They both rely on scent to navigate through their environment. Sperm use chemotaxis to navigate to eggs, and newborn puppies use olfaction to navigate to the milk in their dam’s nipples, from the day their born.

For animals like fish, that reproduce outside of their bodies, it’s obviously crucial to reproductive success for sperm to detect eggs at a distance, and swim to them to fertilize. While less obvious in mammals, where fertilization occurs inside the body, studies suggest that sperm navigate their way to an egg by detecting chemicals in the environment. Sperm don’t just laze about and bump into an egg by accident. Rather, sperm detect the odor of the egg, swim to it, and can then penetrate the egg’s outer layers to fertilize. Here’s an actual photo from an electron microscope of sperm at the surface of an egg¹. 

On a microscopic level, smells are chemicals, which are detected at a distance and attract sperm, which swim towards those chemicals. Sperm don’t have the complex olfactory system of mammals. But animals’ olfactory systems perform a similar function, helping the animals navigate their environments (5). Olfaction helps animals detect chemicals at a distance, and use them to navigate to resources such as food and mates. Microscopically, the scent molecules fit into receptors on the surface of the cells in the animal’s nose, like a lock fits a key². Voshshall’s research showed that the same hOR 17-4 receptor is present in sperm cells, testicles and cells in the nose³. Future research will determine if egg scent could be used to improve the motility of sperm, and if unattractive scents that disorient sperm could be used as contraceptives.

20 Minute Old Sniffer Puppies

Of course, olfaction doesn’t end at conception. Ultrasounds of puppies 2 weeks before birth show them exercising their breathing muscles in the womb, before they are born. This likely allows the puppy to learn about his mother’s unique odor, including the type of food she eats most and the common smells in her environment.

In one experiment, pregnant bitches were fed anise in the last 3 weeks before whelping. Just 20 minutes after birth, and before the pups had even begin to suckle, the puppies moved towards the smell of anise. In contrast, the puppies did not move towards vanilla, which was absent from their dam’s diet. The newborn puppies’ preference for anise may be due to the fact that the puppies “smelled” anise in their dam’s amniotic fluid, and produced more olfactory receptors to detect anise as a result¹.

How does that happen? Mammal’s noses adapt to scents prevalent in their environment using “up-regulation”. Like skin cells, olfactory cells are regularly replaced. When an animal smells a scent frequently e.g. anise, over time his nose adapts by producing more anise receptors in his olfactory cells. He becomes more proficient at smelling anise. So the more a puppy uses his nose to detect a target scent, the easier it is for him to find that scent, with less wasted energy spent searching.

Puppies Smell to Survive

The sense of touch operates in close quarters. You need to be close to a fire to feel its heat. But olfaction allows animals to detect chemicals at a larger distance, providing long-distance information on prospective mates, family members, food sources.

One reason why sniffer dogs are so popular is that they allow humans to answer important human questions, by searching larger areas, faster, and more conveniently than visual inspection by humans. For example, bed bugs can fit in a 1 mm crack in a bed frame, and it’s time consuming and disruptive for an exterminator to take the bed apart to find them. But a reliable bed bug detection dog can rapidly determine upon entering the room, whether there is bed bug scent or not, without moving the mattress.

Scent is key at life stages, but newborn puppies dramatically demonstrate its necessity. At birth puppies are blind, and cannot hear for at least 10 days. A dam simply cannot make every puppy in her litter suckle at her nipples every few hours. In order to survive, each puppy must rely heavily on its sense of smell, most importantly, and supplemented by taste and feel operating in closer proximity⁴. The next time you watch a puppy sleeping and wonder what he’s dreaming of, think less of what you see and more on the scents he’s smelling.

Subscribe to our blog to learn more about scent as we follow a litter of sniffer puppies as they are introduced to scent work.

Resources

1. Sperm and egg cells Powerpoint Images, Wellcome Images https://bigpictureeducation.com/sperm-and-egg-cells-images
2. Press Release: The 2004 Nobel Prize in Physiology or Medicine to Richard Axel and Linda B. Buck”. Nobelprize.org. Nobel Media AB 2014. Web. 18 Oct 2017. http://www.nobelprize.org/nobel_prizes/medicine/laureates/2004/press.html
3. Voshshall, LB. Olfaction: Attracting Both Sperm and the Nose. Current Biology, Vol. 14, R918–R920, November 9, 2004. http://www.sciencedirect.com/science/article/pii/S0960982204008048
4. Bradshaw, John. Dog Sense: How the New Science of Dog Behavior Can Make You a Better Friend, Basic Books, New York, 2011
5. Lucia F. Jacobs. Function of olfaction, Proceedings of the National Academy of Sciences Jun 2012, 109 (Supplement 1).
   https://www.pnas.org/content/109/Supplement_1/10693