My brother and I constantly hear from others that we are "obviously related." We do have similar features, yes, and sometimes we act alike, but is there really a way to be "obviously related?" With the exception of identical twins, humans tend to take for granted that they only recognize someone as a sibling or relative because they are told so; many people claim to be able to "sense" that someone is related to them, but overwhelmingly, humans seem to identify kin by association. Some may claim that other factors are involved, including parent-child bonds or even spiritual connectedness, but do we really know for certain? In examining this topic, what seemed intriguing to me was how organisms other than humans are able to recognize kin. What I was most interested in were the biology and variations of kin recognition across a large range of species, including other non-human mammals, birds, reptiles, amphibians, fish and even insects.
The first issue to consider is the reasoning behind kin selection: Why be able to identify kin? What benefit, if any, is there to an organism if it is able to recognize kin? The two main concepts concerning this issue are nepotism and kin avoidance. What appears to be a beneficial trend among many organisms is their ability to identify their own kin, in order to prevent or lessen the instance of inbreeding. The other reason for kin recognition is for an organism to show favoritism to kin (nepotism) in order to ensure the passage of their genes to future generations. There is a theory that applies to this, called the "inclusive fitness theory," in which an individual unconsciously considers not only its own survival, but the survival of those sharing the same genes. When looked at within individual species, the reasoning for kin identification (and the techniques with which it is accomplished) varies greatly, yet still always relates to nepotism and/or kin avoidance.
I wanted to work my way through organisms much less related to humans than other mammals, in order to gain some perspective. Insects seemed like the best place to start. Eusocial insects, like honeybees (Apis mellifera) and paper wasps (Polistes), seemed to be the insects to focus on while considering kin selection. Female siblings theoretically have a higher level of relatedness to each other ( compared to the usual , so they are more closely related to their sisters than they are their parents) so assisting in the raising of their female siblings instead of investing energy in their own offspring still proves to be indirectly beneficial to the passing on of their genes. With paper wasps, it was found that they could only recognize kin if they were in the presence of their own nest, or otherwise they wouldn't be able to pick up on the chemical cues needed to identify kin. Honeybees are more selective, most likely due to the fact that their queens mate with several different males in contrast to the single male usually mated with by wasp queens. Being able to more closely identify relatednessby smelling themselves and then relating that odor to the similar odors of relativesis beneficial because other female workers may be more closely related than others. After the bees determine the level of relatedness, they are then faced with the option of how to act towards them: tolerant/intolerant or helpful/apathetic?
Concerning kin recognition, fish are not studied as much as other organisms, but that isn't to say that data are lacking. In a study on rainbowfish (Melanotaenia eachamensis), it was found that even though they shoal together in large groups of relatives, they are still able to distinguish between individuals with different levels of relatedness well enough to avoid detrimental inbreeding. Females were found to have a higher preference for associating with female full siblings over female half siblings, and a higher preference for female half siblings over female non-siblings. Females were also found to avoid male relatives. Visual cues played a large role in such kin discrimination, and it is still under speculation whether or not olfactory cues play a small part. Other species of fish are known to prefer schooling in waters occupied by kin.
Some amphibian species use chemical, behavioral, and/or morphological cues to identify kin. Like fish, schooling amphibians prefer waters shared by siblings. The most intriguing cases I found regarding amphibians, though, were those pertaining to cannibalism. Tiger salamanders (Ambystoma tigrinum) and spadefoot toad (Pelobatidae) tadpoles develop into either the typical small-headed morph or a large-headed cannibal morph depending on population characteristics and food availability. When placed with kin and non-kin, cannibalistic tiger salamanders appear to prefer associating with (and eating) non-kin, and they are able to identify relatives through olfaction and chemical cues. Spadefoot tadpoles of the large-head cannibal morph actually take a small bite out of other tadpoles and then use that input to determine whether or not they should eat them due to the amount of kinship. Kin recognition is very beneficial to these amphibians because eating their relatives is obviously detrimental to the survival of their genes.
Reptiles (with the most commonly known exclusion of many species within Crocodylia, who protect and guard their young) are not known for "family values," or social interactions dealing with kin discrimination (at least, not nearly as much as other animals). There are very few cases in which kin are recognizedand treated differently because of itin the lizard world. The green iguana (Iguana iguana) and the black rock skink (Egernia striolata) are two examples of lizards that can recognize their kin (they do so through smell). Snakes, though, until very recently, were thought to not have any sense of kin recognition at all. A study was done with timber rattlesnakes (Crotalus horridus) that showed there to be kin discrimination regarding the behavior of females towards other females from the same litterin contrast to their treatment of females not from the same litter. The females spent more time with the females from the same litter. This concept of these timber rattlesnakes' gathering/association being influenced by kinship is a relatively new idea, and is likely to make scientists reevaluate beliefs concerning snake behavior and the behavior of other reptiles.
There is so much more information available on kin recognition in birds and mammals than any other of the taxa I came across, so I'm going to have to be more selective in my discussion. Many species of bird merely treat the eggs in their nests, and the resulting hatchlings, as their own offspring. One of those species would be the Bank Swallow (Riparia riparia), which uses location (the nest) as an indirect cue with which to identify offspring. After they start to leave the nest, though, the adult swallows begin to recognize their offspring by voice, a direct technique. This seems closer to the idea that many humans have about kinship than any of the previously mentioned organisms, but yet it lacks some of the judgment that we normally would attribute to humans; some birds will raise and feed the offspring of other birds who laid an egg(s) in their nestlike the infamous Cuckoo (certain members of Cuculidae). Other birds don't even recognize eggs and will incubate rocks if they are placed in their nest. We hope, as humans, that we don't make similar judgment errors.
Since humans are mammals, identifying kin recognition in other mammals may supply some insight into how (if at all) humans are able to recognize kin. Female house mice (Mus musculus) relate kinship to odor, and associate more with females that have a similar odor to themselves, and associate less with males that smell similarlyto ensure the passing on of their genes by helping (to raise/protect) nieces and nephews, and to avoid inbreeding. Mexican free-tailed bats (Tadarida brasiliensis) live in massive colonies (containing millions of individuals) but yet they are able to recognize their offspring by olfaction and vocalizations. Among mammals, Belding's ground squirrels (Spermophilus beldingi) are perhaps the most well-known for their kin-recognition abilities. They get close to each other and smell secretions from scent glands on the face, and then are able to tell the relative level of relatedness. This technique is classified as a phenotypic matching. The Belding's ground squirrels were studied after the females were raised apart, so that association was not a factor. Many other mammals are able identify kin by olfaction, vocalizations, or a variety of other techniques.
Now, on to humans. I was surprised to have discovered studies pertaining to olfaction as a device for humans to detect level of kinshipI had never heard of this before in any of the biology or psychology classes I've taken. Apparently individuals had an easier time being able to identify close blood relatives than with strangers or more distant relatives. Less accuracy was shown concerning cousins and half-siblings and there was even less accuracy (or no accuracy at all) pertaining to stepsiblings or stepchildren. This still was surprisingI've never thought of smelling any of my relatives, nor have I wondered if I'd be able to tell my brother from a stranger if I was forced to only use my sense of smell. Perhaps societal views concerning family values or kinship (tracing back thousands or millions of years ago) have made this ability no longer beneficial or useful to the survival of mankind. Maybe we are fully capable of surviving without having to identify our true kin, and that is why as you trace kin recognition through other organisms, more primitive taxa are able to identify kin more readily than those that came later on an evolutionary scale.
Did humans or their ancestors have more highly tuned kin-recognition skills before it became socially irrelevant? Is it possible to find that out? Does anyone really care about this other than me? Many people don't really care whether or not they are related to someone, because it's how they feel that matters, not how closely related they are. This brings me to my final series of questions, which were inspired by watching daytime television:
Hypothetically, would you still feel inclined to deal with your horribly dysfunctional, abusive family, if you found out that you were adopted? Would you still feel the same about someone you disliked if you found out that they were, in actuality, your fraternal twin? And lastly, do you really think your emotions would win out over biology if you were to find out that the person you were planning on marrying was revealed to be your second cousin? Just think about it.
Relatedness does play a large factor in our society and how we live our everyday lives. This could probably be traced back to the hypothetical social structure of early hominidssmall groups of hunters and gatherers who traveled and lived together and were not necessarily related. I started off trying to find out more about kin recognition in humans and other organisms, but I just ended up realizing that in the end, it was a matter far greater than just biology We as a species may not be able to recognize our kin very well (from a biological standpoint), but that is most likely because, simply, we don't have to.