Biology Reference
In-Depth Information
partner's work rate as a cue to how hard it should work. This might lead to matching
responses if increased effort by one parent signalled to the other parent that the brood's
needs had increased (Johnstone & Hinde, 2006).
Future work needs to investigate the relative importance of compensation and cueing
in parental responses. The main message from this example is that there can be fruitful
exchanges between theory and empirical studies; when a model fails to predict observed
behaviour this can often provide insights into aspects of natural history that the theory
has neglected.
Sibling rivalry and parent-offspring
conflict: theory
Everyone is familiar with human family conflicts, if not from personal experience then
from literature and art. So it is perhaps surprising that scientists were slow to recognize
the importance of conflict in the evolution of interactions in animal families. In 1974,
Robert Trivers published a paper which has changed our perception of family life.
Let's begin by returning to Fig. 8.2, where we considered a parent's view of how
investment should be allocated to a particular offspring. Trivers's insight was that the
optimal investment would be different from an offspring's point of view. It is useful to
distinguish two sources of conflict. The first is
intrabrood
conflict. Consider a species
with sexual reproduction and a brood of two offspring (Fig. 8.7a). A parent is related
genetically to each offspring by 0.5 (half an offspring's genes come from the other
parent). Therefore, there is no genetic reason for a parent to favour either offspring
Robert Trivers's
theory
Intrabrood
conflict: each
offspring should
demand more
than its fair share
from the parent's
point of view
(a)
(b)
Intra-brood conflict
Inter-brood conflict
Parent
Parent
0.5
0.5
0.5
Current
offspring
0.5
Offspring 1
Offspring 2
1.0
1.0
0.5
1.0
0.5
Future
offspring
Fig. 8.7
(a)
Intrabrood conflict
. A parent with two offspring in a brood; the parent is
equally related to both offspring (r = 0.5) but each offspring is more related to itself
(r = 1) than to its sibling (r = 0.5, if full sibling). (b)
Interbrood conflict
. A parent with one
offspring per brood. Again it is assumed that the offspring in the next brood is a
full-sibling (same father and mother). The current offspring values itself (r = 1) more than
its future full sibling (r = 0.5), whereas the parent is equally related to both offspring
(r = 0.5).
