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Sexual Selection, Paternal Care, and Concealed
Ovulation in Humans
Beverly I. Strassmann
Changes in the social structure of early humans greatly
enhanced the potential for paternal care to contribute
to offspring success. Selection therefore favored females
who mated with more paternal males. Since paternal
care limits mating effort, males least successful as po-
lygynists would have the most to gain by paternal be-
havior, while the successful polygynists would gain least.
Concealed ovulation may have evolved because it pro-
moted the paternal tendencies of the less polygynous
males who advanced female reproductive success the
most. Since the offspring of indulgent males would have
a competitive advantage over the offspring of more po-
lygynous males, and females revealing the time of ovu-
lation would become increasingly scarce, all males would
eventually pursue a reproductive strategy emphasizing
paternal effort over mating effort.
In nonhuman primates the most polygynous males
mate selectively with females who are likely to be ovu-
lating. Sueh males would probably not mate with females
having dhninished cues to the time of ovulation. The
more paternally prone males could therefore consort
with these females and experience a high confidence of
paternity. Another characteristic of nonhuman primates
is that the most successful polygynists tend to have a
high dominance rank. Thus, female hominlds who mated
with more paternal males may have sacrlflced having
offspring with some of the gene advantages that con-
tribute to dominance. However, subconscious physiolog-
ical and psychological correlates of ovulation in humans
may have tempted females to exploit infrequent, low-
risk opportunities to mate outside the pair-bond with
males of superior genetic fitness. These correlates may
also promote conception irrespective of mating partner,
or they may have helped females avoid rape.
Key Words: Concealed ovulation; Dominance rank;
Estrus; Paternal care; Polygyny; Reproductive stra-
tegies; Sexual selection.
Received April 3, 1980;accepted October 16, 1980
Address
reprint requests to: Beverly I. Strassmann, c/o
Dept. of Entomology, Comstock Hall, Cornell University,
Ithaca, NY 14853.
INTRODUCTION
In the superfamily Hominoidea, human females
are unique in that they may be sexually receptive
at any time of their cycle and do not exhibit a
phase of pronounced physiological or behavioral
estrous cues that overtly signal ovulation to
themselves or to males (Butler, 1974; Alexander
and Noonan, 1979; Benshoof and Thornhill,
1979; Burley, 1979; Hrdy, 1979; Symons, 1979).
Thus, in the evolutionary line leading to humans,
there clearly has been selection to conceal ovu-
lation. Understanding the selective background
that led to the concealment of ovulation in hu-
mans could improve our understanding of inter-
actions between the sexes and provide insight
helpful in improving contraceptive methods. The
present paper will (1) review five recent hy-
potheses on the evolution of concealed ovulation
and (2) employ the theory of sexual selection to
modify the scenario proposed by Alexander and
Noonan (1979). The resulting hypothesis is sup-
ported by behaviors of primates, and it is con-
sistent with a possible explanation of the sub-
conscious physiological and psychological
correlates of human ovulation.
PREVIOUS HYPOTHESES
Burley (1979) proposes that concealed ovulation
evolved to counteract a tendency of hominid fe-
males to deliberately avoid conception. She ar-
gues that the motivations for this behavior, such
as fear of death, fear of pain, and the inconven-
ience of having children, stem from the evolution
of a large cerebrum. However, if a large cere-
brum were responsible for such dramatically
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32 Beverly I. Strassmann
nonadaptive behavior its disadvantages would
probably have outweighed its advantages. More-
over, studies of nonhuman primates make it
doubtful that females in obvious estrus could
have thwarted attempts by males to impregnate
them. Selection for concealed ovulation as a
means of promoting conception therefore would
not have occurred. Burley’s conclusion that
there is an apparent lack of female initiation of
copulation during ovulation is contradicted by
a study reporting an ovulatory peak in female-
initiated sex (Adams et al., 1978). Adams et al.
attribute the conflicting results of earlier studies
(e.g., McCance et al., 1937; James, 1971; Spitz
et al., 1975) to measures of sexual activity in
which the ovulatory rise in female-initiated cop-
ulations is masked by male-initiated copulations.
Hrdy (1979) suggests that concealed ovula-
tion evolved among prehominid females as a
means of confusing paternity so that males
would not be able to selectively kill the offspring
of other males. This model may explain sham
estrus in Hanuman langurs
Presbyt is ~ntelus)
but does not fit the evidence on prehominid so-
cial structure. The primates from whom humans
evolved probably experienced strong competi-
tion between groups, thus enhancing unity and
cooperation within groups. Selection for coop-
erative hunting and predator defense would also
have reinforced group solidarity (e.g., Alex-
ander, 1974). If males in a troop gained by co-
operating with each other they would not have
profited from killing each other’s offspring. As
Hrdy points out, in Savannah baboons (Papio
c~nocephnlus) the males cooperate in predator
defense and females have not evolved concealed
ovulation. According to Hrdy, “any male who
killed offspring belonging to a male still present
in the troop would reduce the stake that male
had in risking his life to defend the troop. Hence
all infants in a troop containing an infanticidal
male (including his own) would have been less
likely to survive.” Moreover, in an affiliative
troop, relatives and consorts of mothers would
help them defend their infants from attacking
males (Smuts, in preparation, cited in Hrdy,
1979). Bygott (1979) asserts that exaggerated
aggression over access to females will be pun-
ished, since selfish males will not be able to par-
ticipate in beneficial coalitions.
Hrdy claims that continuous receptivity was
part of the physiological heritage that prehom-
inid females brought to the new lifestyle, distin-
guishing humans from other primates. She may
mean that concealed ovulation evolved in a com-
mon ancestor to the Hominoidea (great apes and
humans). At such an early stage in evolution,
prehominids might not yet have become partic-
ularly cooperative within groups. This idea is
implausible since humans are the only members
of the superfamily Hominoidea who have con-
cealed ovulation.
Even if cooperation within groups is strong,
it could be argued that concealed ovulation
evolved to counteract infanticide practiced by
males from outside the group. Evidence provided
by Bygott (1979) indicates that chimpanzees
(Pan t roglod.y ty t rs) of the same troop have few
aggressive encounters over females, while in-
tergroup competition for females is fierce and
sometimes involves infanticide. Similarly, infan-
ticide among the Yanoama is often associated
with the exchange of women in intercommunity
warfare (Biocca, 1969). However, when infan-
ticide is practiced on the infants already born to
mothers recruited into a different group, males
of the new troop may be quite certain they did
not sire the infants. In this situation, the evo-
lution of concealed ovulation would only benefit
females who switched groups early in pregnancy
before their condition could be detected by
males. It is, therefore, not surprising that con-
cealed ovulation has not evolved in chimpan-
zees. These arguments imply that concealed
ovulation evolved in humans for a reason other
than intergroup infanticide. Such infanticide
would, however, increase the importance of pa-
ternal care; this could have been a precipitating
factor in the evolution of concealed ovulation
(Alexander and Noonan, 1979).
Symons (1979) offers two scenarios to explain
the loss of estrus in humans. In one, humans
live in promiscuous polygynous bands and males
give meat to females in exchange for sex. Ac-
cording to Symons, this practice caused females
to prolong their stage of sexual attractiveness.
However, males who mated only with females
in true estrus would have been selected for rel-
ative to males who mated with females in either
true or sham estrus. Unless one assumes that
such discriminating males were extremely scarce.
it is questionable whether females who pro-
longed estrus would have been given more meat
and thus been favored by selection.
In another scenario, Symons (1979) and Ben-
shoof and Thornhill (1979) suggest that con-
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Concealed Ovulation in Humans
33
cealed ovulation evolved to facilitate cuckoldry.
The authors assert that females who did not ad-
vertise ovulation would have minimized the ca-
pacity of their mates to monitor their reproduc-
tive cycles and guard them during ovulation. If
at the time concealed ovulation evolved the
human breeding system was monogamous or
harem polygynous (and females sometimes had
access to males who were genetically superior
to their mates), the cuckoldry scenario is cred-
ible. However, it is doubtful that females would
evolve induced ovulation or another trait that
would perfect their ability to cuckold males.
Otherwise their mates would have such low con-
fidence of paternity that selection would dim-
inish their paternal care-whether or not males
discovered infidelities.
Alexander and Noonan’s hypothesis regard-
ing the evolution of concealed ovulation is based
on the following assumptions: (1) At the time
ovulation became concealed human society was
polygynous and consisted of multimale bands.
Several studies link early humans with this social
system (Lee and DeVore, 1968; Jolly, 1972;
Alexander et al., 1979; Brace, 1979; Campbell,
1979). (2) During this period in human evolution
paternal care became an increasingly important
determinant of offspring success. The increasing
importance of paternal care, according to Alex-
ander and Noonan, is associated with intergroup
competition and the growth in group size and
unity. These developments increased the com-
plexity of social interactions and aggravated ju-
venile disadvantages in experience and strength.
Therefore, parents who conveyed to their off-
spring the knowledge and skills needed for making
the best decisions in complex social situations
outreproduced less instructive parents. Other
authors point out that conflicting selection for
bipedality, which favors a narrow pelvis, and
larger brain size, which favors a wide pelvis,
could result in more altricial young whose brains
would grow large after birth (e.g., Fox, 1972:
Benshoof and Thornhill, 1979). The advantages
of a longer learning period could also promote
altriciality (Washburn’ and Lancaster, 1967:
Pfeiffer, 1972). The increasing altriciality of
human young would further enhance the advan-
tages of parternal care.
Alexander and Noonan’s two initial assump-
tions, as well as their important idea that con-
cealed ovulation evolved in the context of fe-
males securing paternal care, are an integral part
of the hypothesis advanced in this paper. Yet
the evolutionary steps they propose are differ-
ent. They suggest that through concealing ovu-
lation females forced desirable males into con-
sort relationships long enough to decrease the
males’ chances of success in securing other mat-
ings. The confidence of paternity of these males,
according to Alexander and Noonan, was si-
multaneously increased because potential com-
petitors no longer knew when females were
ovulating.
Their hypothesis implies that the highest
quality females would be in demand by the high-
est quality males and would enter into prolonged
consortships with these males before subordi-
nate females and males would enter into con-
sortships.’ It seems more likely that males most
successful as polygynists wotdd be the last to
increase their paternal care, not the first.
In order for high ranking males to abandon
their successful polygynous strategy and in-
crease their paternal investment, differentials in
female quality would have to be remarkably
steep. The relevant sample sizes are small, but
some studies of nonhuman primates seem to find
instances in which females differ markedly in
their attractiveness (Hausfater, 1975) or in their
reproductive success (Goodall, 1971), while oth-
ers point toward more uniform female quality
(e.g., Loy, 1971; Kaufmann, 1965). Further
studies are needed to determine whether steep
differentials are likely to have existed among
human females. In the absence of steep differ-
entials, high ranking males who copulated around
the most likely time of ovulation with females
having a precise estrus would probably have the
highest reproductive success.
In addition to reducing the number of estrous
females with whom a male can mate, partici-
pation in a consortship potentially has other dis-
’ “The highest quality females fin terms of ability to bear
and rear offspring, and, perhaps, to enhance their status or
other correlates of reproductive success), would be in demand
by the highest quality males most prone to polygyny; such
females could afford to prolong estrus longer than others with-
out risk of desertion by the male. Low quality females would
be at least partially excluded from mating with the highest
quality males by this ploy. Competition for matings by lower
quality males would be reduced (because top quality males
are already committed), making concealment of ovulation ad-
vantageous to low ranking females in securing at least some
parental care for their offspring” (Alexander and Noonan.
1979, p. 448).
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34
Beverly I. Strassmann
advantages discussed by Hausfater (1975). He
postulates such negative consequences as re-
duced food intake resulting from the need for
constant guarding of the female, and increased
exposure to aggression and harassment from
other males. In his study of yellow baboons
(Pupio cynocephalus), the first ranking adult
males mated and consorted primarily on the op-
timal day for mating. Lower ranking adults con-
sorted on as many potentially fertile cycle days
as possible. According to Hausfater, the strategy
of the first ranking males gives them a relatively
high probability of impregnating females while
minimizing the negative consequences of con-
sortship.
High ranking males who entered into con-
sortships with females who prolonged estrus
would suffer the additional disadvantage of the
higher cost of high confidence of paternity.
When males need to guard their mate only during
the brief period of estrus, confidence of paternity
is least expensive (Benshoof and Thornhill,
1979; Burley, 1979; Symons, 1979). If a male
guards a female continuously, he compromises
his ability to hunt and carry out the other activ-
ities that are at least indirectly an important com-
ponent of paternal care. Although concealed
ovulation would greatly reduce the probability
of matings outside the pair-bond resulting in
pregnancy, the following males would still at-
tempt such matings: unmated males, males with
pregnant mates, and other males as low cost
opportunities arose. Since confidence of patern-
ity would be more expensive for high-ranking
males, it is unclear how concealed ovulation in
Alexander and Noonan’s scheme could increase
paternal investment. In view of the disadvan-
tages of consortships to dominant males, it is
necessary to add to Alexander and Noonan’s
scenario the condition that the first males to pro-
long their consortships would probably be those
having a low probability of securing matings.
Moreover, the females with whom such males
consorted would have to be sufficiently unat-
tractive to the polygynous males that their con-
sorts would have a high confidence of paternity.
CONCEALED OVULATION AND MATE
SELECTION
Due to anisogamy, which may be the ultimate
source of differential parental investment be-
tween the sexes, females are usually a limiting
resource to males (Bateman, 1948; Trivers,
1972). One consequence is a general tendency
toward polygynous breeding systems in which
there is great variance in reproductive success
among males. In their competition to secure suc-
cessful matings with females, conspecific males
have evolved alternative strategies involving
both behavioral and morphological differences.
For a recent review of alternative male strategies
among several classes of organisms see Cade
(1979).
This theoretical framework suggests a pos-
sible selective background for the evolution of
concealed ovulation. If humans were polygyn-
ous at the time concealed ovulation evolved,
some males by definition would have had more
success at securing matings resulting in preg-
nancy. Studies of nonhuman primates indicate
that those males who secure the most matings
around ovulation tend to be dominant while sub-
ordinate males find seeking matings a costly en-
deavor with a lower probability of success (see
Conaway and Koford, 1965; DeVore, 1965:
Kaufmann, 1965; Lindburg, 1971; Loy, 1971:
Hausfater, 1975). Those males who are least suc-
cessful as polygynists would tend to expend
greater paternal effort on the offspring that they
produced with a relatively limited number of fe-
males. In fact, their options for increasing their
reproductive success would resemble those of
females. The successful polygynists, on the con-
trary, would tend to expend less paternal effort
on each of the offspring of a larger number of
females. This argument is based on the assump-
tion that time and energy spent on mating effort
will limit paternal effort (see Alexander and Bor-
gia, 1979). Borgia (1979) formulates the similar
argument that males who are unlikely to be suc-
cessful at securing matings because of their rel-
atively low genetic fitness may enhance their
reproductive success by providing females and
their young with material benefits. Of course,
greater paternal effort on the part of subordinate
males would have to be associated with en-
hanced confidence of paternity, and it would
have to be the best strategy for these males to
pursue in terms of their overall life history.
As the potential for paternal care to be a de-
terminant of offspring success increased so as
to exceed the importance of the male’s genetic
assets which do not contribute to paternal care,
those males least able to monopolize matings
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Concealed Ovulation in Humans
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would have become the most desirable mates.
These arguments suggest that concealed ovula-
tion evolved because it favored the strategy of
the subordinate, more paternal males who pro-
moted female reproductive success the most.
Females sacrificed the genetic benefits offered
by the high ranking males in exchange for the
more important paternal benefits offered by
lower ranking males. Borgia (1979) also arrives
at the conclusion that females sometimes must
compromise potential gain from either the ma-
terial or the genetic benefits offered by males in
order to maximize reproductive success.
How would the concealment of ovulation en-
hance the confidence of paternity of the subor-
dinate males? Subordinate males could probably
keep their consorts from copulating with other
low ranking males, but not from the high ranking
males. However, if females who concealed ovu-
lation were sufficiently unattractive to the high
ranking males, their lower ranking consorts
would have a higher confidence of paternity.
Female Attractivity
Behaviors of nonhuman primates indicate that
females who concealed ovulation by gradually
dampening their estrus signs might have been
unattractive to the dominant males. Observa-
tions of baboons, rhesus monkeys, and chim-
panzees reveal a strong degree of selectivity in
the mating of the dominant males. First ranking
male yellow baboons do not mate on all the po-
tentially fertile days of a female’s cycle, but in-
stead constrain their mating to the optimal day,
sometimes even the optimal time of day (Haus-
fater, 1975). On the optimal day estrus signs such
as sexual swellings are most pronounced. In
other studies of baboons (Papio w-sinus and P.
am is)
and macaques
(Macaca mulatta),
it was
found that the more advanced the estrous con-
dition of the female, the more exclusively was
copulation confined to high ranking males (Hall,
1962; DeVore and Hall, 1965). Moreover, there
are some females with whom the dominant males
do not consort at all. For example, Hausfater
(1975) found that when three female yellow ba-
boons were in estrus, two different first ranking
males did not consort with them even if no other
estrous females were present in the group.
Goodall (1965, 1968) found that female chim-
panzees during their first few sexual swellings
appeared unattractive to mature males, but fre-
quently solicited mounting from adolescents.
Since dominant males cue in on estrus signs and
do not mate with females with partial swellings,
they might pass up opportunities to mate with
females who had begun to reduce their estrus
signs, even though these females would be fer-
tile. This behavior would reflect an inability of
the males to distinguish between females un-
likely to be fertile and females concealing ovu-
lation. Low ranking males would probably mate
with females who dampened their estrus signs
because they mate with females having partial
swellings.
It is probable that at the time concealed ovu-
lation evolved all males including the dominant
ones already wqre somewhat inclined to be pa-
ternal-perhaps bnly through protecting past or
present female c sorts or their young. Such
behavior is thought TO occur to some extent in
all the great apes (Mitchell, 1969) and would re-
duce the time and energy available to the dom-
inant males to seek suboptimal matings. Domi-
nant males would be especially disinclined to
attempt copulations with females appearing su-
boptimal if these females were being vigorously
defended by their low ranking male consorts.
Since their matings with females appearing su-
boptimal would be rare, dominant males would
not experience strong selection for the ability to
distinguish between females concealing ovula-
tion and females unlikely to be fertile. The rare
matings that would occur between dominant
males and females concealing ovulation would
not diminish the paternal behavior of the fe-
males’ indulgent consorts because the latter
would have been copulating with the females
much more regularly and therefore would be
more likely to sire offspring.
In nonhumans there is a close correlation
between female attractivity and estrogen levels
(Beach, 1976; Baum et al., 1977). Hausfater
(1975) suggests that in yellow baboons a sexual
pheromone may be the mechanism governing
cycle day and individual selectivity. If so, it is
unlikely that a female who decreased her non-
hormonal estrus signs but retained her hormonal
estrus signs would be unattractive to dominant
males. Since estrogen is an agent in ovulation,
it is necessarily present in high concentration
during ovulation. Therefore, in the loss of estrus.
the hormonal cues to ovulation could not have
been obscured through a reduction in hormone
production. Symons (1979) suggests that the loss
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36 Beverly I. Strassmann
of estrus may have begun with the continuous
production of estrogen, or a pheromone based
on estrogen. Another possibility is that there was
a slight alteration in an estrogen based phero-
mone that, in association with the dampened
visual signs, deterred high ranking males from
mating with females concealing ovulation. Se-
lection for males to detect the new pheromone
and associate it with ovulation would be espe-
cially weak if it varied among females. The vol-
atile organic constituents of the vaginal secre-
tions of modern humans have been shown to
vary (Preti and Huggins. 1975). The visual signs
of ovulation may have been lost through a neo-
tenous reduction in the sexual skin or in the
sensitivity of the sexual skin to estrogen.
Extended Sham Estrus Versus Direct Loss of
Estrus
It is doubtful that females concealed ovulation
by extending the duration of estrus signs, making
sham estrus indistinguishable from true estrus,
as has been suggested (Alexander and Noonan,
1979: Burley, 1979; Symons, 1979). This ploy
would have been less likely to allow females to
grant paternal males a high confidence of pa-
ternity than would the concealment of ovulation
through the direct loss or disguisement of es-
trous cues. One could expect the most success-
ful polygynists not to copulate with females who
extended their signs because the longer period
of consortship necessary to give a high proba-
bility of conception would reduce the number
of less costly matings these males could secure.
However, females in prolonged sham estrus
would occasionally be exposed to high ranking
males from outside their group. Hrdy (1977,
1979) observed that female langurs in sham es-
trus who are ignored by resident males are often
sought after by extra-troop males. She attributes
this behavioral discrepancy to differences in the
ability of resident and nonresident males to mon-
itor females’ cycles, and thereby distinguish
pseudo-estrus from true estrus. Thus, polygyn-
ous males from outside the’band would probably
attempt copulations with hominid females in
sham estrus. Such males would compromise the
confidence of paternity of the paternal males
who mated with females who concealed ovula-
tion through sham estrus. Moreover, extension
of such estrous cues as sexual swellings would
probably impose on females a great energetic
expense. Thus, ovulation was probably con-
cealed through the direct dampening of its visual
signs and the masking of its hormonal signs.
The Diversion of Mating Effort into Paternal
Effort
Since the offspring of indulgent males would re-
ceive the most paternal care, they would have
a competitive advantage over the offspring of
more polygynous males. Compared to the pa-
ternal strategy, the polygynous strategy would
become increasingly less successful. Females
who concealed ovulation would, on the average,
have more indulgent mates and achieve greater
reproductive success than females who adver-
tised ovulation. Eventually, females with a con-
spicuous estrus would no longer exist, nor would
there be males exploiting this physiology by em-
phasizing mating effort over paternal effort. Fe-
male willingness to mate would gradually have
extended throughout the menstrual cycle be-
cause as long as receptivity was keyed to the
ovulatory phase ovulation would not have been
concealed.
Polygynous behavior would not disappear,
but each male would mate with fewer females
on the average and would exhibit more paternal
care. Several factors explain why polygynous
behavior would not be lost entirely. First,
greater access to resources that are a component
of paternal care would enable some males to
have more mates or seek more matings than
would be the case for most males. Second, there
are diminishing returns from paternal care after
a certain level of indulgence. Third, although
paternal care restricts mating effort overall, sit-
uations would arise in which males could secure
additional mates or copulations without invok-
ing a disadvantageous trade-off with paternal
care.
Paternal Investment: Male Versus Female
Advantages
Burley (1979) has argued that concealed ovula-
tion would not evolve to induce males into pro-
viding paternal care because males would in-
crease their paternal investment anyway when
this behavior became advantageous for them.
This assertion ignores the probable existence of
a stage in human evolution in which increased
paternal investment would have benefited fe-
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Concealed Ovulation in Humans
7
males greatly but would not yet have been
strongly selected for among males. In a poly-
gynous primate species, subordinate males would
have had the most to gain by paternal behavior
but their low confidence of paternity would have
restricted the development of their paternal
tendencies. It cannot be assumed that the high
ranking males would have increased their pa-
ternal investment since these males would al-
ready have been achieving high reproductive
success through a reproductive strategy empha-
sizing mating effort. If these males increased
their investment, time and energy constraints
would then have reduced the total number of
matings they secured, making increased invest-
ment a marginal advantage at best. It could even
have been disadvantageous because it would
probably have increased the number of matings
secured by lower ranking males or raised the
confidence of paternity of lower ranking males,
enabling them to be more paternal. Therefore,
a female adaptation that increased the advan-
tages of paternal investment to males could have
been favorably selected.-The hypothesis ad-
vanced in this paper is that concealed ovulation
enhanced the advantages of paternal care to sub-
ordinate males by increasing their confidence of
paternity. Assuming that paternal care was a
more important determinant of offspring success
than the dominance status of the father, con-
cealed oculation would have benefited females
and therefore been positively selected.
PREDICTIONS
If concealed ovulation evolved because it in-
creased the reproductive advantages that males
could gain from paternal effort relative to mating
effort, certain predictions should follow:
If paternal care were sufficiently important
in determining offspring success so that a
male’s genes unrelated to paternal care were
relatively trivial, then one would expect the
ability of females to control conception to be
relatively poorly developed and cuckoldry to
be rare.
If, however, genes irrelevant to paternal care
were still significant in determining repro-
ductive success, one would expect females
to retain the ability to exploit occasional
ow
risk opportunities to mate outside the pair-
bond with males of superior genetic fitness
(Alexander, 1975).
In either case, conscious knowledge of ovulation
would be selected against because it would
hinder the ability of females to deceive males
and promote paternal care (see Alexander and
Noonan, 1979; Benshoof and Thornhill, 1979).
In the second case, there would be selected (or
never lost in the first place) subconscious phys-
iological and psychological correlates of ovula-
tion that increase the probability that isolated
matings with more fit males result in pregnancy.
An example is facilitated sperm transport men-
tioned by Benshoof and Thomhill (1979).
PHYSIOLOGICAL AND PSYCHOLOGICAL
CORRELATES OF OVULATION
A review of the subject of ovulation reveals an
array of physiological correlates. Concomitant
with the various hormonal changes around ovu-
lation are various changes in the cervical secre-
tions that increase the ability of sperm to mi-
grate. When coupled with sexual arousal in the
female, they increase the probability of concep-
tion (Levin and Wagner, 1977). Hormonal changes
in the ovarian cycle of the human female have
been shown to effect the CNS and cause changes
in sensory stimuli. Diamond et al. (1972) found
that visual sensitivity built up to a peak at mid-
cycle and remained high until menstruation
when it dropped precipitously. Women taking
nonsequential oral contraceptives, and men
tested by the same procedures, did not undergo
significant visual threshold changes. Laws (1977)
discovered that changes in the neuroendocrine
system lower the acoustic reflex threshold of
women using oral contraceptives or at day 16 in
their menstrual cycle. Semesuk et al. (cited in
Diamond et al., 1972) also found changes in
acoustic sensitivity to be related to the menstrual
cycle. Olfactory sensitivity of women for invo-
latile compounds has been correlated with the
menstrual cycle and estrogen production, with
the greatest sensitivity occurring around ovu-
lation (Le Magnen, 1950; Schneider et al., 1958;
Good et al. 1976; Mair et al., 1978). Sensitivity
to pain was found to decrease during ovulation
by Buselli et al. (cited in Diamond et al., 1972).
In addition to the physiological changes, a
periodicity in psychological state is related to
the ovarian cycle. Several studies show that
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8
Beverly 1. Strassmann
aggression, anxiety, inner-directed hostility, and
fatigue are lowest at mid-cycle while libido is
highest (McCance et al., 1937; Benedek and
Rubenstein, 1939; Altmann et al., 1941: Hart,
1960; Gottschalk et al., 1962; Moos, 1969:
Luschen and Pierce. 1972). Other studies show
a premenstrual or postmenstrual rise in con-
scious sexual desire (e.g., Kinsey, 1953). The
curve showing that sexual arousal peaks during
ovulation found by Altmann et al. (1941) and
Moos (1969) corresponded closely to a
urve
for
visual sensitivity (Diamond et al., 1972). Adams
et al. (1978) found an increase in female initiated
sexual behavior during ovulation.
Tests of the physiological and psychological
correlates of ovulation in nonhuman primates
would be helpful in determining whether these
traits are vestigial, or attributes that selection
has actively retained or even heightened in hu-
mans. Of
course, analysis of these correlates is
complicated
since one explanation could not ac-
count for all of them. Beyond the idea that these
correlates are simply vestigial remnants of es-
trus, there are at least three possible explana-
tions for their occurrence. The marked improve-
ment in acoustic, visual, and olfactory sensitivity
during ovulation may have improved the ability
of females to avoid rapists operating by stealth
or ambush. In the literature on ovulation, the
correlates are widely regarded as a mechanism
to increase the probability of conception (e.g.,
Diamond et al., 1972). Diamond et al. assert that
the psychological and sensory changes could in-
crease the probability of coitus occurring during
ovulation because of the combined effect of
greater sensitivity to arousing stimuli and de-
creased sensitivity to pain. A third possibility is
that the psychological and sensory changes
tempt females to mate outside the pair-bond with
males having superior genes. This explanation
supports the hypothesis that concealed ovula-
tion evolved because it allowed females to pro-
mote paternal care among the males most prone
to it anyway, but who were not necessarily the
optimal genetic fathers of their offspring. How-
ever, matings outside the pair-bond would not
be expected to be sufticiently successful that the
paternal males would have a low confidence of
paternity.
I thank Richard Alexander for introducing the topic of
concealed ovulation in one of his many interesting
classroom lectures. This paper has benefited greatly
from informative and discerning comments he offered
on all drafts. I also appreciate the helpful suggestions
of Gerald Borgia, Katharine Noonan, Donald Symons.
and Randy Thornhill.
Nancy
Burley, Lee Benshoof,
and Randy Thornhill obligingly sent me their prepub-
lication manuscripts. Joan Strassmann provided much
encouragement.
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