MANY instincts are so wonderful that their development will probably appear to the reader a difficulty sufficient to overthrow my whole theory. [page 317]
In this episode of Discovering Darwin we covered Darwin's chapter on Instinct and how Chuck attempted to explain how animals exhibit complex behaviors that are not learned. More importantly Darwin was trying to outline how behaviors could evolve in the same way he explained the evolution of physical traits.
It is not too difficult to study the evolution of physical traits because we often have fossil evidence of their transformation. Last episode we discussed the evolution of whales and the plethora of fossil evidence that has allowed researchers to reconstruct the evolution of the terrestrial ancestor of whales to the streamlined marine mammals we see today.
Behaviors are harder to imagine through the lens of natural selection because we can only see those behaviors that are exhibited by extant (living) organisms and behaviors can evolve much faster than physical traits. Cultural evolution can allow individuals within their lifetime to adopt a new behavior that they learn from others. One of the wonderful examples of this is dolphins using a sponge to protect their rostrum (beak) as they hunt for prey in coral structures. They can pass this idea on to other dolphins and you can track the rapid transmission of this behavior through a population.
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| Picture from http://www.livescience.com/21989-dolphin-sponge-tools-culture.html |
Darwin was not interested in learned behaviors in this chapter but he was interested in behaviors that are known at birth or at specific developmental times in the organism's lifespan. Darwin called these behaviors instinct. In exploring this idea Darwin focused on three major examples of innate/instinctual behaviors:
1. Cuckoos and their behavior to dump their eggs in other birds nests.
2. "Slave making" ants species which capturing of other ant species to become sources of forced labor in their own colonies.
3. Honey bees and their complex, mathematically efficient, honeycomb making behavior.
Cuckoo
Cuckoos exhibit a behavior known as "brood parasitism" where they lay their eggs in the nest of a host species and leave the eggs to be incubated and hatched by the host. The host also raises the cuckoo baby as their own until it is strong enough to fledge from the nest. Below is a dramatic photo showing the poor host Reed Warbler dutifully feeding the ginormous common cuckoo (Cuculus canorus) nestling.
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| "Reed warbler cuckoo" by Per Harald Olsen - Own work. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Reed_warbler_cuckoo.jpg#/media/File:Reed_warbler_cuckoo.jpg |
You may wonder why the Reed warbler does not recognize the cuckoo offspring is not their own offspring? What you are really asking is why has the Reed warbler not evolve the ability to recognize its own offspring from another species? If you were switched at birth in the hospital, do you think your mother would know? What ways do we know our offspring are actually ours? Only through hospital tagging or non-interrupted contact are we to "know" the offspring we have are the ones we gave birth to.
Why have we not evolved an ability to recognize our own offspring? Probably because there has not been a selective advantage to recognize our offspring because over evolutionary time it has been rare for humans to be in a situation where we must recognize our newborn from other unrelated newborns. Since that ability is rarely useful, selection has not favored it in our species. In the same manner, birds that nest individually associate those eggs in their nest as being their own. Recognition has not evolved because there is little selective advantage for that ability. However there are birds that do exhibit an amazing ability to discern their own specific offspring among a throng of others. Colonial nesting birds like albatross and penguins have an unerring ability to discern their own chick from maddening crowds because selection has favored that ability.
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| baby penguins awaiting the return of their parents to feed them. Image from Mike Johnson http://www.dailymail.co.uk/travel/travel_news/article-2871252/Tourist-s-incredible-photographs-reveal-stunning-beauty-remote-Antarctic-island-teeming-thousands-penguins-seals.html |
In nesting birds like the Reed warbler it would normally be rare for it to be stuck raising an unrelated offspring so they have not developed the ability to recognize that the over-sized baby is not really their own offspring. Instead the poor Reed warbler probably thinks she has the largest and healthiest reed warbler baby in the world. Feed it some more!
Because nesting birds are less discerning in recognizing their young, brood parasitism has the opportunity to evolve and based upon phylogenetic analysis it seems it has. Brood parasitism has evolved independently seven times in the evolution birds resulting in 75 species out of the 8600 known species of birds exhibiting forms of brood parasitism from occasional indiscretions to those species which never raise their own offspring, instead relying totally on other species to incubate and raise their young.
This extreme form of brood parasitism intrigued Charles Darwin in that a cuckoo could be born in a Reed warbler nest, be raised by Reed warblers, fledged from the nest and go off to grow up and retain its identity as a cuckoo bird and not a reed warbler or whomever was its host species. The identity of the cuckoo was innate, instinctual and expressed itself in the adult females when they reached reproductive age.
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| Image from https://whyevolutionistrue.wordpress.com/2011/03/04/mimicry-the-nefarious-cuckoo/ |
Darwin predicted the cuckoo species we see this complex behavior in various stages of complexity, transitions if you will. There are some cuckoos which specialize in parasitizing a single species of birds while other cuckoo species are generalist and parasitize a wide variety of host species. The research suggests it is difficult to discern if the evolution of cuckoo behavior went from specialist (single host species used) to generalist (many potential host species used), or vice versa. In addition, within the common cuckoo (Cuculus canorus there are families (gentes) who specialize on parasitizing a single host species and the cuckoo egg color has evolved to better mimic their hosts eggs. In the picture below see the cuckoo egg indicated by the arrow in a variety of host nests while showing the great variation in egg coloration within the single species of cuckoo.
By what steps the instinct of F. sanguinea originated I will not pretend to conjecture. But as ants, which are not slave-makers will, as I have seen, carry off the pupæ of other species, if scattered near their nests, it is possible that such pupæ originally stored as food might become developed; and the foreign ants thus unintentionally reared would then follow their proper instincts, and do what work they could. If their presence proved useful to the species which had seized them—if it were more advantageous to this species to capture workers than to procreate them—the habit of collecting pupæ, originally for food, might by natural selection be strengthened and rendered permanent for the very different purpose of raising slaves.[page 338]
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| Polyergus mexicanus workers return from a successful raid with captured pupae of the host species, Formica subsericea. Urbana, Illinois, USA Photo from http://www.alexanderwild.com/Ants |
Sarah discussed the intriguing behavior of slave making ants and how Darwin thought it evolved and how modern science has supported, somewhat, his original view. The biggest hurdle for slave making to evolve is that each ant species relies on species-specific and colon-specific pheromones for individuals within a colony to recognize each other. Individuals from other colonies, and even more so, individuals from other species will smell distinctively different from the slave making ants so how do they suppress aggression towards their newly captured indentured pupae? It was proposed that ants should raid closely related species so that their pheromones are more likely to be similar to reduce aggression between raiders and potential slaves. This is known as the Emery Rule.
Interestingly, scientists since Darwin have been studying the evolution of slave making behavior and much is still to be figured out. The closing remarks on a wonderful review paper about the evolution of social parasitism and slave making behavior in ants makes the following observation:
"Though slave-making ant species have been studied for more
than 150 years, many problems are still open, the most prominent obviously
being the evolution of slave raiding
itself. Recent investigations have given contradictory results concerning, for
example, the mechanisms of chemical
integration of slave makers and their hosts in a mixed society, the pattern of sex allocation
in slave makers, and coevolution between social parasite and host….Considering
that almost all slave-making ant species
are listed as threatened by the World
Conservation Union (IUCN), investigations on
their behavior, population structure, and genetic variability may help
us learn more about how endangered they really
are and if and how they can better be protected." (D'Ettorre & Heinze 2001)
Josh ended the program with a discussion of the perfection of honeycombs and how is that bees make such perfect hexagon shapes?
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| stockphoto from http://www.npr.org/sections/krulwich/2013/05/13/183704091/what-is-it-about-bees-and-hexagons |
Josh explained that cell shapes within the colony can range from a circular shape to the beautifully distinct hexagon shape we associate with honey bees. Two major hypothesis, which are not mutually exclusive, have been proposed to explain the hexagon shape. The first is the efficiency hypothesis which argues that wax is expensive to produce so bees would evolve to be the most efficient in building their combs and hexagon shape requires the least amount of material and produces the least amount wasted space.
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| PHOTOS BY KATHY KEATLEY GARVEY, UC, DAVIS |
Notice this circular form above has many gaps between the cells in contrast the compact arrangement in the hexagon com.
The second hypothesis Josh brought up was the idea that in the bee hive, the bees body heat melts the wax and the wax then forms a shape that requires less energy to maintain so the straight edges between the cells forms as an outcome of soft wax reaching a low energy resting state. In this model the bees are not intentionally creating a hexagon shape but it emerges from their constant activity and body heat.
It is interesting that the questions that Darwin outlined as interesting issues for evolution are still be investigated and we are beginning to understand these instinctual behaviors better because of the evolutionary framework that Darwin gave us over 150 years ago.
The opening theme to Discovering Darwin is "May" by Jared C. Balogh. http://freemusicarchive.org/music/Balogh/Revitalized_Eyes/MAY
Interlude music is Rhapsody In Blue Part 1 by Paul Whiteman and George Gershwin Published 1924 https://archive.org/details/rhapblue11924
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