Monogamy is generally
considered the norm among humans, but it is surprisingly uncommon in
nature. This unique mating system is
usually limited to the class of Aves. As
a result, studies on the evolution of monogamy and its driving forces of
selection have largely been done on birds.
However, monogamy has also arisen selectively in other taxa in various
forms. One such form is social monogamy,
which accounts for about 90% of birds (Klug et al. 2013). Social monogamy involves a mated pair that
remain together and cooperate, though there may be some extramarital
copulation. The opposing form would be
genetic monogamy where the mated pair remain together and only mate with each
other (Klug et al. 2013). Humans would
be best represented by sequential monogamy, where we have sequential mates and
are dedicated to one throughout the duration of the relationship. Each of these forms retain the same
disadvantage of restricting reproduction and gene flow. Accordingly, it should be more advantageous
to engage in polygamy and mate with as many as possible to spread one’s genes
farther. How, then, has the evolution of monogamy occurred multiple times?
Though many hypotheses
have been proposed, the bi-parental care hypothesis offers the most support so
far (Tumulty et al. 2013). This
hypothesis suggests that the relationship between the parents and offspring has
ultimately produced the monogamous relationship between the parents. Understanding the foundations of parental
care will help clarify this hypothesis.
Parental care is a
complex form of parental investment. All
organisms practice some method of investment in their offspring that involve
the expenditure of time and energy. These
investments can be simple, physiological processes like the formation of yolk
or placenta which is obligatory of the parent organism. Others can be complex, behavioral processes
like nest-building, guarding, and teaching. Parental care represents the latter, which is a
subset of parental investment that focuses mainly on altruistic behavioral
patterns associated with investment into offspring rather than just physiological
investments (Klug et al. 2013).
Parental care can then be
divided into various types, including maternal, paternal, and bi-parental care
approaches. Maternal care is
characteristic of many vertebrates, specifically mammals since most energetic
investments are provided by the female in terms of placental nourishment and
lactation (Klug et al. 2013). Paternal
care, though not as common, does occur frequently in many species of fish and
amphibians; in particular, territorial behavior in males translates well into
the guarding and brooding of eggs (Klug et al. 2013). Finally, bi-parental care is an interesting case
of shared parental investment by both sexes.
Like monogamy, it is most common in birds and is likely due to the
shared capabilities of both males and females to incubate and feed their young
(as opposed to mammals where the only the female can) (Klug et al. 2013). Bi-parental care is much more complicated
than either maternal or paternal care as it requires the investment and
cooperation of two separate parties into the care of young. This leads to varying provision rates and
selection based on fair investment.
Bi-parental care can be supported by either similar or complimentary
builds of the sexes. Most species of
birds utilize the similar build strategy as described above which clearly
supports bi-parental care, however, forms of complimentary builds are seemingly
less common and can be demonstrated by the Onthophagus
Taurus, a species of dung beetle.
These sexually dimorphic insects have radically different shapes that
enable them to perform different parental tasks that the other could not (Hunt
and Simmons, 2002). These traits of cooperation
among mated individuals also underlie the foundation of monogamy.
traits underlying the systems of bi-parental care and monogamy appear to go
hand in hand and may support the influence of one over the other. The question I want to answer with this
research is: Do systems of bi-parental
care favor monogamous mating systems?
To answer this question,
I have looked into research that studied bi-parental care in conjunction with
monogamy. I purposefully sought articles
with varying study species other than
birds that display bi-parental care.
My rationale for excluding birds is their commonality and relatively
well-known association of bi-parental care and monogamy. In attempting to prove that one system favors
the other, I would prefer to use study species where this is not already
assumed. With that being said, I have
hypothesized that bi-parental care will favor a monogamous mating system.
and bi-parental care are often associated with more intelligent species that
are thought to be better capable of cooperation, as seen in higher level
mammals and birds. However, research by
Tumulty et al. (2013) from East Carolina University disproves this
misconception. They studied the mimic
poison frog, Ranitomeya imitator, an
amphibian which practices both social and genetic monogamy and provides
bi-parental care to its offspring. These
arboreal anurans offer a unique look into the development of bi-parental care
since ancestrally this species and its extant relatives rely on paternal care
(Tumulty et al. 2013), meaning that the evolution of bi-parental care in this
species involved incorporating the female as a caregiver. According to Tumulty et al. (2013), the males
of this species, along with its close relatives, carry tadpoles between various
pools of water suspended in plant leaves high off the ground. The males protect the pools and keep their
offspring safe. The female’s role, which
is unique to this species, involves laying trophic eggs (unfertilized,
nutrient-rich eggs) into the pools to feed their developing offspring. Tumulty et al. (2013) noted that there was a
negative correlation between parental care and breeding pool size, which showed
that bi-parental care was most often associated with the smallest of
pools. This suggests that bi-parental
care in these frogs was driven by the exploitation of an unused niche. Larger, nutrient-rich pools, were utilized by
related species, but were less common than the small, nutrient-poor ones. R.
imitator makes use of the latter through bi-parental care and the added
contribution of the female to make up for the poor nutrients in the small
pools. Tumulty et al. (2013) used this
organism and its distinctive traits to test the bi-parental care hypothesis.
involved monitoring the growth of R.
imitator tadpoles in conjunction with male removal studies to determine how
necessary males are for their offspring’s survival. Four field sites located in the tropical
forest outside of Chazuta, Peru were utilized over two 4-month sessions during
the mimic poison frog’s breeding season.
Tumulty et al. (2013) installed artificial, identical pools made with
PVC pipe in the four field sites to accurately monitor tadpole growth. A total of 23 families, each consisting of a
male, female, and tadpoles, were arbitrarily designated to either the control
group (n = 11) or the experimental group (n = 12) and were monitored for six
weeks. Three weeks into the monitoring
process, the adult males were removed from the experimental group. Comparisons regarding parental care, number
of trophic eggs, tadpole growth and survival made between the two groups
yielded significant results. It was
shown that the experimental group suffered a significant decrease (85%) in
reproductive success after the male removal when compared to the control
group. The data accumulated by Tumulty
et al. (2013) also showed a strong correlation between the number of trophic
eggs fed to the tadpoles and the survivorship of the tadpole. Though trophic egg deposition is a female
task, these points connected through observation of the mated pairs where the
male called to the female and displayed behaviors that encouraged the laying of
trophic eggs. Without the male
vocalizing and leading the female, she was much less likely to feed the
tadpoles, establishing the importance of both the male and female in this form
of bi-parental care.
Tumulty et al. (2013) go on to further
describe how this system of bi-parental care not only encourages but necessitates
social monogamy and, in 11 out of 12 cases, genetic monogamy as well. This is due to the fact that widowed parents
of R. imitator are unable to adequately
care for their offspring on their own, thus selecting for extensive bi-parental
care which directed their evolution of monogamy (Tumulty et al. 2013).
to prove that bi-parental care does not support monogamy often prove difficult
since these terms are almost always associated in the species that practice one
or the other. However, Brotherton and
Rhodes (1996) from Cambridge University, managed to find a study species that
did not follow this pattern of association.
This led Brotherton and Rhodes (1996) to investigate Kirk’s dik-dik (Madoqua kirkii), a small, monogamous
species of dwarf antelope, in order to determine whether this species also
practices bi-parental care.
kirkii display obvious forms of maternal care in the form of lactation, so
the bulk of this study focused on determining the importance of males to the
offspring’s survival. To do so, three
hypotheses were tested to assess multiple ways in how males could benefit their
offspring. In this observational study
of 23 dik-dik pairs in the Etosha National Park of Namibia, the researchers
looked to see if males helped their young through either resource defense,
infanticide defense, or predation defense.
terms of resource defense, Brotherton and Rhodes (1996) found that, while males
are territorial towards other males, they showed tolerance of other females who
shared their territory. Meaning that the
defense of resources for offspring is an unlikely scenario since the male is
willing to share with females that he is not mating with. In terms of infanticide defense, they found
that males of this species do not practice infanticide. Brotherton and Rhodes (1996) have also recorded
several cases of males forming monogamous bonds with females who had been sired
by now dead males, where the new male partner showed no ill intentions towards
the offspring that were not his own. In
terms of predation defense, the dik-dik has limited ability to defend its young
except in the form of a sentinel or lookout, due to its small size. Observations have shown that mothers do
perform this behavior actively to protect their young, but this pattern has not
been seen in the males of the species.
considering the above hypotheses, Brotherton and Rhodes (1996) declared that
there was no solid empirical evidence for any of them. The researchers went on to declare that their
study demonstrated the first case of monogamy in a mammal without any paternal
care. Brotherton and Rhodes then
suggested that the dik-dik may have evolved to be monogamous as a result of a
male mate-guarding strategy. This opposes the bi-parental care hypothesis for
the evolution of monogamy since in this scenario, it appears to have evolved
without bi-parental care.
final source looks into bi-parental care and obligate monogamy in marsupials,
which rarely practice monogamy according to Runcie (2000). Her observational study focused on Petropseudes dahli, a large species of
possum known as the Rock-Haunting Possum.
Runcie (2009) studied their social organization which showed initial
signs of a monogamous lifestyle. The aim
of her study was to determine the degree of social monogamy in the species by
answering four main questions: Whether
the maintenance of the bond is equal between both sexes, which sex controls the
decision-making, the extent of paternal care, and whether their territories are
exclusive or overlapping.
(2000) conducted this research in the Kakadu National Park in northern
Australia with 16 possum groups by analyzing and recording various behaviors
associated with pair-bond maintenance and parental care. Her observational studies concluded that
pair-bond maintenance was usually initiated by the males of the pair-bond,
indicating that females may be selective based on these tendencies. In terms of decision-making, Runcie (2000)
noted that neither sex conducted more decision-making or group-direction than
the other, which provides further evidence for a cooperative, monogamous
system. Most importantly, her study
observed that males provide extensive parental care in addition to the mother in
the form of protection and general rates of interaction, displaying bi-parental
care associated with a monogamous mating system. Finally, Runcie (2000) determined that the
territories of the possums were discrete and non-overlapping, providing further
evidence for their monogamous lifestyle.
(2000) research indicated that P. dahli
is both monogamous and practices bi-parental care. She suggested that the evolution of monogamy
in this species most likely evolved because of the need of bi-parental care and
the extensive amount of time the parents must dedicate to the offspring. Runcie (2000) also stated a few other
hypotheses that may explain the evolution of monogamy in this species. Though her study does support the bi-parental
care hypothesis, it does not offer empirical evidence that bi-parental care is
necessary to maintain the monogamous bond.
originally hypothesized that systems of bi-parental care would favor monogamous
mating systems. Two of the research
articles I utilized for this topic did end up supporting my hypothesis, with
only one opposed. Overall, I think that
my hypothesis is well supported with not only the data in these specific studies,
but also in several other studies cited by these very researchers.
first article I looked into by Tumulty et al. (2013) offered the strongest
support for my hypothesis. This was a
dedicated, two-year, field study that performed experimental manipulations to
determine the importance of bi-parental care, as opposed to the others which
conducted only observational studies.
The experimental data obtained from Tumulty et al. (2013) provides
empirical evidence for the necessity of both sexes in the care of offspring and
connects this cooperation with the obligation of monogamy. According to Tumulty et al. (2013), utilizing
the small, nutrient-poor pools requires the joint-care of both sexes for the
highest rates of reproductive success.
This means that a male from this species would be less reproductively
successful if he engaged in the practice of polygamy. Thus, to be able to contribute most towards
the next generation, it is most advantageous to practice bi-parental care,
which in turn leads to monogamy in this species.
addition to their clear and concise conclusion, Tumulty et al. (2013) suggested
some future studies that could be performed on this species. The researchers would want to conduct the
same study with much larger sample sizes to improve their data collection and
clarify other observed relationships in their study that were not significant,
but could have been given a larger sample size.
second study I looked into was very interesting and was the one paper that
opposed my hypothesis. Brotherton and
Rhodes (1996) research suffers from being dated more than twenty years, and
only being an observational study with little to no experimental manipulation. However, this experiment does discover a
novel trait of monogamy persisting without bi-parental care in a mammal. This does not necessarily rule out my
original hypothesis though. Bi-parental
care can still favor a monogamous system even in this case. The dik-dik may have a one point practiced
bi-parental care and monogamy, but during the course of its evolution, the
paternal care may have become redundant and unnecessary. This paper also fails to provide any
substantial explanation for how monogamy evolved otherwise, and as such, I do
not weight this paper as great as the other two. Future studies in related species to the
dik-dik may also reveal past social behaviors and clues to the origin of their
monogamy. More research is needed to
fully prove that bi-parental care is not the cause.
final research I looked into was also supportive of my hypothesis, but not as
strongly as Tumulty et al. (2013) did.
Runcie’s (2000) research suffers from a very small sample size and only
observational study, with no experimental manipulations. While these observations are valuable and set
a great precedent for future research, they do not offer empirical evidence in
favor of my hypothesis. And while her
observations do support the bi-parental care hypothesis most, Runcie (2000)
also offers mild support for some other explanations for the evolution of monogamy. Future research into this species may want to
conduct male removal studies to assess the importance of paternal care on
reproductive success similar to Tumulty et al. (2013).
conclusion, it can be generally assumed that systems of bi-parental care favor monogamous
mating systems. As described in these
articles, monogamy is supported when dedication to offspring results in further
proliferation of ones genes than polygamy would result in. Thus bi-parental care should be a leading
cause of monogamous mating systems across all taxa.