Love and Biology

From Ihle, Kempenaers, and Forstmeier

From Ihle, Kempenaers, and Forstmeier

If two people marry each other to mutually benefit each other’s rise to the top of the political ladder or because their combined wealth will give their child a worry free life, what’s love got to do with it? If a man married his wife because his mother arranged it, would that affect the affection he showed towards their son? Researchers of the German Max Planck Institute for Ornithology studied zebra finches to find out if being able to freely choose one’s mate, ensured reproductive success.

The zebra finch is a monogamous species of bird that tends to stay with its chosen mate for life. With only one mate, however, the rate at which baby finches can be produced is limited. With the goal of creating offspring that will continue to pass down genes, many organisms including lions, deer, and crickets choose the polyandrous route and mate with numerous partners. The married zebra finch is not at a disadvantage, however. Fitness is a means of gauging success in passing on genes. This study found the relative fitness of finch pairs that chose their mate and compared it to those whose mate was chosen for them.

160 zebra finches freely chose a partner of the opposite sex. Following, half of the pairs were assigned to a random partner instead of their chosen one. Two more rounds of either allowing pairs to stay together or force choosing a partner for them followed this. After each pairing cycle, the couples were allowed to court and mate (get to know each other and have sex). Either good parenting or good genes would be the factor to determine if the group of finches choosing their own mate or the group with their mate chosen for them as having more fitness.

The study found the pairs of finches whom chose their own partners (C) were 37% more reproductively fit than pairs with mates force chosen for them (F). Many factors played into this:

– (C) vs. (F)
– Divorce rate: 8% vs. 24%
– Disappeared or buried eggs: 12% vs. 19%
– Offspring mortality: 32% vs. 52%
– Male courting outside his pair: Once every five hours vs. once every three hours

The higher fitness of zebra finch pairs that chose their own mates (C) supports the behavioral compatibility model. The finch pairs may have chosen each other based on harmony in parenting or general behavior according to the study. The downfalls seen within the (F) pairs were lower nest attendance by males, reduced motivation to care for young, chronic stress with a forced mate, lower female responsiveness, and higher male cheating rates with compatibility not increasing as marriages lengthened.

Is love the ultimate motivator for ensuring family success? Studies with people follow similar conclusions to this study in that emotionally satisfying relationships showed more commitment and less domestic violence. Lions can have their harem. I for one am a proponent of love (and so is science it seems).

Malika Ihle, Bart Kempenaers, Wolfgang Forstmeier. Fitness Benefits of Mate Choice for Compatibility in a Socially Monogamous Species. PLOS Biology, 2015.
DOI: 10.1371/journal.pbio.1002248