Updated: Oct 19, 2019
The selection process by which females of a species will select their partners for reproduction are typically defined by three criteria:
The distribution of mate quality
The costs of searching for a mate
The quality of the individual choosing a mate
The general rule when selecting a partner for reproduction is therefore that less desirable individuals (low quality) should be less discriminatory when choosing a mate, but generally selection and choice when trying to reproduce of the standard technique for offspring survival (Gibson and Langen, 1996).
While it is generally accepted that costs and benefits will impact a choice, there are species which demonstrate alternative method to ensure reproductive success other than relying solely selection techniques.
Standard mate selection techniques
During mating, males have a literal fight on their hands. What is generally seen in sexual competition for animal species is males competing to win reproductive access to a female. Of course, there are exceptions, but generally females of the species have more to lose from a fitness perspective and are therefore generally ‘sought after’. Techniques to ensure reproductive success during female selection from males will result in conflict among males (Amqvist and Rowe, 2013).
Some species even have males will apparent handicaps which would at first glance demonstrate a negative impact on survival fitness and therefore be a non-desirable trait. Zahavi in 1975 outlined that a handicap could in fact demonstrate robustness of an individual, given that the quality of the male is tested, and survival with a handicap demonstrates a high fitness, which in turn will make it sexually appealing to potential female mates.
Case N: Land snails
Land snails (Arianta arbustorum) are a hermaphroditic group of species that demonstrate random mating and show no behaviours that demonstrate any form of mate selection. Studies show that land snail species also do not possess any indicators for kin identification which means that snails will actively reproduce with full/ part siblings, as well as non-siblings with no preference for or against any particular conspecific (Baur and Baur, 1997).
The study demonstrated that inbreeding with full and half siblings had difference in reproductive success of those who bred with non-siblings. This random breeding technique would typically result in genetic weaknesses from inbreeding, however due to the reproductive behavior that the snails will mate with multiple partners, and generally any other land snail they encounter will result in the reduced genetic inbreeding effects in wild populations. This clearly demonstrates that random breeding and non-selective can have a positive impact on reproductive success in species.
Case N: The Natterjack Toad
Sexual success of the male natterjack toads (Bufo calamita) appears to be related to bdoy size and call volume. Male toads with larger body sizes all maintained larger territories and gave off louder calls than those of small body size. Females that were selective with their mates almost always chose the larger males based on the volume of their calls, which implies that mate selection follows the standard pattern where louder toads tend to be larger and maintain larger territories, demonstrating a higher survival value, despite having calls that could attract predators (Arak, 1987).
The same paper also stated however that one half of the females that arrived at the pond mated with the first male they encountered. This demonstrates a passive approach to reproduction; the females had no preference to male partner size, call volume or territory. Arak explains that there was no competition for males, so there was no reason for half the females to be non-selective based on male numbers as there were plenty of males in the population. He did, however, note that on cold nights, females did not venture to the pond at all. This can be explained by looking at optimum temperatures for oviposition and a successful offspring hatch rate. Females that mate with the first male they encounter could therefore be attempting to increase offspring success rates by quickly taking advantage of warmer evenings in case the weather cools too much for the remainder of the breeding season.
If both of these reproductive selection (or lack of) behaviours are occurring with a 50:50 ratio, then both of them must be equally successful. This does however, demonstrate that reproductive selection techniques extend beyond that of mate selection with an equally successful outcome.
Case N: Atlantic Cod
Fish species that reproduce by spawning are impacted to a degree by male competitiveness. Larger males in Atlantic cod species (Gadus morhua) tend to hold the higher offspring rates, despite all males in the population releasing the sperm-filled milt where females deposit their eggs (Bekkevold et al, 2002).
Although this shows that there is a higher chance of larger males producing offspring, the act of spawning itself is random. This demonstrates that in the case of this particular species, random breeding by means of spawning still benefits the larger (and therefore the fittest) of the species overall, and therefore that random breeding is not detrimental to the reproductive success of the individual.
Random breeding, spawning and inbreeding
Of course there are cases where random breeding has impacted a species negatively. In the case of spawning marine fish, inbreeding is typically avoided with large breeding populations and the movement of shoals throughout the non-breeding seasons (O’Leary et al, 2013). This backs up the above point that spawning as a random breeding technique is beneficial to the individual and the species.
The same study by O’Leary et al, however, shows that in the case of the winter flounder (Pseudopleuronectes americanus) who breed in the estuaries in New York, USA, inbreeding has become an issue and is very common in every breeding population observed and tested in the study. The historically common fish has only recently become prone to inbreeding issues such as genetic disease and this is starting to affect population.
From the case studies outlined, we can see that random breeding as a reproductive technique is as successful as techniques which include complex breeding behaviours seen among species with female selection and male competition. However, such as with the case of the winter flounder, there are issues with random breeding techniques which seem to follow a specific trend: inbreeding. This comes with its own issues such as the introduction of genetic weaknesses and diseases, which clearly can impact a species very suddenly.
Overall, this shows us that while random breeding is successful in many species, the best reproductive method to ensure the fitness of a species or population in relation to longevity is with selective behaviours to ensure the highest fitness of the offspring and therefore the population.
Amqvist, G. and Rowe, L. (2013) Sexual Conflict; monographs in behavior and ecology. Princeton University Press and Princeton Oxford, UK.
Arak, A. (1987) Female mate selection in the natterjack toad: active choice or passive attraction? Behavioural Ecological Sociobiology, Vol. 22, 5. 317-327.
Baur, B. and Baur, A. (1997) Radom mating with respect to relatedness in the simultaneously hermaphroditic land snail Arianta arbustorum. Invertebrate Biology, Vol. 116, 4. 294-298.
Bekkevold, D., Hansen, M. M. and Loeschcke, V. (2002) Male reproductive competition in spawning aggregations of cod (Gasus morhua, L.) Molecular Ecology, Vol. 11, 1. 91-201.
Gavrilets,S., Arnqvist,G., and Friberg, U. (2001) The evolution of female mate choice by sexual conflict. Proceedings of the Royal Society of Biological Sciences, Vol. 26, 1466. 531-539
Gibson, R. M. and Langen, T. A. (1996) How do animals choose their mates? Trends in Ecology and Evolution, Vol. 11, 11. 4668-470.
O’Leary, S. J., Hice, L. A., Feldheim, K. A., Frisk, M.G., McElroy, A. E., Fast, M. D. and Chapman, D. D. (2013) Severe inbreeding and small effective number of breeders in a formerly abundant marine fish. PLOS, Vol. 8, 1.
Zahavi, A. (1975) Mate selection – A selection for a handicap. Journal of Theoretical Biology, Vol.53, 1. 205-214.