Contents - Konrad Lorenz Institute

Steven M. Platek
An Evolutionary Model of the Effects of Human
Paternal Resemblance on Paternal Investment
A
Abstract
Species with fertilization internal to females face an
asymmetry in parental certainty: whereas females are
always 100% certain that they will share approximately
50% of their genes in common with their offspring,
males can never be certain of paternity. As a
consequence of this uncertainty, investment in offspring
is similarly asymmetrical and should be, according
to evolutionary meta-theory, proportional to
the confidence of paternity on the part of males. There
are three ways in which a male can attempt to limit
cuckoldry. First, he can monitor his mate during the
time that she is fertile and engage in tactics that limit
her opportunities to engage in extra-pair copulations
(EPCs). Second, males have been designed in such a
way that if an EPC does occur and results in extra pair
paternity (EPP), physiological and behavioral processes
out of his conscious control help to limit cuckoldry—namely,
sperm competition and semen
displacement. Finally, post-parturition he can make
assessments about whether the child shares genes in
common with him by estimating the degree to which
his offspring resemble him. Here I model the third process,
the effect that paternal resemblance might have
on a male’s decisions about how or whether to invest
in putative offspring. Additionally, I speculate about
the effects resemblance might have in situations of
known non-paternity (i.e., step-children and adoption),
instances of suspicion about paternity of any
one offspring, and the effects of kin selection and kin
resemblance.
Key words
Paternal investment, paternal resemblance, social
mirror, evolutionary psychology, fluctuating asymme-
fter making a choice
about with whom to
mate, females are faced
with two potential dilemmas:
1) what is the likelihood
that this male is going
to invest resources in
offspring produced, and
2) does the male provide
“good genes,”—that is,
offer genetic resiliency to
pathogens that will facilitate
the offspring’s development
and reproductive
success
Males, on the other
hand, are faced with a different
dilemma: making
sure that the offspring
they invest resources in
shares copies of his genes.
Males have evolved psychological
mechanisms
that help to reduce the
likelihood of unknowingly
raising another
man’s child. The first
mechanism is to attempt
to limit his mate’s opportunities
for extra-pair
copulation (EPC) that
could eventuate in extrapair
paternity (EPP).
Males appear to have
evolved three sets of psychological
mechanisms
designed to reduce, limit,
and control female infidelity. For example, mate
guarding and jealousy during a female’s most fertile
period will help to reduce and possibly prevent a female’s
chances of becoming inseminated by another
male (BUSS 2000). If
mate guarding fails to
prevent an EPC from
eventuating in EPP, males
unknowingly employ a
number of competitive
strategies that help to ensure
that his sperm reach
the egg, and not another
male’s. These strategies
include, but may not be
limited to, sperm competition
(BAKER/BELLIS 1995)
and semen displacement
(GALLUP et al. 2003).
Sperm competition entails
a number of strategies
that enhance placement
and volume of
sperm in the vagina; e.g.,
ejaculating small
amounts of spermicidal
fluids that act to kill an
interloper’s sperm
(BAKER/BELLIS 1995) and
increasing semen volume
as a function of jealousy
and time spent apart
(BAKER/BELLIS 1995;
POUND et al. 2002), which
effectively increases the
statistical probability that
a male will father the
child. Semen displacement
is the result of interplay
between a morphological
adaptation (the
shape of the penis, in particular the glans penis) and
behavioral adaptations that capitalize on that morphological
adaptation; i.e., the morphology of the
human penis allows a male to employ his penis to
Evolution and Cognition ❘ 189 ❘ 2003, Vol. 9, No. 2