLUTIONARY NECESSARY? 1
Abnormal or Evolutionary Necessary?
A Review of the Literature of Non-reproductive Sexual Behaviors in Bottle-nose Dolphins
and Other Cetaceans
Cornelia Yu
Hiram College
Author Note
This paper was prepared for my senior capstone study, advised by Professor Thomas
Koehnle.

Abstract
Bottle-nose dolphins are one of the most common marine mammals we study for
behavioral analysis. One of the reasons why we study the Bottle-nose Dolphins is that these
animals have a large brain that reaches 1886 g average, and the greatest part of the total mass
is the cerebrum which leads to interesting behaviors. Bottle-nose dolphins have a variety of
different behaviors; however, the sexual behavior of these animals is bizarre. Generally, when
we talk about sexual behavior of animals we assume the purpose is to reproduce. However,
within the group of the Bottle-nose Dolphins there are a lot of different non-reproductive
behaviors including masturbation, interspecific interactions, spontaneous ejaculation and
homosexual relationships. This multi-variable study is therefore focused on analyzing data,
observations and experiments on the motivation, existence and evolutionary advantages of
these behaviors in the Bottle-nose Dolphins, with support from other animals especially
cetaceans including other types of dolphins and whales.

Abnormal or Evolutionary Necessary?
A Review of the Literature of Non-reproductive Sexual Behaviors in Bottle-nose Dolphins
and Other Cetaceans
When doing research on behavior of Bottle-nose Dolphins, McBride & Hebb’s study is
always the first one pops out. This paper was published in 1948 and was cited by a lot of later
studies on Bottle-nose Dolphins’ behavior. As McBride & Hebb (1948) mentioned, the total
mass of the brain of the Bottle-nose Dolphins appears to be considerably larger than human
brains, with a cerebellum proportion lower than human brain, the greatest part of the total
mass is therefore in the large cerebrum, which indicates a lot of complicated voluntary
behaviors of these animals (p.111). As a result, Bottle-nose Dolphins are viewed as a typical
example of Cetaceans for behavioral studies.
As we all know, most of the animal behaviors exist due to an evolutionary advantage. So
how do non-reproductive sexual behaviors help the group? Bonnet et al. (2016) mentioned
that homosexual behaviors between Macedonian tortoises can actually help them to improve
their sexuality and their ability to recognize females, which will be an advantage in later
reproduction. For bulls, lack of breeding chances might be a cause of their spontaneous
erection and masturbation behaviors since this is a way for them to maintain their sexuality,
and attempts to stop these behaviors can adversely affect their sexual arousal, breeding
behavior and semen (Perumal et al., 2013, p.646). Female albatross are commonly pair
bonded in a remarkably stable way according to Zuk and Bailey (2008), for there are
behaviors such as mutual preening and mate guarding engaged and will provide them a
higher chance for reproduction (p. 658). Masturbation in Cape ground squirrels are not as

well understood since it usually happens after a male had copulated with a female and seems
to be a waste of sperm (Waterman, 2010) and for male Rhesus Macaques the reason seems to
be clearer since the high dominance ranking individuals are observed with higher mating
frequency and therefore have lower masturbation frequency and the lower dominance ranking
individuals are completely the opposite (Dubuc, 2013, p.1010). All these facts make it
interesting and important to study non-reproductive sexual behaviors in animals.
On the other hand, when a certain type of non-reproductive behavior was studied in a
certain type of animals, the Bottle-nose Dolphins actually have all these behaviors mentioned
above as well as interspecific interactions (McBride & Hebb, 1948, p.114; Herzing &
Johnson, 1997). In fact, they are the only animals you could find with all sorts of
non-reproductive behaviors other than the primates.
Even though spontaneous ejaculation is a very rare event in Bottle-nose Dolphins
(Morisaka et al., 2013), these animals are still very commonly observed for their
masturbation behavior (McBride & Hebb, 1948, p.114). In addition, the Bottle-nose Dolphins
are also well known for their same sex bonding relationship especially between males
(Bagemihl, 1999, p. 342; Connor & Smolker, 1995, p.213). It seems that all these behaviors
can be analyzed by observation and comparison with their results and possible causes, which
may give us a clue to lead to the motivation of these behaviors even though the animals
cannot talk.
Behaviors
As mentioned above, masturbation and homosexual interaction are the most commonly
observed non-reproductive sexual behaviors in all types of animals. It is obvious that a higher

masturbation rate is a result of lower mating rate; it has also been mentioned in some of the
articles that the behavior of masturbation is a way for the animals to maintain their ability of
breeding.
For Bottle-nose Dolphins specifically, there are a couple of different ways they could
carry out the masturbation behaviors. According to McBride and Hebb (1948), male
Bottle-nose Dolphins are observed doing masturbation on the floor of the tank as well as
against other males (p.114); one individual tended to “hold his erect penis in the jet of the
water intake for prolonged intervals” (p.114); a young male was seen on several occasions
“scooping up an eel at the bottom of the tank and swimming zigzag all the way across the
bottom of the tank with the eel wrapped about its penis”, and “swimming upside down at the
top of the tank, catching and towing a feather with its erect penis” (p.115). In Malikides’
video (2012), a Bottle-nose Dolphin was filmed masturbating with a beheaded fish. Moreover,
the Bottle-nose Dolphins are not the only Cetaceans found having masturbation. Baiji, or
Chinese River Dolphin, are also commonly found having masturbation behavior during
breeding season (Chen et al., 2002).
Interspecific interactions are another highly mentioned type of behaviors carried out by
the Bottle-nose Dolphins. As mentioned by McBride and Hebb (1948), male Bottle-nose
Dolphins have a lot of sex play with sharks and turtles (with turtles, they insert their penis
into the soft tissues at the rear of the turtle shell), attempting for copulation (p.114). These
behaviors can also be viewed as a type of masturbation. Herzing and Johnson (1997) has
done specific observations on interspecific interactions between Atlantic spotted dolphins and
Bottle-nose Dolphins, and these behaviors are not only sexual but also forming interspecific

coalitions having the two types of dolphins help protecting each other (p. 92). Within one
coalition, males would have confrontation during interspecies sexual and aggressive behavior
as well as erection attempts towards each other (p.91-p.92). The two species will also work
together to protect pregnant females while travel as well as having alloparental care (p.93).
These behaviors make it possible for the two different types of dolphins to coordinate with
each other, and therefore increase their survival rate both when facing danger and caring the
young (p.95).
Spontaneous ejaculation, defined as “the release of seminal fluids without apparent
sexual stimulation”, is a way less observed behavior in Bottle-nose Dolphins (Morisaka et al.,
2013). The possible functions of the behavior have three major directions studied by the
scientists: 1) a type of masturbation releasing surplus spermatozoa; 2) an element of sexual
display; 3) misuse of inhibitory neural control system during drowsiness and sleep (Morisaka
et al., 2013). This is defined different from masturbation basically because there’s no
observation of rubbing erect penis against anything, and the seminal fluid is just released
during their process of gentle swimming (Morisaka et al., 2013). According to Morisaka et al.
(2013), this is “the first report of spontaneous ejaculation in an aquatic mammal”. This
behavior “may have a ubiquitous physiological function considering the phylogenetically
wide spread nature of this phenomenon”. However, the author admitted that this behavior is
very hard to observe, but the reports are necessary for understanding the phenomenon as well
as “reveal its evolutionary function”.
Homosexual relationships are well observed behaviors among all sorts of animals
including a lot of different types of Cetaceans. In Bottle-nose Dolphins, this behavior is very

primary carried out by both males and females, observed both in wild and captivity and
including a lot of different behaviors including courtship, affectionate, sexual and
pair-bonding (Bagemihl, 1999, p.342). It has been observed that a female would persistently
approach a male when sexually excited and rubbing her body against his, but then the aroused
male would leave the female and attempt copulation with another male (McBride & Hebb,
1948, p.114). Female sometimes even “ride” over each other’s dorsal fin with one inserting
her fin into the other’s vulva or genital slit (Bagemihl, 1999, p.343). “Oral” sex is also
observed with one animal rubbing and nuzzling the other’s genitals with its snout or beak
(Bagemihl, 1999, p.343). Male Bottle-nose Dolphins often form life-long pair-bonds with
each other, starting from a very young age (the two partners are usually at the same age); this
is observed typically in all-male groups where homosexual activity is common(Bagemihl,
1999, p.344). This is a very strong bond that the two males will spend the rest of their lives
together. They become constant companions traveling together, take turns guarding or
remaining vigilant while their partner rests, and defend their mates against predators and
protect them while they are healing from wounds. If one dies, the other one may try to search
for a new partner, which usually falls since most of the dolphins do have a partner and
without a partner there’s a lower chance for them to live under threats. Sometimes there are
also bonds with three males forming a trio together (Bagemihl, 1999, p.344).
Homosexual interactions are very frequent and regular in wild dolphins, especially for
younger Bottle-nose Dolphin males (Bagemihl, 1999, p.345). With a same sex partners,
Bottle-nose Dolphins usually don’t mate until they are 20-25 years old and even after they
start mating heterosexually, they still retain their primary male pair-bonds (Bagemihl, 1999,

p.345). Furthermore, not more than half of the males in the same community are
heterosexually active each mating season if they have a same sex partner; those who do not
form same-sex pairs may have a more exclusively heterosexual orientation (p.345).
However, according to Connor and Smolker (1995), the male-male bonds in Indian
Ocean Bottle-nose Dolphins are not as stable as mentioned by Bagemihl (1999). They found
that there’re actually seasonal changes in the stability of the bonds. During winter time (May
through August in Indian Ocean, especially in July and August) more partner changes than
during spring and summer time (p.214). September is the start of the breeding season in this
area, and start from this time the instability of the bond dramatically drops (p. 213). The
authors believe that this change may have something to do with the level of testosterone in
their blood since it changes during breeding season; however, this hypothesis has not been
proved yet (p.215).
On the other hand, there are also several other Cetaceans showing homosexual behaviors.
In fact homosexual behavior seems to be very often happens between Cetaceans according to
Bagemihl (1999, p.339-359) since they were all described as either “primary” or at least
“moderate”. Amazon River dolphins and Killer Whales only have the males of the specie
involved in homosexual behaviors while Spinner Dolphins, Gray Whales, Bowhead Whales
and Right Whales all have both males and females involved, and the Spinner Dolphins also
have interspecific interactions with Bottle-nose Dolphins as well (p.343). These activities are
mostly mating, courtship and affectionate, with only the Gray Whales and Killer Whales
having pair-bonding behavior like the Bottle-nose Dolphins. However, none of these
Cetaceans was observed with homosexual parenting behavior. Some of the male specific

activities may involve up to 6 males together at one time (p. 356).
Motivations
Since animals cannot talk, it seems very hard for us to learn the motivation of their
behaviors. The good part of studying sexual behavior is that it has been well proved that these
behaviors can be motivated by hormones. This is also a direction of possible motivations
which scientists are mostly studying about on non-reproductive sexual behavior of animals.
Connor and Smolker (1995) slightly mentioned that the stability of male-male bonds may be
led by the level of testosterone in an animal’s blood (p. 215), and Chen et al. (2002) actually
found that Baiji’s behavior of masturbation matches with the peak of their blood testosterone
level (p.614). Chen et al. (2002, p. 614) also mentioned that Baiji’s blood testosterone level
changes seasonally, with a major peak in spring around April and a smaller peak around
October. The blood testosterone concentration is comparatively static between November and
January of the next year, since the periodicity of the Baiji’s blood testosterone level has been
observed.
Another interesting part of studying non-reproductive sexual behavior is that how these
behaviors are oriented. For example, for homosexuality, according to Bagemihl (1999) only
those individuals who do not have a same sex partner will be heterosexual aroused during
mating season, which lead to the fact that only 5 or 6 calves are born to a community each
year (p.345), so I would assume that the homosexual behaviors are more learnt than inborn,
for only heterosexual oriented individual will actually attempt to give birth.
Evolutionary advantages
If these sexual behaviors are not directly reproductive purposed, why they are biological

constant, appearing generation after generation? Or in other words, are there any evolutionary
advantages of these behaviors?
As mentioned by Bailey and Zuk (2009, p.443), “it is clear that same-sex sexual
behaviors occur in a wide variety of animal taxa, so predicting their evolutionary effects
should be a key goal of future research.”
Spontaneous erection and masturbation is a way for animals to maintain their sexuality
when do not get a chance to mate according to Perumal et al. (2013, p.646) and Morisaka et
al. (2013), so are some of the homosexual behaviors among males (Bonnet et al. 2016). With
a huge amount of males with same sex partners, there will be a smaller amount of calves born
and need to be protected and taken care of within a Bottle-nose Dolphin community
(Bagemihl 1999, p.346). In the review paper by Bailey and Zuk (2009), a couple of possible
explanations for animal homosexual behaviors are listed, including social glue, intersexual
conflict, practice, kin selection, indirect insemination, over dominance, sexually antagonistic
selection, etc. (p. 442). Bagemihl (1999) also mentioned that another possibility is that
homosexual behavior may help regulates the population size of a certain type of animal
(p.172). However, even clues are found for all these possible causes, it has not been proved
clearly that any of these explanations are exactly the case.
Bailey and Zuk (2009) also noted that for Laysan albatrosses, female-female pairing
increases the potential fitness benefits since the result of this is that both partner of the couple
will participate and “provide care for the offspring afterward than there would be if all pairs
in the population were opposite sex”, since male albatrosses usually do not participate in
taking care of the offspring (p. 443). The homosexual behavior actually provides better care

for the offspring, and therefore can be an evolutionary advantage compare with normal
heterosexual partners.
As Bagemihl (1999) mentioned, homosexual behavior might be multi sourced (p.182).
The cause of these behaviors can be social, behavioral, individual, etc. (p.175-181) other than
genetically oriented (p.182). But a lot of advantage such as offspring caring and population
regulating caused by homosexual behavior are observed and proved in the past (p. 173). As a
result of these, it is fair enough to judge non-reproductive sexual behaviors as “advantaged”
on the process of evolution.
Discussion—why this is important and further research directions
It is as important to study animals’ non-reproductive sexual behavior as to study their
reproductive sexual behavior. This will help us to understand evolution better on the
behavioral aspect as well as parenting behavior. Analyzing hormone motivation of these
behaviors also helps us to have a better understanding of the hormone itself, for there could
be “by-products” of these hormones other than general reproduction.
In addition, non-reproductive sexual behavior in animals is more than academic or
science for a lot of people. According to Bailey and Zuk (2009), media articles and websites
have cited documenting same-sex behaviors among animals to deal with gay rights issues in
humans. Understanding non-reproductive sexual behavior is a general characteristic in a lot
of animals will provide us a perspective of view that is more just viewing human behaviors as
well.

References
Bagemihl, B. (1999). Biological exuberance: Animal homosexuality and natural diversity.
New York: St. Martin's Press.
Bailey, N. W., & Zuk, M. (2009). Same-sex sexual behavior and evolution. Trends in
Ecology & Evolution, 24(8), 439-446. doi:10.1016/j.tree.2009.03.014
Bonnet, X., Golubović, A., Arsovski, D., Đorđević, S., Ballouard, J., Sterijovski, B., . . .
Tomović, L. (2016). A prison effect in a wild population: A scarcity of females induces
homosexual behaviors in males. Behavioral Ecology BEHECO, 00(00).
doi:10.1093/beheco/arw023
Chen, D., Zhao, Q., Wang, K., Zhang, X., Wei, Z., Kuang, X., . . . Wang, D. (2002).
RELATIONSHIPS BETWEEN SEXUAL MASTURBATION BEHAVIOR AND
SERUM TESTOSTERONE LEVELS OF A CAPTIVE MALE BAIJI. Acta Zoologica
Sinica, 48(5), 611-617.
Connor, R. C., & Smolker, R. A. (1995). Seasonal changes in the stability of male-male
bonds in Indian ocean bottlenose dolphins (Tursiops sp.). Aquatic Mammmals, 21(3),
213-216.
Dubuc, C., Coyne, S. P., & Maestripieri, D. (2013). Effect of Mating Activity and Dominance
Rank on Male Masturbation Among Free-Ranging Male Rhesus Macaques. Ethology,
119, 1006-1013. doi:10.1111/eth.12146
Herzing, D. L., & Johnson, C. M. (1997). Interspecific interactions between Atlantic spotted
dolphins (Stenella frontalis) and bottlenose dolphins (Tursiops truncatus) in the Bahamas,
1985-1995. Aquatic Mammmals, 23(2), 85-99.
Malikides, A. (2012, December 26). Dolphin self masturbates with beheaded fish. Retrieved
April 1, 2016, from https://www.youtube.com/watch?v=TvLZxG6pB6U
McBride, A. F., & Hebb, D. O. (1948). Behavior of the captive Bottle-nose Dolphin, Tursiops
truncatus. Journal of Comparative and Physiological Psychology, 41(2), 111-123.
Morisaka, T., Sakai, M., Kogi, K., Nakasuji, A., Sakakibara, K., Kasanuki, Y., & Yoshioka,
M. (2013). Spontaneous Ejaculation in a Wild Indo-Pacific Bottlenose Dolphin (Tursiops
aduncus). PLoS ONE, 8(8). doi:10.1371/journal.pone.0072879
Perumal, P., Vupeu, K., Khate, K., Veraselvam, M., Verma, A. K., Nahak, A. K., &
Rajkhowa, C. (2013). Spontaneous Ejaculation and Masturbation in Mithun (Bos
frontalis) Bulls. International Journal of Bio-resource and Stress Management, 4(4),
645-647.
Waterman, J. M. (2010). The Adaptive Function of Masturbation in a Promiscuous African
Ground Squirrel. PLoS ONE, 5(9). doi:10.1371/journal.pone.0013060
Zuk, M., & Bailey, N. W. (2008). Birds gone wild: Same-sex parenting in albatross. Trends
in Ecology & Evolution, 23(12), 658-660. doi:10.1016/j.tree.2008.08.004

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