Within the same species, we can
observe morphological differences between male and female. These differences,
called sexual dimorphism, can be so important that the two genders are mistaken
for two different species. In 1758, a famous naturalist, Linné, classified male
and female mallard duck as two separate species. The first naturalists, years
later, who observed these two “species” copulate together were very surprised.
The female, in gray-brown tones, is
blended in its environment, while male with a bright green head is clearly
visible. The females colour can be explained by natural selection. A process,
which over time will select the most adapted individuals to their
environment. Thus adorned, she is hard
to spot by predators, giving her an advantage over more colorful individuals.
Why does the male of this same species have bright colors? This is explained by
Male (foreground) and female (second
plan) mallard duck.
The sexual selection, as Darwin
wrote, “depends on the
advantage which certain individuals have over other individuals of the same sex
and species, in exclusive relation to reproduction”. That means in access to mating
partners. Some morphological or behavioural traits are an asset to seduce a
mating partner. These traits are called secondary sexual characters and are
submitted to the sexual selection. Primary sexual characters are genitals and
body parts used in parental care like mammary glands in mammals.
Secondary sexual characters are very
diverse: size, colour, behaviour, some attributes such as antlers in deers,
antenna length and cerci morphology in some insects.
A: Elephant seals, male is three times bigger than female.
B: Deer, spotted to these antlers, and doe. C: Great Capricorn beetle, male
antenna are longer than female. D: mandarin duck, male colours are brighter
than female. E: Indian peafowl, male has a very colorful tailwhich he uses to seduce females. F: Common earwig, male
cerci are more curved than female.
Secondary sexual characters have evolved
by selection pressure. In general, this is done by sex, which invests the most
for each descendant, over other sex. Parental investment of females being
greater than parental investment over males for most species, secondary sexual
characters are mainly expressed by males. When parental investment is almost
the same between the two genders, there is a mutual sexual selection: each sex
exerts a selection pressure on the other.
There exist two types of sexual
selection: intra-sexual selection and inter-sexual selection.
Intra-sexual selection is access to
mating partner by competition between individuals of the same sex. Inter-sexual
selection is access to mating partner by choice of sex exercising a selection
pressure over the other sex.
Competition between individuals of
same sex can be done by direct and violent clashes like with cervidae(https://www.youtube.com/watch?v=FMRO__1YH5o),
or ritualized as in some snakes (https://www.youtube.com/watch?v=UGjwnf0r6-U).
When there is a choice by sex
exercising a selection pressure (female in general), this one can base its
choice on benefits that it will obtain from its mate (male in general). These
benefits can be direct or indirect.
Direct benefits are nutritive
resources, territory quality, protection or parental care brought by a second
Nutritive resources provided by a
second mate can take the form of nuptial gifts. Male nursery web spiders bring
a nuptial gift wrapped in silk containing food resources. Once a female grabs a
gift, male can begin to copulate with her. Females can decide to stop
copulation at any time. It has been demonstrated that bigger gifts, longer
copulation time and greater number of eggs fertilized by male. This tends to
show that females are sensitive to nutritive resources provided by male.
nursery web spider bringing a silk nuptial gift.
Figure showing respectively the relationship between size of
the nuptial gift and duration of copulation (left figure) and between the duration of
copulation and percentage of fertilized eggs (right figure).(STÅLHANDSKE,
Quality territory can mean food
resources are numerous, predation risk is lower and the temperature environment
allows an optimal growth of young, especially for aquatic environments.
Second mate can provide protection.
An individual who feeds is not very vigilant of surrounding predators. Having a
mate with highly developed vigilance allows it to feed itself longer. Female
grey partridge performs precisely a choice on this behaviour trait.
Male grey partridge in position of
vigilance. (credit : Nicolas Bestel)
Mate protection can also be provided
against congeners. In many species, males to copulate ardently solicit females.
Even after forming a stable couple, this sexual harassment can continue, hence
the need for a female to mate with a socially dominant male.
Parental care that will be provided
by the second partner can be appreciated by the presence of some traits in the
latter. Color intensity found in some animals is a secondary sexual characterreflecting
their ability to find food in their environment. Therefore, the pigments we see
are pigments found in the dishes they eat. This is particularly the case of the
flamingo whose colour derives prawn.
In blue tits, this phenomenon is
observable with yellow colour on breast. Males and females that express an
intense yellow colour will feed their chicks more often than dull ones.
Blue tits feeding its chicks.
between the proportion (%) of caterpillars delivered to the young and adult
plumage yellow coloration (carotenoid chroma) for male and female blue tits. ( et al, 2012)
In this species, both genders can
choose a mate in the function of parental care quality that it will bring.
Direct benefits are various and
include the immediate advantages provided to mates and offspring.
Indirect benefits are benefits
genetically transmitted to offspring. Benefits can simply be a transmission of
secondary sexual character making the son, in adulthood, just as attractive as
the father, if not more. In other case, in addition to being attractive to the
opposite sex, secondary sexual character provides an immune advantage. This is
particularly the case with the yellow color seen previously in blue tits. In
addition to being a sign of good nutrition for individuals, it is also a sign
of a good immune system.
These indirect benefits provided to
each descendant are at the origin of the evolution of secondary sexual
characters, the essence of sexual selection. Several models can explain this
evolution. They will be detailed in a future article.
J. – 1990, Females choose vigilant males: an experiment with the monogamous
grey partridge Perdrix perdrix.
2. DARWIN C. – 1871, The Descent of Man and Selection in
Relation to Sex. John Murray, Lond Research.
,– Plumage yellowness predicts
foraging ability in the blue tit Cyanistes
caeruleus. Biological Journal of the Linnean
4. STÅLHANDSKE P. – 2001, Nuptial gift in the
spider Pisaura mirabilis maintained
by sexual selection. Behavioral
5. TRAIL P.W. – 1985, Courtship disruption
modifies mate choice in a lek-breeding bird. Science, n° 227, p. 778-780.