Biology Reference
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(a)
(b)
(c)
Fig. 7.4 (a) Male southern elephant seal. (b) Two males fighting for harems of females. (c) The male is huge
compared to the female. These are two subadults. Photos © Oliver Krüger.
the largest harems; in one long-term study in California by Burney Le Boeuf and
Joanne Reiter (1988), each year as few as five out of 180 competing males were
responsible for 48-92% of the matings with 470 females. Adult males were from
three to seven and a half times as heavy as adult females. While most females first
bred at four years of age, males delayed attempting to breed until 6-9 years old,
building up their size and strength so they had a chance in the intense fights for
females. Whereas a female's reproductive success depended on her ability to nurture
offspring, a male's depended on his mating success resulting from fights and defence
of harems.
The evolution of sexual dimorphism leading to larger male size, strength or weaponry
is, therefore, easy to explain because these traits increase a male's success in male-male
combat for mates.
Larger and
stronger males
often gain the
most females
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