Once Grud was deep asleep, something started to happen to his body. After consuming the whole Tusk Pig, his body had enough nutrients and biomass for the mutation to complete the densification of his body and had some extra to fill out some mass on his emaciated looking body.
At the same time the mutating agent in his body was extracting the beneficial genes of the rodent and Tusk Pig he ate earlier. And if you are asking, "Why was Grud's body not affected by all the meat he ate back at the tribe?" The answer is simple. The Rock Skull tribe are terrible cooks, all their meats were cooked past well done like the barbaric culinary philistines that they are. After cooking it through for so long, any genetic data that could have been extracted by the mutating agent in Grud's body were all damaged beyond recognition and passed off as normal biomass.
Another feature of the mutation agent is gene and DNA manipulation and assimilation to optimize the host body in doing what the host is aiming to do. When the mutation agent is first injected into the body it would optimize what is already in the body until a new compatible gene and DNA is introduced to the system, where the mutation agent can extract and manipulate what is needed and the incompatible or useless gene and DNA are processed as biomass.
And at the moment, the host body is aiming to hunt, reproduce and protect tribe and family. With those objectives, the mutation agent can definitely help with. From the rodent, the mutation agent managed to mutate Grud's nervous system to have response time now three times faster than what it used to be, to help with hunting.
And because zoologists were not a thing yet, there was another thing that nobody knew about this rodent, it was the males of the species were sex machines during mating season, they would literally have as much sex as they could get until their heart failed from overstress.
And the mutation agent decided that would help with the host's goal of reproducing by lowering the refractory period to seconds and increasing the sperm and semen production drastically which would synergise with what he got from the Tusk Pig.
As for what the mutation agent got from the Tusk Pig for Grud in terms of the goal to hunt and protect? It highly increased his sense of smell, slight increase in bone mass and skin becoming tougher. As for the reproduction side, the mutation agent will use the information from the host's mind and in the host mind, bigger is better.
So the mutation agent got to mutating, the Tusk Pig's penis was not the right shape to be compatible with the host's body so it discarded the gene that controlled the shape but it could adapt the gene that controlled the length which was very long relative to the Tusk Pig's body, had to bulk up the girth to support the new length and grow more foreskin to cover the tip.
And lastly was the testicles, the Tusk Pig's testicles were the size of large oranges but taking into account the space between Gurd's legs, it was not practical to the mutation agent and scaled them down to the size of normal oranges.
What the mutation agent did not take into account was that with the now rapid sperm and semen production, his testicles would inflate to the impractical large orange size, but with an organ that was practically outside the body, if it fits it sits. All the mutation agent had left to do was expand his scrotum to accommodate his new twins.
While all these changes were happening his mind was retreating back to the safe place in his head called sleep to avoid all the growing pains. And the growing part was no joke, before Grud went to sleep he looked like a malnourished teenager on the verge of death. After the mutation was done, he looked like a healthy but skinny young man with wiry muscles. By the time his mind deemed it safe for his body to return to consciousness the sun was already high in the sky.
When Grud opened his eyes and breathed in the afternoon air and froze. Did he really need a bath that bad? He just washed himself last night, 'it may not be the most thorough wash he ever did but it should not smell this bad.' It was after a while that he could actually pinpoint where certain smells were coming from and there was one smell he recognized it was the smell of semen and it was coming from himself. He looked down at his crotch to see his twitching dick standing straight up at an angle, dripping precum and managed to flip his leather wrapping on his belly through pure stiffness.
He stared at his dick not believing what he was seeing and after sitting up to get a better look. He grabbed his dick to check if it was real or not and it felt very real. After roughly measuring his new dick, it measured in at slightly over ten inches long and the girth was almost the size of his wrist but the more surprising and worrying thing he saw were his balls, they were the size of large oranges and instead of hanging in mid air just below his dick, they were now sitting comfortably on the floor. But more than any of that, he was horny as fuck and needed to bust a fat nut. So he did the only thing he knew to do in the situation, he had to jack off right now.
As he began beating his meat, he realized his old reliable way of doing it was not enough, he had to double hand this job. And as he did his first backstroke, his tight foreskin peeled back releasing another inch of dick being contained by the foreskin and revealed a fat flared out mushroom head.
After about a minute of pumping, he could not hold back his new sensitive equipment, his huge balls literally flexed up towards his body and he shot the biggest load of his life. He shot rope after rope of semen at the tree root wall and because of the light peeking through this leaf roof, he noticed that the semen he shot was a lot thicker than the last time he masturbated. Even with the huge volume of sticky semen on the wall, it barely dribbled down the wall.
After the first shot, his balls were still the size of large oranges and his dick was still raring to go. "This is going to take awhile"
Over the course of just over an hour of furious masturbation. Grud produced over two liters of semen before his balls calmed down due to the lowering water content of his body. He painted the wall of his shelter white and would need to move his shelter to the root next to his current "unpainted" wall.
Now that his dick and balls had calmed down, he stood up to go about his day and his head crashed through the makeshift roof of his shelter that he fit in perfectly before. He noticed he had grown about a head taller and was almost as tall as the bigger men from the tribe.
Now that he finally had a proper meal without his hyper metabolism taxing him so hard, a good long sleep and post nut clarity, he thought back to the day before until now. "What in the Spirit's name is happening to me? I managed to eat a whole Tusk Pig, raw, which should have made my stomach sick. My dick and balls have grown so much and I do not think the amount of seed I shot out was normal. And on top of that I have grown so much taller."
As he made his way out his shelter into the sunlight, he noticed how much his body filled out, he no longer looked anorexic but skinny and athletic. Just as he finished inspecting himself, his stomach growled from hunger and he finally noticed how thirsty he was from losing so much water content just now.
As Grud made his way to the stream he noticed a problem. As he was walking, his dick, even though it was now flaccid and smaller than when he woke up, was hanging half way down to his knees with the tip of his dick peeking out from under the leather wrapped around his waist and was flopping around as he walked but the bigger problem was his balls.
They were knocking around between his thighs and making his walking awkward. To fix this, he undid his leather wrapping, pulled it up in between his legs to secure his dick and balls out of the way of his thighs and secured everything together in a loincloth fashion with some nearby vines.
After taking care of that, he went to the stream to drink his fill and decided to test his new body while searching for a meal for the day. During his brief walk to the stream, he felt his body being lighter and more powerful then he ever had before.
So he started the test with a run, as he took off, his foot dug into the ground due to his now higher weight and density as he sprinted past trees. Grud was amazed at the speeds he was reaching, he may just be able to outrun anyone in the tribe.
While he was running, he got the bright idea to test his jumping capabilities and as he put said bright idea into motion, he did not bother starting off small and went for a full powered jump. Which flung him one and a half times his height forward straight into a tree branch to his stomach, knocking the wind out of him and snapping the branch as thick as his forearm to fall with him.
When he landed on his ass with a tree branch clutched to his midsection, he expected some scrapes and bruises but as he checked his body, besides some dirt, he was undamaged. He poked around his own body and found that his muscles were a lot harder and skin a lot tougher. "Whatever is happening to me, it has made me stronger. Are the Spirits preparing me for what is to come? If so, I thank you Great Spirits!"
"Now for some hunting" As Grud was done with his testing, he decided to go get some food. And with his now improved body he had no problem running down and killing five rabbits that were too far away from their burrow. He looked at the sky and noticed that he better get back to camp to relocate his shelter before it gets dark or he will need to sleep next to the cum wall he made.
After reaching camp and dealing with the shelter, he started a fire, skinned and gutted the rabbits for cooking. Just as he was about to start cooking, a thought came to him. "The shaman told me that I have to eat the heart of the Big Tooth Cat to gain great power…. What if I am also gaining power from the hearts of the animals I ate yesterday?" Grud pondered as he removed the organs from the rabbits. "I guess there is only one way to find out." After he skewered the rabbits and let them cook on the fire, he popped the small rabbit hearts in his mouth like candy and swallowed them. In a way, Grud was right in his assumption that consuming the heart was the way to gain power but all the mutating agent needed was that sweet undamaged genetic information.
And with the eating of raw hearts out of the way, he cooked and ate his rabbit dinner. After dinner he went to the stream to wash himself, when he undressed he noticed his balls were back to their bloated size and his dick was leaking precum even though he was flaccid. He sighed and tried to ignore the backed up pressure he felt in his balls, washed himself and his spear and went to sleep.