Why do we love? An empirical test…

Archetypal lovers Romeo and Juliet portrayed by Frank Dicksee

Yeah love is indeed a mysterious thing and has always captured our imaginations. One of the most famous tragic love stories was the Romeo and Juliet by William Shakespeare. Tragic in the sense that the main protagonists die at the altar of their own love. So, what makes love so special? Or indeed as a biologist i ask what is the need for love. Just look what goes into the love process. Endless dating games, elaborate preparations, endless flirtations, also many humiliations and finally if you are lucky the one acceptance.

But wouldn’t it be simpler to just think about procreation alone, i.e., reproduce for the sake of propagation?? Since, evolutionary struggles dictate that there exists differential reproduction and hence propagation of one’s own genes is the thing which ultimately matters. So, then why do we go for this protracted cycle?

To answer this question albeit in an indirect way authors – Malika Ihle, Bart Kempenaers and Wolfgang Forstmeier all from Department of Behavioral Ecology and Evolutionary Genetics, Max Planck Institute for Ornithology, Seewiesen, Germany conducted a remarkable experiment. The results of this experiment was published recently in PloS Biology – Fitness Benefits of Mate Choice for Compatibility in a Socially Monogamous Species.

As we know that to actually conduct a cost/benefit analysis of love is easier said than done and there would be innumerable ethical concerns regarding the bounds of experimentation with humans. This present study however, used a model animal in an elegant experiment which was designed to find the reproductive consequences of mate choice.

The Experiment

The model species used here was the –zebra finch (Taeniopygia guttata, a native bird of Australia).

Adult male at Dundee Wildlife Park, Murray Bridge, South Australia

Adult male at Dundee Wildlife Park, Murray Bridge, South Australia

They started off with a population of 160 birds that had recently been isolated from the wild, and then set them up on a sort of speed-dating session, with groups of 20 females to choose freely between 20 males (See figure 1 below). Once the birds had paired off, half of the couples (the “chosen” or C group) were allowed to live happily ever after. For the other half, however, the authors intervened like overbearing Indian parents, and split up the happy pair to forcibly pair them up with other broken-hearted individuals (the “non-chosen” or NC group). The bird couples of both C and NC groups were then left in aviaries to breed. The authors then measured the couple’s behaviour and the number and paternity of dead embryos, dead chicks and surviving offspring.

Experimental Design

Figure 1: Experimental Design


Relative fitness estimates (mean ± SE) of males (n = 84) and females (n = 84) from chosen and non-chosen pairs

Figure 2: Relative fitness estimates (mean ± SE) of males (n = 84) and females (n = 84) from chosen (C) and non-chosen (NC)pairs

The first batch of results is elegantly shown in the figure above.The overall reproductive fitness (measured as the final number of surviving chicks) was 37% higher for individuals in chosen pairs than those in non-chosen pairs. But since reproductive fitness is the sum total of different effects which add up to the total number of offspring produced, it’s vital to look at those parameters and understand the mate choice in C group affected the fitness. To start off the authors noted that both the C and NC group laid similar number of eggs which suggests that their initial investment towards egg laying is not affected by the group they are in and also oblivious to the mismatched mate picked up by the authors. But the nests of NC group had almost three times as many unfertilized eggs as the chosen ones, and a greater number of eggs that were neglected (either buried or lost).

The authors in their earlier studies had known this fact that embryo deaths happened mainly due to genetic incompatibility between the parents, however the egg hatching related deaths happened due to behavioural incompatibility. So, the next step was to compare these two phenomena in the two C and NC groups. They found that though the embryo mortality was similar in both the groups, however mortality of the hatched chicks was comparatively
higher in the NC couples. This suggests that its the behavioral incompatibility
between the non-chosen (NC) parents, and not genetic incompatibility which might be the driving factor behind the observed reduction in overall fitness (Fig. 3, below).

Embryo (A) and offspring (B) mortality rates (parameter estimates [mean ± SE]) in chosen and non-chosen pairs.

Figure 3: Embryo (A) and offspring (B) mortality rates (parameter estimates [mean ± SE]) in chosen (C) and non-chosen(NC) pairs.

So, the next question which the authors asked was if it’s the behavioural incompatibility which leads to greater hatchling death then can it be observed during the elaborate courtship rituals which happened before pairing? What they found was that although the NC and C couples spent almost similar time in courtship rituals the NC group females were far less receptive to NC males and also tended to copulate lesser compared to C group. Harmonious behaviour during courtship in zebra finches have been studied in detail and is taken as sum total of these: friendliness, mutual following, synchronous activity etc. So, a couple showing these behaviours in a greater amount would be termed as the ones who show behavioural compatibility and in anthropomorphic terms ”are in love”. An analysis of this behaviour among the C and NC couples showed that on an average the NC couples showed far less such behaviour than the ones in C group.Apart from these results, when the chicks hatched what was seen that greater proportion of males in NC group showed infidelity than in C group and the majority deaths of chicks which happened in the critical period of first 48 hours was due to lesser paternal care in NC group than in C. 


The authors in the end ascribe this difference in reproductive fitness to the behavioural incompatibility between the two groups. They also mention – ‘‘The mechanisms behind such behavioural compatibility, in terms of willingness or ability to cooperate with certain individuals and in terms of coordination between partners need further study, in particular in the context of offspring provisioning.”

In humans, some studies suggest that individuals are more satisfied, more committed, and less likely to engage in domestic violence, when involved in a love-based rather than an arranged marriage (2,3,4). The challenge there is also to find out whether stable and happy marriages result from motivation to cooperate (and to identify what stimulates such feelings, see 5-8), or from congruence in terms of partners’ intrinsic behavioural types [9].


  1. Ihle M, Kempenaers B, Forstmeier W. Fitness Benefits of Mate Choice for Compatibility in a Socially Monogamous Species. PLoS Biol. 2015; 13(9): e1002248. doi:10.1371/journal.pbio.1002248
  2. Xu XH, Whyte MK (1990) Love matches and arranged marriages—A Chinese replication. Journal of Marriage and the Family 52: 709–722.
  3. Sahin NH, Timur S, Ergin AB, Taspinar A, Balkaya NA, Cubukcu S (2010) Childhood trauma, type of marriage and self-esteem as correlates of domestic violence in married women in Turkey. Journal of Family Violence 25: 661–668.
  4. Regan PC, Lakhanpal S, Anguiano C (2012) Relationship outcomes in Indian-American love-based and arranged marriages. Psychological Reports 110: 915–924. PMID: 22897093
  5. Asendorpf JB, Penke L, Back MD (2011) From dating to mating and relating: predictors of initial and long-term outcomes of speed-dating in a community sample. European Journal of Personality 25: 16– 30.
  6. Honekopp J (2006) Once more: Is beauty in the eye of the beholder? Relative contributions of private and shared taste to judgments of facial attractiveness. Journal of Experimental Psychology-Human Perception and Performance 32: 199–209. PMID: 16634665
  7. Meltzer AL, McNulty JK (2014) “Tell me I’m sexy . . . and otherwise valuable”: Body valuation and relationship satisfaction. Personal Relationships 21: 68–87. PMID: 24683309
  8. Todd PM, Penke L, Fasolo B, Lenton AP (2007) Different cognitive processes underlie human mate choices and mate preferences. Proceedings of the National Academy of Sciences of the United States of America 104: 15011–15016. PMID: 17827279
  9. Rammstedt B, Spinath FM, Richter D, Schupp J (2013) Partnership longevity and personality congruence in couples. Personality and Individual Differences 54: 832–835.

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