Biology Reference
In-Depth Information
different physiological and life history constraints which may predispose one sex to care
more. Secondly, ecological conditions and mating opportunities will influence the costs
and benefits of care for each sex. We shall illustrate these two themes by contrasting
patterns of care in birds, mammals and fish.
Birds
As we saw in the first chapter, reproductive success in birds can be limited by the rate at
which food is delivered to the nest. When two parents can feed twice as many young as
a single parent, both male and female will increase their reproductive success by staying
together. If either sex deserted, they would approximately halve the output of the last
brood and would also have to spend time searching for a new mate and nest site before
being able to start again. So monogamy and parental care by both male and female are
not hard to understand.
When the constraint of having to have two parents feeding the young is lifted, it is
usually the male who deserts and the female who is left caring for the brood. Comparative
evidence shows that polygyny often occurs in fruit and seed eaters, probably because
these food supplies become so seasonally abundant that one parent can feed the young
almost as efficiently as two (e.g. weaver birds, Chapter 2). Why is it the male who deserts?
There are two factors which may be important. Firstly, the male has the opportunity to
desert before the female. With internal fertilization, she is left literally holding the babies
inside her. Secondly, the male can often gain more by desertion than the female because
his lifetime reproductive success depends more on his number of matings (Chapter 7).
Biparental care
common in birds
Mammals
In mammals, females are even more predisposed to care for the young. The offspring
often have a prolonged period of gestation inside the female, during which the male can
do little direct care (though he can protect and feed the female). Once the young are
born they are fed on milk and only the female lactates. Because of these constraints on
the opportunity to care for offspring and also because, with internal fertilization, the
male can desert first, it is not surprising that most mammals have parental care by the
female alone, with the male deserting to seek further matings.
Monogamy and biparental care occur in a few species where the male contributes to
feeding (carnivores) or to carrying the young (e.g. marmosets). Perhaps it is surprising
that male lactation has not evolved in these cases (Daly, 1979).
Female only care
common in
mammals
Fish
In the bony fish (teleosts), most families (79%) have no parental care (Gross & Sargent,
1985). In those families which do care for the eggs or young, it is usually done by one
parent; biparental care occurs in less than 25% of the families which show care.
Compared with the elaborate care of offspring by birds, parental care in fish is a simple
affair often consisting of just guarding or fanning eggs. These tasks can usually be done
effectively by one parent alone. Which parent will provide care? Table 8.2 shows that
