Biology Reference
In-Depth Information
they stay in the pride for two to three years before they, in turn, are driven out by new
males. A male's reproductive life is therefore short.
The lion pride thus consists of a permanent group of closely related females and a
smaller group of separately interrelated males present for a shorter time. Brian
Bertram (1975) considered two interesting observations about reproductive behaviour
in a pride.
Female lions show
synchronous
oestrus
(1)
Lions may breed throughout the year but, although different prides may breed at
different times, within a pride all the females tend to come into oestrus at about
the same time. The mechanism, or causal explanation, is likely to be the influence
of pheromones on oestrus cycles (Stern & McClintock, 1998). But why are
lionesses designed to respond in this way? One adaptive advantage of oestrus
synchrony is that different litters in the pride are born at the same time and cubs
born synchronously survive better. This is because there is communal suckling
and, with all the females lactating together, a cub may suckle from another
female if its mother is out hunting. In addition, with synchronous births there is
a greater chance that a young male will have a similar-aged companion when it
reaches the age at which it leaves the pride. With a companion a male is more
likely to achieve a successful take-over of another pride (Bygott
et al
. 1979; Packer
et al
. 1991).
(2)
When a new male, or group of males, takes over a pride they sometimes kill the cubs
already present (Fig. 1.2b). The causal explanation is not known but it may be the
unfamiliar odour of the cubs that induces the male to attack them. But, whatever
the mechanism, why are male lions designed to respond in this way?
Males kill cubs
after take-over
The benefit of infanticide for the male that takes over the pride is that killing the cubs
fathered by a previous male brings the female into reproductive condition again much
more quickly. This hastens the day that he can father his own offspring. If the cubs were
left intact then the female would not come into oestrus again for 25 months. By killing
the cubs the male makes her ready for mating after only nine months. Remember that a
male's reproductive life in the pride is short, so any individual that practises infanticide
when he takes over a pride will father more of his own offspring and, therefore, the
tendency to commit infanticide will spread by natural selection.
The take-over of a pride by a new coalition of adult males also contributes to the
reproductive synchrony of the females; because all the dependent offspring are either
killed or evicted during the take-over, the females will all tend to come into oestrus again
at about the same time (Packer & Pusey, 1983b). Interestingly, the sexual activity of the
females is most intense during the first few months after a take-over. The females play
an active role in soliciting copulations from several males and this appears to elicit
competition between different male coalitions for the control of the pride, with the
result that larger coalitions eventually become resident. This is of adaptive advantage to
the female because she needs protection from male harassment of her cubs for over two
years in order to rear her cubs successfully (3.5 months gestation plus 1.5-2 years with
dependent young) and only large male coalitions are likely to remain in the pride for
more than two years. High sexual activity in females at around the time of take-overs
may therefore incite male-male competition and so result in the best protectors taking
over the pride (Packer and Pusey, 1983a).
